Air-to-Water Heat Pumps 

With Radiant Delivery in 

Low-Load Homes

C. Backman, A. German, B. Dakin, and 

D. Springer 

Alliance for Residential Building Innovation

December 2013
NOTICE

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iii

Air-to-Water Heat Pumps With Radiant Delivery in Low-Load Homes

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program 

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway 

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

C. Backman, A. German, B. Dakin, and D. Springer 

Alliance for Residential Building Innovation

Davis Energy Group, Team Lead 

123 C Street

Davis, California 95616 

NREL Technical Monitor: Michael Gestwick

Prepared under Subcontract No. KNDJ-0-40340-03 

December 2013 
iv

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v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary.................................................................................................................................... x

Acknowledgments .................................................................................................................................... xii

1 Introduction........................................................................................................................................... 1

1.1 Background and Motivation ............................................................................................................ 1

1.2 Objectives and Research Questions................................................................................................. 2

2 Technology and Project Description.................................................................................................. 4

2.1 Air-to-Water Heat Pump Products................................................................................................... 4

2.1.1 Hydronic Distribution Options ........................................................................................... 5

2.2 Test House Measure Details ............................................................................................................ 5

2.3 System Costs.................................................................................................................................. 11

3 Methodology ....................................................................................................................................... 14

3.1 General Technical Approach ......................................................................................................... 14

3.2 Measurements................................................................................................................................ 15

3.2.1 Monitoring Data Points..................................................................................................... 15

3.2.2 Short-Term Tests .............................................................................................................. 21

3.3 Equipment...................................................................................................................................... 21

3.3.1 Data Logger and Sensor Types and Specifications........................................................... 21

3.4 Computation of Monitoring Variables........................................................................................... 22

3.4.1 Heat Pump Performance ................................................................................................... 22

3.4.2 Heat Pump Water Heating Efficiency............................................................................... 24

3.4.3 Indoor Comfort and Dew Point......................................................................................... 24

3.4.4 Additional Data................................................................................................................. 25

3.5 Modeling Methodology ................................................................................................................. 25

3.5.1 Calibration Process........................................................................................................... 25

3.5.2 Climate Zone Evaluation .................................................................................................. 30

4 Results................................................................................................................................................. 33

4.1 Monitoring Results and Discussion ............................................................................................... 33

4.1.1 Heating Performance ........................................................................................................ 33

4.1.2 Domestic Hot Water Performance .................................................................................... 35

4.1.3 Cooling Performance ........................................................................................................ 36

4.1.4 Nighttime Precooling Strategy.......................................................................................... 39

4.1.5 Distribution System Performance ..................................................................................... 41

4.1.6 Latent Cooling, Dehumidification, and Condensation...................................................... 43

4.2 TRNSYS Modeling Results........................................................................................................... 45

4.3 Cost Effectiveness.......................................................................................................................... 51

5 Conclusions and Recommendations ............................................................................................... 52

6 References .......................................................................................................................................... 55

Appendix A: Analysis Details .................................................................................................................. 57

Appendix B: Short-Term Testing and Commissioning Results ........................................................... 58

S.E.E.D................................................................................................................................................. 58

System/Sensor Commissioning ..................................................................................................... 59

HERS Tests.................................................................................................................................... 60

Cana...................................................................................................................................................... 61

System/Sensor Commissioning ..................................................................................................... 62

Airflow Calibration and Balance ................................................................................................... 63

Appendix C: S.E.E.D. House Mechanical Systems Control.................................................................. 65

Appendix D: Additional TRNSYS Model Results ................................................................................... 66
vi

List of Figures

Figure 1. Schematic of air-to-water cooling system with MM distribution ........................................... 2

Figure 2. Completed Cana test house....................................................................................................... 6

Figure 3. Completed S.E.E.D. test house.................................................................................................. 6

Figure 4. Schematic of AWHP with MM distribution ............................................................................... 8

Figure 5. The Monobloc Altherma installed at the Cana house ............................................................. 9

Figure 6. The hydronic installation at the Cana house. The mechanical room on the left shows the 

storage tank for DHW and piping to both the radiant floor and the fan coil. On the right is a 

manifold for the radiant floor. ............................................................................................................. 9

Figure 7. West elevation of S.E.E.D. house showing sun screen over window and AWHP.............. 10

Figure 8. S.E.E.D. house hydronic equipment and piping installed in the garage............................. 10

Figure 9. Sensor locations for measuring HP system performance (water side measurements only)20

Figure 10. HP calibration step showing alignment between model and monitoring data for a 

heating event (left) and cooling event (right) .................................................................................. 26

Figure 11. Heating event (left) and cooling (right) temperature comparison during flow events .... 28

Figure 12. Interior temperature comparison .......................................................................................... 29

Figure 13. Energy input required to change slab temperature for heating (left) and cooling (right)30

Figure 14. Calculated full load COP of the S.E.E.D. house HP in space heating versus outdoor drybulb temperature and EWT and compared to manufacturer-rated specifications (N = 1,241)... 34

Figure 15. Calculated full load COP of the Cana house HP in heating mode versus outdoor drybulb temperature and LWT and compared to manufacturer-rated specifications (N = 1383) .... 35

Figure 16. Calculated full load COP of the Cana house HP in DHW mode versus outdoor dry-bulb 

temperature and LWT and compared to manufacturer-rated specifications (N = 1607)............. 36

Figure 17. Calculated full load EER of the S.E.E.D. house HP (condenser + pump) in space cooling 

versus outdoor dry-bulb temperature and lLWT and compared to manufacturer-rated 

specifications (n = 8,208)................................................................................................................... 38

Figure 18. Calculated full-load EER of the Cana house HP (condenser + pump) in space cooling 

versus outdoor dry-bulb temperature and LWT and compared to manufacturer-rated 

specifications (N = 337)...................................................................................................................... 39

Figure 19. Daily average EER and energy use comparison for C&C (N = 82) versus constant set 

point (N = 30) operating strategies in radiant floor delivery mode................................................ 40

Figure 20. Interior temperature and HP operation for two hot days comparing C&C versus 

constant set point operating strategies in radiant floor delivery mode ....................................... 41

Figure 21. Radiant floor distribution efficiency versus outdoor dry-bulb temperature for S.E.E.D 

house (left) and Cana house (right).................................................................................................. 42

Figure 22. Daily average HP (condenser + pump + fan) energy use comparison for radiant floor 

delivery (N = 82) versus MM delivery (N = 71) modes in C&C operation ...................................... 43

Figure 23. Evaluation of S.E.E.D. house radiant slab condensation potential during 2012 monsoon 

season (MM operation) ...................................................................................................................... 44

Figure 24. Monitored S.E.E.D. house evaporator coil condensate flow (MM operation)................... 45

Figure 25. Tucson modeled indoor conditions during cooling season .............................................. 47

Figure 26. Plugged Altherma filter screen.............................................................................................. 62

Figure 27. Mechanical system controls schematic ............................................................................... 65

Figure 28. Sacramento indoor conditions over the course of the year as compared to ASHRAE 55-

2010 comfort ....................................................................................................................................... 67

Unless otherwise noted, all figures were created by the ARBI team.
vii

List of Tables

Table 1. HVAC Measure Specifications .................................................................................................... 7

Table 2. S.E.E.D. House System Total and Incremental Costs............................................................. 12

Table 3. Cana House System Incremental Costs................................................................................... 12

Table 4. Mature Market Incremental Cost Estimates (Dry Climatea).................................................... 13

Table 5. Evaluated Cooling Strategies.................................................................................................... 14

Table 6. S.E.E.D. House Monitoring Points List..................................................................................... 16

Table 7. Cana House Monitoring Points List.......................................................................................... 18

Table 8. Sensor Specifications ................................................................................................................ 21

Table 9. HP Flow Parameters................................................................................................................... 26

Table 10. Fan Coil Parameters................................................................................................................. 27

Table 11. Slab and House Characteristics.............................................................................................. 27

Table 12. Evaluated Cities and Climate Zones....................................................................................... 30

Table 13. House Characteristics as Modeled in TRNSYS..................................................................... 31

Table 14. Design Heating and Cooling Loads (Btu/h) ........................................................................... 31

Table 15. Equipment Assumptions Used in the TRNSYS Model.......................................................... 31

Table 16. Distribution Efficiency Comparison ....................................................................................... 42

Table 17. TRNSYS Model Predictions for Tucson, AZ. ......................................................................... 46

Table 18. Tucson TRNSYS Results Comparison ................................................................................... 48

Table 19. Sacramento TRNSYS Results Comparison ........................................................................... 49

Table 20. Sacramento TRNSYS Results Comparison with C&C Operation June–September from 4 

a.m.–6 a.m. .......................................................................................................................................... 49

Table 21. Denver TRNSYS Results Comparison.................................................................................... 49

Table 22. Houston TRNSYS Results Comparison (Stand-Alone DH, DH Waste Heat to Conditioned 

Space).................................................................................................................................................. 50

Table 23. Houston TRNSYS Results Comparison (DH Waste Heat Rejected to the Outside)........... 50

Table 24. Climate Zone Annual HVAC Energy and Utility Cost Savingsa............................................ 51

Table 25. Cost-Effectiveness Evaluation................................................................................................ 51

Table 26. Statistical Results from Multiple Linear Regression of Condenser Power and Capacity in 

Space Cooling in Relation to OAT and LWT.................................................................................... 57

Table 27. Statistical Results from Multiple Linear Regression of Condenser Power and Capacity in 

Space Heating in Relation to OAT and LWT .................................................................................... 57

Table 28. Hydronic System Flow Rate With Various Zones Calling ................................................... 59

Table 29. Sacramento TRNSYS Results Comparison with Dehumidification Control....................... 66

Table 30. Denver TRNSYS Results Comparison With Dehumidification Control............................... 67

Unless otherwise noted, all tables were created by the ARBI team.
viii

Definitions

ACH Air changes per hour

ACM Alternative Calculation Methodology

AH Air handler

AHRI Air-Conditioning, Heating, and Refrigeration Institute

AMY Actual meteorological year

ARBI Alliance for Residential Building Innovation

AWHP Air-to-water heat pump

BA Building America

BEopt Building Energy Optimization

Btu British thermal unit

C&C Cool and coast

CEC California Energy Commission

CI Confidence interval

COP Coefficient of performance

CZ Climate zone

DEG Davis Energy Group

DH Dehumidifier

DHW Domestic hot water

DOE U.S. Department of Energy

EAT Entering air temperature

EER Energy efficiency ratio

EIA U.S. Energy Information Administration

EPS expanded polystyrene

ERV Energy recovery ventilator

EWBT Entering wet-bulb temperature

EWT Entering water temperature

HERS Home Energy Rating System

HP Heat pump

HSPF Heating seasonal performance factor

IECC International Energy Conservation Code

kWh Kilowatt-hour
ix

LWT Leaving water temperature

MM Mixed-mode

OAT Outdoor air temperature

PV Photovoltaic

PWR Power

RCC Reverse cycle chiller

RH Relative humidity

S.E.E.D. Super Energy Efficient Design

SEER Seasonal energy efficiency ratio
x

Executive Summary

Vapor compression heating and cooling system performance varies substantially with changing 

operating conditions. Performance can be improved by reducing the “thermal lift” (the difference 

between condenser and evaporator temperatures) of the system, reducing compressor cycling, 

and improving distribution efficiency. Air-to-water heat pumps (AWHPs) that substitute a 

refrigerant-to-water heat exchanger for the customary refrigerant-to-air indoor air coil and that 

use hydronic distribution can be used to facilitate these potential efficiency improvements.

Space conditioning represents nearly 50% of average residential household energy consumption 

(U.S. Energy Information Administration 2009), highlighting the need to identify alternative 

cost-effective, energy efficient cooling and heating strategies. As homes are built better, there is 

an increasing need for strategies that are particularly well suited for high performance, low-load 

homes. Due to their efficiency advantages, AWHP systems are particularly well suited for lowload homes.

This research evaluates air-source AWHPs applied to radiant floor and mixed-mode (MM) 

distribution systems. The MM strategy consists of hydronic distribution using a small fan coil 

connected in series with and upstream of a radiant floor system. Two monitoring projects in hotdry climates were initiated in 2010 to test this strategy. One of the projects, the Cana house, is a 

three-bedroom, 3,270-ft2 straw-bale house located in Chico, California, which uses a threefunction Altherma heat pump. The second project is the Super Energy Efficient Design 

(S.E.E.D.) house, which is a 1,935-ft2

, single-story spec home in Tucson, Arizona. The heat 

pump at the S.E.E.D. house is a built-up system with an Aqua Products refrigerant-to-water heat 

exchanger. The systems in each test house were fully instrumented and monitored over 1 year to 

capture complete performance data over the cooling and heating seasons. Results are used to 

quantify energy savings, cost effectiveness, and system performance using different operating 

modes and strategies. A calibrated TRNSYS model was developed and used to evaluate 

performance in various climate regions. 

Monitoring results demonstrated seasonal space heating coefficients of performance over the 

full monitoring period of 3.26 and 4.18 at the S.E.E.D. house and Cana house, respectively. 

Measured heating performance of the Altherma heat pump was very comparable to manufacturer 

specifications; however, water heating performance was much lower than expected as a result of 

poor heat transfer between the heat pump supply loop and the storage tank and regular operation 

of the electric resistance backup heater. Seasonal energy efficiency ratios over the monitoring 

period in space cooling were 11.2 and 10.8 at the S.E.E.D. house and Cana house, respectively. 

This is a substantial improvement over measured performance in the field of residential air 

conditioners with ducted air delivery of 5.5 to 8.5 energy efficiency ratios (Proctor et al. 2011). 

Performance was most dependent on outdoor air conditions with less than expected sensitivity 

between efficiency and entering water temperature on the load side. Data were not able to 

confirm expected performance improvements of the hydronic system due to reduced thermal lift 

from high supply temperatures in cooling and lower supply temperatures in heating. 

Measured distribution efficiencies of the radiant floor distribution averaged 96%. This is 

approximately equivalent to a ducted distribution system with ductwork located inside 
xi

conditioned space (94%), but is much higher than the 76% estimated for typical attic-located 

tight ducts (≤ 6% air leakage).

TRNSYS modeling estimates up to 31% annual HVAC energy savings compared to an air-to-air 

heat pump with tight ducts located in the attic and up to 28% compared to the same base case 

with ducts located within conditioned space. Percent savings are higher for cold climates or hot 

climates with high heating loads as modeling results indicate higher radiant distribution 

effectiveness for heating than for cooling. Some form of dehumidification is required in all but 

the driest climates. Monitoring and modeling results from the dry Central Valley of California 

indicate that neither floor condensation nor interior comfort is a concern with radiant-only 

cooling distribution. In other dry climates, such as Tucson, Arizona, some dehumidification is 

required during the humid monsoon season, which can be accomplished with the MM

distribution strategy. A control strategy to optimize performance could incorporate a humidistat 

control on the fan coil to switch from floor cooling to MM cooling only during periods of rising 

indoor moisture conditions. Due to high latent loads, the model found that radiant cooling is not 

appropriate in humid climates.

Substantial cooling energy savings can be achieved from a precooling operating strategy that

shifts air-conditioner daytime operation to cooler nighttime hours and utilizes the house thermal 

mass to ride out most peak afternoon cooling events. Monitoring results from the S.E.E.D. house 

show 27% savings from precooling operation at a daily maximum outdoor temperature of 90°F 

and up to 40% savings at a maximum temperature greater than 100°F. TRNSYS modeling 

estimates 17%–43% seasonal cooling energy savings from precooling in hot-dry climates. 

Seasonal percent savings can be lower than daily savings on hot days due to overcooling on 

milder days. An optimized solution could minimize this by employing a “smart” precool strategy 

that monitors weather conditions and changes the precooling set point accordingly. 

AWHPs with radiant or MM delivery are an effective and efficient means of providing space 

heating and cooling in residential buildings in certain climates. This strategy presents a viable 

alternative to locating ductwork in conditioned space, which may not be feasible in all homes 

due to architectural challenges, while providing the comfort, thermal storage, improved 

distribution, and reduced noise benefits of radiant slab delivery. Based on a cost benefit analysis 

over a 30-year mortgage using mature market costs, the strategy was found to be cost effective 

only in cold climates and not in hot-dry climates. However, there are various benefits provided 

by the AWHP radiant system that are not factored into a cost analysis. These include increased 

occupant comfort, improved distribution, noise reductions, and peak load reduction. In addition, 

as electricity prices increase this may move the technology into cost effectiveness for additional 

climates. 

Current system costs are high; however, there is justification to anticipate lower incremental 

costs as this strategy gains wider market acceptance. Cost reductions can be expected with

increased contractor familiarity and reductions in manufactured equipment costs from volume 

production. Further research focused on development of packaged AWHPs as well as packaged 

controls for zoned systems is necessary. This will be a driver for cost reductions and simplified 

installation procedures, as well as for ensuring consistent levels of quality and gaining market 

acceptance from contractors and installers.
xii

Acknowledgments

Davis Energy Group would like to acknowledge the U.S. Department of Energy Building 

America program for its funding and support of development of this technical report as well as 

the research that informed it. In addition, we would like to acknowledge builders Michael 

Ginsburg of La Mirada Homes and Robin Trenda of Chico Green Builders, Mark Sadler at 

Daikin AC, and Michael Erickson at Hydronic Heat Pumps for their cooperation throughout the 

design, construction, and monitoring stages of this project.
1

1 Introduction

1.1 Background and Motivation

Space conditioning represents nearly 50% of average residential household energy consumption

according to the 2009 Residential Energy Consumption Survey (U.S. Energy Information 

Administration [EIA] 2009). Identifying cooling and heating strategies that address the need for 

more cost-effective, energy efficient residential systems will lead to reductions in overall 

residential energy consumption and help further progress toward Building America (BA) goals. 

Vapor compression heating and cooling system performance varies significantly with changing 

evaporator and condenser temperatures, evaporator airflow, equipment cycling, and other factors

such as system charge and airflow. Installed heat pump (HP) system performance can be 

improved by reducing “thermal lift” (the difference between condenser and evaporator 

temperatures) of the system, reducing compressor cycling, and improving distribution efficiency. 

Air-to-water heat pumps (AWHPs) that substitute a refrigerant-to-water heat exchanger for the 

customary refrigerant-to-air indoor air coil and that use hydronic distribution can be used to 

facilitate these potential efficiency improvements.

As homes are built better, there is an increasing need for heating and cooling strategies suited for 

high performance, low-load homes. Forced-air furnace systems are typically oversized for these 

applications, resulting in reduced operating efficiencies and delivery effectiveness. Ductwork 

located in attics results in large distribution losses due to conduction and air leakage.

AWHP systems are an energy efficient space conditioning solution that, through additional field 

research and coordination with manufacturers, has the potential to lead to a market-ready product 

that cost-effectively provides comfort in homes with efficient, safe, and durable operation. 

Radiant delivery increases distribution efficiencies and hydronic systems with higher cooling 

supply temperatures and lower heating supply temperatures improve system efficiency and are 

well designed for low-load homes. Radiant cooling distribution is most appropriate to hot-dry 

climates; however, when combined with a dehumidification strategy it may be applicable in 

humid climates. The technology can be implemented in both single and multifamily residences. 

While it may be appropriate for some deep-retrofit projects, the focus of this research is new 

construction.

Critical path milestones related to high performance HVAC and delivery systems identified by 

the BA Space Conditioning Standing Technical Committee members include the following:

• Identify low-cost space conditioning distribution strategies with negligible conductive, 

radiant, and leakage losses

• Demonstrate market-ready, high-efficiency, small-capacity heating and cooling 

equipment for low-load situations.

Through detailed monitoring of two test houses and TRNSYS modeling, this research project 

evaluates air-source AWHPs applied to mixed-mode (MM) distribution systems that are capable 

of moderating condensing/evaporating temperatures and reducing thermal lift. The MM

distribution strategy consists of hydronic distribution using a small fan coil connected in series 

with and upstream of a radiant floor system (see Figure 1). Chilled water is piped first to the 
2

small fan coil, which provides latent and sensible cooling and significantly reduces the size of 

ducting needed. The water is then delivered to the radiant floor tubing, which will provide the 

bulk of the sensible cooling. Piping chilled water to the fan coil first warms the water entering 

the slab and removes moisture in the supply airstream, reducing the risk of condensation on the 

floor surfaces.

Figure 1. Schematic of air-to-water cooling system with MM distribution

The radiant system provides primary distribution, thermal storage, and zoned comfort. Due to the 

thermal mass requirements, houses must be built on slab foundations with exposed or tile floors 

(minimal carpeting) or incorporate other strategies of adding sufficient mass to the house.

The effectiveness of a MM forced-air/radiant cooling system was evaluated in a previous BA

study in Borrego Springs, California, with results showing significant improvements in cooling 

efficiencies (Springer et al. 2008). Two nearly identical homes were equipped with the same 

model 13 seasonal energy efficiency ratio (SEER) condensing unit, one connected to a 

conventional direct expansion evaporator coil and ducted distribution system, and the other to a 

refrigerant-to-water heat exchanger with MM distribution. Over the test period from July through 

September 2007, energy efficiency ratios (EERs) of 5.1 and 10.3 were measured for the standard 

system and the chilled water system, respectively. It was theorized that the reduced thermal lift 

resulting from the relatively high evaporator temperature of the chilled water system was 

responsible for the substantial reduction in compressor power. Because the slab underside was 

uninsulated, little benefit was seen in the way of seasonal energy savings, resulting in 

distribution inefficiencies due to the significant downward energy loss. 

1.2 Objectives and Research Questions

The primary objectives of this study are to evaluate air-source AWHPs applied to MM systems 

that use radiant cooling and heating as the primary means of distribution, along with an upstream 

forced-air cooling coil for humidity control (latent cooling), and determine how well this strategy 

performs in various climates. 

CONDENSING

UNIT

EVAPORATOR

FAN COIL

RADIANT DISTRIBUTION

(Plate Heat

Exchanger)

(Tubing in Slab Floor)

Return

Air

Supply

Air
3

Efforts are made to provide conclusions to the following research questions in this report:

1. What are the average effective heating coefficients of performance (COPs)? What is the 

efficiency of the integrated water heating/space heating system in heating mode 

(Altherma) and how does it compare to manufacturer’s specifications?

2. What are the average effective cooling EERs, and can the dramatic improvement in 

performance relative to typical forced-air-only systems seen in previous testing be 

replicated?

3. How effective is nighttime precooling in improving HP efficiencies and reducing cooling 

energy use?

4. How does the distribution efficiency of the MM system compare to that of a typical 

forced-air delivery system with ducts in unconditioned space?

5. Is the fan coil and the latent cooling it provides necessary for dehumidification and to 

prevent floor condensation in a hot-dry climate, or, can the forced-air delivery be 

eliminated completely? 

6. Can TRNSYS reliably predict performance of this HVAC strategy?

7. In what climate zones is this strategy applicable?
4

2 Technology and Project Description

2.1 Air-to-Water Heat Pump Products

AWHPs operate on the same mechanical principles as air-to-air HPs, but instead of connecting 

outdoor units to an indoor refrigerant-to-air heat exchanger coil as split-system air-to-air HPs do, 

AWHPs employ a refrigerant-to-water heat exchanger and generate hot or chilled water. To 

distribute heating and cooling they circulate the water through fan coils or radiators. 

Numerous manufacturers offer AWHPs. These may be offered as “packaged” products that 

incorporate both the refrigerant-to-water heat exchanger and the outdoor unit, such as the Daikin 

Altherma,

1 the Multiaqua MACH,2 the Unico UniChiller,3 and the SpacePak Chiller.4 However, 

the Multiaqua models provide cooling only, not heating. Other companies, such as Aqua 

Products,

5 manufacture individual refrigerant-to-water heat exchangers that can be coupled to 

any commercially available outdoor unit. The Daikin Altherma product can be configured as a 

split system in which the indoor unit includes the refrigerant-to-water heat exchanger, pump, 

controls, and other hydronic components, or as a monobloc-package unit with all of the 

components located with the outdoor unit and supply and return piping run to the building. The 

monobloc-package units are factory charged and only require water and control connections. The 

Daikin units have inverter-driven compressors and variable-speed outdoor fans. These systems 

can produce water that is typically in the range between 40°F and 130°F. 

Of the systems discussed above, all are currently available in the United States and several more 

options are sold in Asian and European markets. Other manufacturers include LG and Fujitsu. 

The two systems evaluated under this research are the Aqua Products reverse cycle chiller (RCC)

installed in the S.E.E.D. house and the Daikin Altherma Monobloc installed in the Cana house.

Advantages of these systems over air-to-air HPs include the following:

• With the use of inverter-driven compressors (Daikin) or buffer tanks they can operate at a 

very wide range of capacities and accommodate low-load buildings.

• They can be operated down to very low outdoor temperatures and when used with buffer 

tanks or radiant floor slab distribution do not require resistance heat during defrost cycles.

• They can utilize building mass, particularly radiant floor slabs, to shift load and improve 

performance by operating when the outdoor temperature is lower in summer and higher 

in winter.

• Systems are easily zoned without the penalties experienced with air-based systems.

Daikin and others offer a domestic water heating option that allows the full capacity of the HP to 

be applied to water heating. A three-way valve switches between space heating and cooling, and 

water heating.

 1 See www.daikinac.com/residential/altherma-system-configuration.asp?sec=productsandpage=53. 2 See www.multiaqua.com/index.htm. 3 See www.unicosystem.com/Home/Products/UniChillerRC/tabid/80/Default.aspx. 4 See http://spacepak.com/air-conditioning-products.asp#chb. 5 See www.aquaproducts.us/products/reverse-cycle-chiller.html
5

2.2 Hydronic Distribution Options

AWHPs open up several options for distribution. As with air-to-air split system HPs, fan coils 

and ducting can be used for distribution. They can also deliver heating and cooling to a variety of 

hydronic distribution systems including radiant floor, ceiling, and wall panels; ductless fan 

convectors; traditional radiators; and baseboard convectors. To capitalize on the opportunity to 

improve performance through reduced thermal lift and load shifting, their best application is with 

radiant floor systems. The Alliance for Residential Building Innovation (ARBI) team researched 

various hydronic distribution systems to determine their market potential in a feasibility study

report (Springer et al. 2012). 

Because of the potential for moisture damage from condensation, radiant floor cooling must only 

be used with exposed concrete slabs or slabs with ceramic tile or stone coverings. Carpeting and 

wood floors increase the risk of floor condensation and provide a better medium for mold 

growth, and vinyl flooring acts as a vapor barrier to trap the condensed moisture. The 

recommended minimum floor surface temperature is about 65°F. The risk of condensation is 

reduced when used in well-insulated, tightly constructed houses in dry climates. Since radiant 

cooling is strictly “sensible,” moisture removal can be accomplished by introducing a small fan 

coil in series and upstream of the radiant panels to provide latent cooling and moisture removal. 

In all but the driest climates some method for dehumidification is highly advised because indoor 

moisture sources can elevate dew point temperatures even in well-constructed and properly 

ventilated houses. Delivering chilled water to the fan coil first facilitates moisture removal, and 

increases the temperature of the water entering the radiant panel, making it less likely to 

approach the dew point temperature. The relative percentage of cooling provided by the coil and 

floor can be varied through adjustment of the air handler fan speed. This strategy was shown to 

provide a dramatic improvement in condensing unit EER (~200% increase in EER compared to a 

13 SEER unit with conventional ducted distribution) in prior BA research conducted at Borrego 

Springs (Springer et al. 2008). 

2.3 Test House Measure Details

Two projects were initiated in 2010 to test MM AWHP systems. One of the projects, the Cana 

house, is a three-bedroom, 3,270-ft2 straw-bale house located in the hot-dry Northern California 

climate of rural Chico (see Figure 2). The owners wanted radiant floor heating and an integrated 

system to effectively heat and cool their home. The home is located in a rural area with no access 

to natural gas on site. The Cana house uses the Daikin Altherma inverter-driven three-function 

AWHP for space heating and cooling as well as domestic water heating. By substituting the

Altherma for a conventional furnace, air conditioner, and water heater, the need for propane gas 

was eliminated, and electric heating allows more of the building energy use to be met by on-site 

photovoltaic (PV) generation. 
6

Figure 2. Completed Cana test house

The second test house is the Super Energy Efficient Design (S.E.E.D.),

6 which is a 1,935-ft2

, 

single-story spec home located in the hot-dry climate of Tucson (German et al. 2012; see 

Figure 3). The builder, Michael Ginsburg of La Mirada Homes, developed this prototype design 

with the goal of providing exceptionally efficient yet affordable homes. The S.E.E.D. test house

uses an Aqua Products RCC for space heating and cooling only. The RCC packages a 

conventional Ruud 13 SEER HP with a refrigerant-to-water heat exchanger. 

Figure 3. Completed S.E.E.D. test house 

 6 More information on the S.E.E.D. house can be found at the builder’s website: 

http://lamiradahomes.net/lamirada_homes_seed.htm
7

Both test homes were completed in 2011 and are currently occupied. Both systems have been 

fully instrumented and are monitored over 1 year to capture complete performance data over the 

cooling and heating seasons. The two test sites also provide information on costs, installation, 

equipment and system operation and durability, and contractor training needs. Results are used to 

quantify energy savings, cost effectiveness, and system performance using different operating 

modes and strategies. Table 1 lists the basic home characteristics and the specifications for the 

AWHP systems installed in the two test houses. For additional details on the S.E.E.D. house see 

ARBI’s “Super Energy Efficient Design (S.E.E.D.) Home Evaluation” (German et al, 2012).

Table 1. HVAC Measure Specifications

Measure Cana Test House S.E.E.D. Test House

Basic Building Characteristics

Building Type/Stories Single family, one story Single family, one story

Conditioned Floor Area, Ft2 3,270 1,935

Number of Bedrooms 3 4

Heating/Air-Conditioning Type 

and Efficiency

Altherma Monobloc HP: 

COP 4.37 and EER 9.4

Ruud HP w Aqua 

Products RCC: heating 

seasonal performance 

factor (HSPF) 8.4 and

EER 11 (Ruud rating)

Heating Distribution MM radiant floor and

fan coil Radiant floor

Cooling Distribution MM radiant floor and fan 

coil

MM radiant floor and

fan coil

Duct Location and Insulation Attic R-6, < 6% total 

leakage Conditioned space, R-6

At both test houses space heating and cooling are provided by the AWHP, with primarily 

delivery through a radiant floor and additional delivery via a fan coil (for cooling only at 

S.E.E.D.; see Figure 4). The fan coil is sized to provide about half the required cooling capacity. 

During the winter, in the Cana house the fan coil is operated at low speed with approximately 

75% of heating being delivered through the radiant floor. At the S.E.E.D. house, a bypass valve 

at the fan coil sends the hot supply water directly to the floor and the fan is only used if requested

for air movement purposes. A dedicated mechanical ventilation system is still required in both 

scenarios. In both houses the fan coil is located in the mechanical room adjacent to the outdoor 

condensing unit, so pipe run lengths are short and pipe losses are not significant.

The Daikin Altherma has not been tested according to the U.S. Department of Energy (DOE)

testing procedures for central air conditioners and HPs (Title 10 of the Code of Federal 

Regulations part 430, subpart B, appendix M) because the test procedure does not account for 

operational characteristics of air-to-water pumps or the integrated domestic hot water (DHW)

component. In March 2011, DOE approved an alternative testing method based on the European 

testing methods and standards (European Standards EN 14511 and EN 15316) for rating the 

Altherma equipment based on EER and COP. Daikin has received approval from the California 

Energy Commission (CEC) for a compliance option that allows for modeling within the Title 24 

Building Energy Code (CEC 2012). Cooling efficiencies were tested at outdoor conditions of 
8

95°F and a supply water temperature of 64°F while heating effiencies were tested at outdoor 

conditions of 45°F and a supply temperature of 95°F. Under these conditions operational 

efficiencies are 9.42 EER for cooling and 4.37 COP for heating. 

Figure 4. Schematic of AWHP with MM distribution

Figure 5 shows the attached DHW tank and three-way valve for switching between space 

conditioning and DHW mode for the system in Chico. No buffer tank was installed because the 

thermal mass of the slab and the variable capacity capabilities of the Altherma prevent the HP

from short cycling. Figure 6 shows a picture of the installed unit at the Cana house and in 

Figure 7 the hydronic distribution system can be seen.
9

Figure 5. The Monobloc Altherma installed at the Cana house

Figure 6. The hydronic installation at the Cana house. The mechanical room on the left shows the 

storage tank for DHW and piping to both the radiant floor and the fan coil. On the right is a 

manifold for the radiant floor.

In the S.E.E.D. test house the AWHP, manufactured by Aqua Products, consists of a standard 

efficiency 13 SEER Ruud HP perched on a module that contains the evaporator coil and 

temperature controls. As is the case with the Altherma, because the Ruud is installed with 

nonmatched heat exchanger coils it is not rated by the Air-Conditioning, Heating, and 

Refrigeration Institute (AHRI). Average rated efficiencies for this unit with a matched standard 

indoor evaporator coil are 11 EER and 8.4 HSPF. The Aqua Products HP does not have the 

variable capacity capabilities of the Altherma. A 30-gal buffer tank was installed in the hydronic 
10

loop to minimize equipment short cycling. Figure 7 shows a picture of the installed unit. The fan 

coil is located in an insulated closet and all ductwork is in conditioned space. Figure 8 shows the 

installed hydronic equipment and piping. The small tank on the far right is the buffer tank for 

space conditioning. The large storage tank in the middle with the drainback tank above it is for 

the solar DHW. The hydronic fan coil can be seen on the far left in the closet.

Figure 7. West elevation of S.E.E.D. house showing sun screen over window and AWHP

Figure 8. S.E.E.D. house hydronic equipment and piping installed in the garage
11

2.4 System Costs

At the initiation of the research study, Daikin was one of the only manufacturers selling 

packaged AWHPs in the United States market, the first of which was installed in 2008. While 

other companies have since entered the market, no major U.S. manufacturer has yet begun to 

make AWHPs. Currently, these systems are priced significantly higher than standard HPs of 

similar efficiencies. For systems like the Altherma that also provide DHW, part of the 

incremental cost can be attributed to this other end use. 

Current high costs of packaged equipment are largely the result of an emerging market and 

limited consumer demand for this technology. Additionally, the controls and capabilities of the 

Altherma are quite sophisticated, adding costs that may or may not be warranted for residential 

applications. For single- or dual-zone, low-load homes with high building thermal mass, 

expensive inverter-driven compressor technology with modulating capacity does not provide the 

magnitude of savings that might be expected in other applications. The same is true of outdoor 

air reset in milder climates. Costs are expected to come down as these systems gain market 

acceptance through increased contractor familiarity and reductions in manufactured equipment 

costs due to volume production. Control simplifications should result in a much more costeffective product without significantly compromising efficiencies. Costs can also be reduced 

through the strategy used in the S.E.E.D. home in which a standard HP is used in conjunction 

with a refrigerant-to-water heat exchanger. The incremental cost in this case is attributed only to 

the heat exchanger, the radiant floor system, and any accessories. However, the limitation of this 

strategy is that there are no AHRI-certified matched combinations of commercially available 

HPs with refrigerant-to-water heat exchangers, limiting credit under building energy codes for 

high efficiency equipment. However, if these systems gain market acceptance, manufacturers 

may move to develop an AHRI testing method or identify other similar acceptance for codes, 

such as the Daikin CEC compliance option for the Altherma unit (CEC 2012). Previously, the 

federal minimum efficiencies for space conditioning HPs and electric resistance storage water 

heaters had to be used as the efficiency descriptors.

There are also a number of other nonenergy benefits of radiant floor delivery that are difficult to 

incorporate into the cost-benefit analysis, including occupant comfort, noise reductions, and peak 

load reduction. Radiant distribution provides additional comfort over air distribution system 

since it better regulates the mean radiant temperature within the space. Often with forced-air 

systems the air may be within a reasonable temperature range but the surfaces within the space 

may not be, causing occupants to feel too hot in the summer and too cold in the winter. In poorly 

insulated homes this can result in the occupant turning the thermostat down in the summer and 

up in the winter to compensate.

Complete as-built cost information for the MM delivery AWHP system was provided by the 

builder for both the S.E.E.D. home and the Cana home and are presented in Table 2 and Table 3, 

respectively. The base case is assumed to be an air-to-air HP of standard efficiency (federal 

minimums) with ducted forced-air delivery (tight ducts, R-6). Base case system costs are 

estimated from a combination of Davis Energy Group’s and Building Energy Optimization’s 

(BEopt’s) cost databases. For the S.E.E.D. house, total as-built HVAC equipment costs were 

higher than expected primarily due to the high costs from Aqua Products for the packaged air-towater condensing unit. Costs could be significantly lowered by purchasing only the refrigerant-
12 

to-water heat exchanger from Aqua Products, sourcing the HP through the contractor’s regular 

supplier, and assembling the unit on site.

Table 2. S.E.E.D. House System Total and Incremental Costs

Building Component Base Case

Specifications

As-Built 

Specifications

Base Case 

Cost

As-Built 

Costs

Incremental 

Costs

Slab 4-in. slab mono 

pour 

4-in. slab footing & 

stem + R-10 

underslab & edge 

insulation

$15,000 $19,020 $4,020 

Radiant Floor, 

Manifolds, Zone 

Controls, and Valves

None Per plan – $9,844 $9,844 

HVAC Equipment

7.7 HSPF/13 

SEER, 4-ton, R-6 

ducts in attic

Aqua Products RCC, 

2-ton $7,624 $15,182 $7,558 

Total Costs $22,624 $44,047 $21,423 

Table 3. Cana House System Incremental Costs

Building Component Base Case 

Specifications

As-Built 

Specifications

Base Case 

Cost

As-Built 

Costs

Incremental 

Costs

Slab – 

2-in. expanded 

polystyrene (EPS) 

edge + 11⁄4-in. EPS 

underslab

– $4,510 $4,510 

Radiant Floor, 

Manifolds, Zone 

Controls, and Valves

none Per plan – $9,083 $9,083 

HVAC Equipment

7.7 HSPF/13 

SEER, 5-ton + 

storage water 

heater

Altherma 

Monobloc HP, 4-

ton 

$11,810 $30,818 $19,008 

Total Costs $11,810 $44,411 $32,601

Slab insulation and hydronic system costs were quite comparable between the two test houses. 

The HVAC equipment costs at the Cana house are significantly more due to the high cost of the 

Altherma system. As-built costs include the DHW portion, and therefore a storage water heater is 

added to the base case costs. More HP downsizing potential was possible but at the time of 

installation, a 4-ton Altherma was the smallest unit available.

In dry climates where dehumidification may not be necessary, the incremental cost could be 

reduced through elimination of the fan coil and all ductwork. Coupling this with the potential 

mature market cost savings including increased contractor familiarity and reductions in 

manufactured equipment costs due to volume production, Table 4 presents estimated mature 
13 

market incremental costs for this system installed in a 2,400-ft2 BA Benchmark7 house. 

Following are justifications for the proposed reductions. 

• Slab: No cost savings are expected for this component (International Energy 

Conservation Code [IECC] Climate Zones 1–3). IECC climate zones 4–8 prescriptively 

require slab edge insulation, which would reduce incremental costs. 

• Radiant Floor: Based on 2011 RSMeans (RSMeans, 2010) pricing, $6,400 was 

estimated. This is about 65% of the cost incurred at test houses and is justified primarily 

by reduced labor costs in a production environment. This cost includes the incremental 

costs for manifolds, pumping, and controls. 

• HVAC Equipment: The primary cost savings here are attributed to the elimination of 

the ducted system. The cost savings of $4,100 assume $1,200 for the refrigerant-to-water 

heat exchanger, $860 for the high efficiency HP over standard efficiency less $6,100 for 

elimination of ductwork and the air handler (based on BEopt and local contractor pricing) 

and $100 for HP downsizing by 1⁄2 ton due to elimination of load associated with ducts in 

an attic. 

Table 4. Mature Market Incremental Cost Estimates (Dry Climatea) 

Building Component Proposed Specification Incremental Cost

Slab R-10 underslab and edge 

insulation $4,000 

Radiant Floor, Manifolds, Zone 

Controls, and Valves

Radiant floor system with 30-

gal buffer tank $6,400 

HVAC Equipment High efficiency HP (12.7 

EER, 8.8 HSPF), ductless ($4,100) 

Total Incremental Cost $6,300 a Assumes elimination of air handler and ducted distribution. 

 

7 Benchmark as defined by the Building America House Simulation Protocols (Hendron and Engebrecht 2010).
14

3 Methodology

3.1 General Technical Approach

The general approach of this research plan is to employ system commissioning, short-term tests, 

long-term monitoring, and detailed analysis of results including model calibration and 

simulations to identify the performance attributes and cost effectiveness of AWHPs with radiant 

or MM distribution. Long-term monitoring continued for a minimum of 1 full year. Monitoring 

commenced in the second quarter of 2011 for the S.E.E.D. house and the third quarter of 2011 

for the Cana house and concluded for both projects in the fourth quarter of 2012. 

The specific approach for evaluating the efficiency of the two AWHP systems is to measure 

heating and cooling energy delivered by the HPs, electrical energy consumed, and the seasonal 

operating conditions under which they are functioning, including outdoor air temperature, indoor 

air temperature, and HP entering and leaving water temperatures. These data allow for 

development of equipment performance maps that can be used in modeling, and for comparing 

performance to conventional systems. Monitoring data are used to calibrate TRNSYS models, 

which have been used to develop seasonal estimates of energy savings in various climates and 

under various conditions. Of key interest is whether the fan coils are needed to prevent floor 

condensation, and whether the mass of the floor slabs can be used to improve performance by 

shifting times of operation. Indoor temperature and relative humidity (RH) measurements are 

used to determine the dew point temperature, and the surface temperature of the slab is 

monitored to determine whether condensation may occur. Supply and return air enthalpies are 

calculated at both sites to identify latent cooling.

Control settings for the heating, cooling, and ventilation systems were verified, and the 

operations of the HP, controls, zone valves, fan, and other components were checked. Long-term 

monitoring was also used to provide “continuous commissioning” and to identify failure of any 

components.

The specific approach to evaluate the effect of using the floor slab mass to shift cooling 

operation to night and early morning to improve system performance was tested in the S.E.E.D. 

home during the 2011 and 2012 cooling seasons. This “cool and coast” (C&C) strategy takes 

advantage of the exposed floor mass and cooler outdoor nighttime temperatures to operate the 

HP at more favorable outdoor conditions. Table 5 describes the operating strategies tested and 

identifies thermostat set points.

Table 5. Evaluated Cooling Strategies

Cooling Strategy Thermostat Set Point

C&C, Precooling 78°F with 73°F setback from 

12:00 a.m. to 6:00 a.m.

Constant Set Point Fixed 76°F
15

3.2 Measurements

The two sites are equipped with data loggers and modems for continuously collecting, storing, 

and transferring data via telephone lines or cellular communications. Sensors are scanned every 

15 s, and data are summed or averaged (as appropriate) and stored in data logger memory every 

15 min. Automated scripts are used for dialup, data retrieval, range checking, and cleaning. A 

minimum of 1 year of 15-min interval data was collected for each test site. 

3.2.1 Monitoring Data Points

Table 6 and Table 7 list all data points for the S.E.E.D. test house and the Cana test house, 

respectively. Key HP water side monitoring data points are shown in the piping diagram in

Figure 9, and are nearly identical for the two test sites. As indicated in Figure 9, there are some 

differences in the design of the systems, most notably that the Cana HP produces DHW in 

addition to hot and chilled water for space conditioning, whereas the Tucson system has standalone DHW using a solar water heater with electric resistance backup for domestic water 

heating. Additionally, the Tucson system has a buffer tank installed in the loop to reduce 

compressor short cycling during low-load conditions, whereas the Altherma system at the Chico 

site has an inverter-driven compressor that modulates HP capacity to match the load, eliminating 

the need for a buffer tank.

Flow and temperature sensors in the piping allow for separately calculating heating and cooling 

delivery via the radiant floor and the fan coil. In addition to the sensors shown in Figure 9, 

temperature and RH sensors are included in the supply and return air plenums for calculation of 

sensible cooling, total cooling from measured enthalpies, and by subtraction, latent cooling 

supplied by the air system. Condensed water retained on the coil and reevaporated during fan 

operation makes it difficult to determine whether any latent cooling is actually occurring. A 

tipping bucket rain gauge installed at the Tucson site measures the volume of any water that 

leaves the condensate drain.

Indoor temperature and RH sensors are located in the individual zones (two for Cana and three

for S.E.E.D.). Each of the houses is also equipped with sensors near the surface of the slab, at the 

bottom of the slab, and below the underslab insulation. By comparing the floor temperature with 

the calculated dew point temperature it can be determined whether there is any potential for 

condensation to occur on the floor and when the floor temperature will be at its lowest. These 

sensors are also used to estimate the rate of heat transfer from the slab to the ground below.
16

Table 6. S.E.E.D. House Monitoring Points List

Abbreviation Description Location Sensor Type Sensor 

Manufacturer/Model

OAT

RHO

Temperature, air, outdoor

RH, air, outdoor

Northwest side of covered 

rear patio, in shade, on 

underside of patio roof

Resistance 

temperature 

detector (RTD), 

4–20 mA

RH, 4–20 mA

R.M. Young 41372LF

TAI1

RHI1

Temperature, air, indoor, east

RH, air, indoor, east

West wing, next to T1, 

outside bath 2, mount 

approx. 4 ft, 6 in. high

RTD, 4–20 mA

RH, 4–20 mA Vaisala HMW60

TAI2

RHI2

Temperature, air, indoor, living

RH, air, indoor, living

Great room, next to T2, on 

west wall of dining area, 

mount approx. 4 ft, 6 in. 

high

RTD, 4–20 mA

RH, 4–20 mA Vaisala HMW60

TAI3

RHI3

Temperature, air, indoor, 

master bedroom

RH, air, indoor, master bedroom

Master bedroom, next to 

T3, on south wall, mount 

approx. 4 ft, 6 in. high

RTD, 4–20 mA

RH, 4–20 mA Vaisala HMW60

TAS

RHS

Temperature, air, air handler 

(AH) supply

RH air, AH supply

Supply plenum, 

mechanical room

RTD, 4–20 mA

RH, 4–20 mA Vaisala HMD60

TAR Temperature, air, AH return Return plenum, 

mechanical room

RTD, 4–20 mA

RH, 4–20 mA Vaisala HMD60 RHR RH air, AH return

TWHL Temperature, water, HP leaving AH, mechanical room Immersion TT Thermex

TWFS Temperature, water, floor supply AH, mechanical room Immersion TT Thermex

TWHE Temperature, water, HP return Mechanical room Immersion TT Thermex

TSF1 Floor surface temperature—zone 2 Living, floor surf near 

t-stat Contact TT Omega

TSF2 Slab bottom temperature—zone 2 Above insulation near 

t-stat Contact TT Omega

TSF3 Below-slab insulation 

temperature—zone 2

Below insulation near 

t-stat Contact TT Omega

EHP Energy, HP At outdoor unit Power meter Wattnode/

WNB-3D-240-P
17

Abbreviation Description Location Sensor Type Sensor 

Manufacturer/Model

EHSE Energy, total house Main service panel Power meter Wattnode/

WNA-1P-240P-PV

EFAN Energy, AH fan AH, laundry Power meter Wattnode/

WNA-1-P-240P

EPV Energy, PV system Main service panel Power meter Wattnode/

WNA-1P-240P-PV

FWS Flow, HP system Mechanical room Flow meter Onicon F-1300

EGEN Energy, house to grid Main service panel Power meter Wattnode/

WNA-1P-240P-PV

FWC Condensate flow Mechanical room RainGauge

SZ1 Zone 1 status Mechanical room

Current status 

meter Hawkeye

SZ2 Zone 2 status Mechanical room

Current status 

meter Hawkeye

SZ3 Zone 3 status Mechanical room

Current status 

meter Hawkeye
18

Table 7. Cana House Monitoring Points List

Abbreviation Description Location Sensor Type Sensor 

Manufacturer/Model

OAT

RHO

Temperature, air, outdoor

RH, air, outdoor

Mount on north side of 

building, mount in shade

RTD, 4–20 mA R.M. Young 41372VF RH, 4–20 mA

TAI1

RHI1

Temperature, air, indoor, zone 1

RH, air, indoor, zone 1 Near Z1 t-stat RTD, 4–20 mARH, 4–20 ma Vaisala HMW60Y

TAI2

RHI2

Temperature, air, indoor, zone 2

RH, air, indoor, zone 2 Near Z2 t-stat RTD, 4–20 mARH, 4–20 mA Vaisala HMW60Y

TAS

RHS

Temperature, air, AH supply

RH, air, AH supply

Supply plenummechanical roomRTD, 4–20 mA

RH, 4–20 mA GenEastern MRHT3-2-1

TAR

RHR

Temperature, air, AH return

RH, air, AH return

Return plenummechanical roomRTD, 4–20 ma GenEastern MRHT3-2-1 RH, 4–20 mA

TWHL Temperature, water, HP leaving Mechanical room Immersion TT Thermex

TWFS Temperature, water, floor supply Mechanical room Immersion TT Thermex

TWHE Temperature, water, HP returning Mechanical room Immersion TT Thermex

TWCS Temperature, water, cold water 

supply Mechanical room Immersion TT Thermex

TWHO Temperature, water, DHW supply Mechanical room Immersion TT Thermex

TWMS Temperature, water, master shower 

hot water supply

Master bath shower 

hot water supply Surface TT Thermex

TSF1 Floor surface temperature—zone 2 Gallery - On floor surface 

near t-stat Contact TT Omega

TSF2 Slab bottom temperature—zone 2 Gallery - Above insulation 

near t-stat Contact TT Omega

TSF3 Below slab insulation 

temperature—zone 2

Gallery - below insulation 

near t-stat Contact TT Omega

TSF4 Floor surface temperature—zone 1 MBed - on floor surface 

near t-stat Contact TT Omega

TSF5 Slab bottom temperature—zone 1 MBed - above insulation 

near t-stat Contact TT Omega

TSF6 Below slab insulation MBed - below insulation Contact TT Omega
19

Abbreviation Description Location Sensor Type Sensor 

Manufacturer/Model

temperature—zone 1 near t-stat

PAS Pressure, air supply plenum Supply plenummechanical room Ptd, 4–20 mA

SDMP Status, Nightbreeze damper NB control panelmechanical room

24 VAC relay Omron

SZD1 Status, zone 1 damper NB control panelmechanical room

24-VAC relay Omron

SZD2 Status, zone 2 damper NB control panelmechanical room

24-VAC relay Omron

SVDW Status, valve, DHW three-way Mechanical room 24-VAC relay Omron

EFAN Energy, AH ran Mechanical room-NB Power meter Wattnode/WNA-1P-240P

EHP Energy, HP At outdoor unit Power meter Wattnode/WNA-1P-240-

P

FWS Flow, HP system Mechanical room Flow meter Onicon F-1300

FWD Flow, DHW Mechanical room Flow meter Dwyer

SPC Status, HW recirculation pump Mechanical room

Current status 

meter Hawkeye

EWH Energy, water heater electric 

element Mechanical room Power meter Wattnode/WNA-1P-240P

EHSE

EPV

EGEN

Energy, total house

Energy, PV

Energy, generated to grid

Main service panel

Main service panel

Main service panel

Power meter

Power meter

Power meter

Wattnode/

WNB-3D-240P(PV)
20

Figure 9. Sensor locations for measuring HP system performance (water side measurements only)

TWHL

FWS

TWHE

EWH

TWFS

ZONE VALVES

TWHO

FWD

SPC

TWCS

HEAT PUMP

BUFFER TANK WATER HEATER

FAN COIL

RADIANT FLOOR

CIRCUITS

AIR-TO-WATER

TUCSON SITE ONLY CHICO SITE ONLY

EHP

COLD WATER

SUPPLY

DHW RETURN

DHW SUPPLY

230 V

EFAN

115V

230V

3-WAY HOT WATER VALVE CHICO SITE ONLY

FWC
21 

3.2.2 Short-Term Tests

Davis Energy Group and the project Home Energy Rating System (HERS) raters completed 

short-term tests. These tests are listed below: 

• A duct pressurization test measures duct leakage at 25 Pa using an Energy Conservatory 

duct blaster. 

• Air handler air flow is measured using an Energy Conservatory fan flow meter. Data 

from this test are used to establish a relationship between fan power consumption rates 

and airflow delivered. 

• A water flow test measures flows with different zone valves operating. 

A single airflow measurement was made at the Tucson site, which uses a single-speed permanent 

split capacitor blower motor. Multiple measurements were made at the Chico site to correlate 

airflow rate with both supply plenum pressure and fan watt draw, the latter of which was

monitored continuously. This site uses a variable-speed electrically commutated motor blower, 

which changes speed depending on cooling stage and modulates torque to maintain a constant 

airflow.

3.3 Equipment

3.3.1 Data Logger and Sensor Types and Specifications

Data Electronics data loggers are used to collect and store monitoring data. A Model DT-800 is 

used for both sites. Standard specifications for the sensor types used are listed in Table 8. Sensor 

selection was based on functionality, accuracy, cost, reliability, and durability. Signal ranges for 

temperature sensors correspond to listed spans. 

Table 8. Sensor Specifications

Type Application Mfg/Model Signal Span Accuracy

RTD Outdoor 

temperature and RH

R.M. Young 

41372LF 4–20 mA 14°–140°F

0%–100%

± 0.5°F

+ 2% RH

RTD Indoor/duct 

temperature/RH Vaisala HM*60 4–20 mA 23°–131°F

0%–100%

± 0.36°F

+ 2% RH

Type T 

Thermocouple

Immersion water 

temperatures

Surface/air 

temperatures

Gordon Watlow 

Type T special 

limits Omega

~11 mV @ 

500°F 

Range =

–328° to 662°F 

–99° to 500°F 

0.4% 

Special 

limits of 

error

24 VAC Relay

Fresh air damper 

status, zone damper 

status

Hawkeye Dry contact n/a n/a 

Small Power 

Monitor 

Fan and condenser 

power 

WattNode 

WNA-1-P-240-P Pulse

Current 

Transformer 

Amps (CTA)/40

± 0.5% 

Large Power 

Monitor

Total house power, 

PV production

Watt Node

WNB-3D-240-P Pulse CTA/60CTA/120 ± 0.5% 

Flow Meter Water flow Onicon F-1300 Pulse Varies by meter ± 0.5%

Pyranometer Insolation LiCor Analog Varies by sensor ± 5%
22 

3.4 Computation of Monitoring Variables

3.4.1 Heat Pump Performance 

Heating and cooling energy delivery are measured using a water flow meter and supply and 

return temperature sensors. HP delivery efficiencies and seasonal performance for both heating 

and cooling are then calculated. A performance map of HP sensible and total capacity and power 

relative to outdoor temperature and supply water temperature is to be developed and compared to 

manufacturer data.

Total HP heating and cooling delivered is computed by the data logger program on 15-s 

intervals, using Equation 1. Heating and cooling delivered to the fan coil and the radiant floor 

system is calculated using water side measurements according to Equation 2 and Equation 3. For 

the Altherma system the status of the DHW three-way valve, SVDW, is monitored to identify 

whether the unit is in water heating or space conditioning mode. When in water heating mode all 

energy is directed to that load and the heating energy delivered to the storage tank is calculated 

with Equation 1. Values are positive for heat addition (heating) and negative for heat extraction 

(cooling). 

Equation 1: QHP_total = FWS * |TWHL – TWHE| * 8.33 (Btu)

Equation 2: QHP_fan coil = FWS * |TWHL – TWFS| * 8.33 (Btu)

Equation 3: QHP_radiant floor = FWS * |TWFS – TWHE| * 8.33 (Btu)

where

FWS = HP system flow (gallons / monitored time period) 

TWHL = HP supply temperature (°F)

TWHE = HP return temperature (°F)

TWFS = radiant floor supply temperature (°F). 

The value of 8.33 in Equation 1 represents the product of the specific heat of water, 1.0 Btu/°Flb, and the density of water, 8.33 lb/gal, at a representative water temperature. Over the range of 

expected temperatures the <0.5% variation is considered to be within acceptable measurement 

error. 

Energy delivered by the fan coil is also calculated using air side measurements. Temperature and 

RH sensors in supply and return plenums are used to measure sensible, latent, and total cooling 

delivery by the forced-air components. The density of air is calculated using the supply air 

temperature in Equation 4. Equation 5 calculates sensible cooling as well as total heating 

capacity of the fan coil.

Equation 4: Dair = (518.67 / (459.67 + TAS)) * 0.075028

Equation 5: 

where

Dair = density of air at 500 ft altitude (ft3

/lbm)

cfm = calibrated airflow rate (cubic feet per minute)

TAS = supply air temperature (°F)

TAR = return air temperature (°F). 

Q ̇air_sensible = cfm * |TAS – TAR| * Dair * 0.24 Btu/°F-lb * 60 min/h (Btu/h)
23

Total fan coil cooling (sensible plus latent) is calculated based on calculated enthalpies of the 

supply and return air streams. Equation 6 through Equation 10 represent a noniterative 

approximation of enthalpy based on supply/return air temperature and RH. 

Equation 6: X = (18.678 - Tc / 234.5) * Tc / (Tc + 257.14)

Equation 7: Y = 1 + X + 0.5*X2 + 0.16393*X3 + 0.041667*X4 + 0.0123457*X5

Equation 8: Pw = RH * 6.112 * Y / 100

Equation 9: W = 0.6219 * Pw / (1013.26 – Pw)

Equation 10: h = 0.24 * Tf + W * (1060.9 + 0.443 * Tf)

where

Tc = supply or return air temperature in °C

Tf = supply or return air temperature in °F

RH = percent relative humidity of the supply or return air

Pw = water vapor partial pressure (hPa)

W = humidity ratio

h = enthalpy of supply or return air (Btu/lbm).

Total cooling load is calculated according to Equation 11.

Equation 11: Q

.

total = CFM * (hsupply - hreturn) x Dair (Btu/h)

Due to moisture reevaporated from the indoor coil during off cycles, total air-side cooling 

calculations may not provide an accurate representation of latent cooling. In the Tucson house, to 

determine whether the fan coil is removing any moisture from indoor air, flow from the 

condensate drain, FWC, is measured using a tipping bucket rain gauge. Net latent cooling is 

calculated using Equation 12. 

Equation 12: Qlat = 1080.84 * mH2O (Btu)

where

mH2O = mass of condensate (lb)

1080.84 = latent heat of condensation at 45°F (Btu/lb).

Total power input to the HP system is the sum of the outdoor compressor unit, the circulation 

pump, and the indoor fan as shown in Equation 13. In the Tucson house, system pump energy is 

calculated from the one-time measurement of power draw and filtering power on HP system 

flow, FWS. The Altherma system uses an internal pump, the energy use of which is captured in 

total unit energy, EHP (i.e., EPUMP = 0). Equation 14 and Equation 15 are then used to calculate 

EER and COP, respectively.
24 

Equation 13: Ehp = EHP + EFAN + EPUMP (kWh)

Equation 14: Cooling Operation: EERcooling = (QHP_total)cool / Ehp 

Equation 15: Heating Operation: COPheating = (QHP_total)heat / (Ehp * 3,412)

where

EHP = HP energy (kWh)

EFAN = air handler fan energy (kWh)

EPUMP = HP circulation pump energy (kWh). 

3.4.2 Heat Pump Water Heating Efficiency

For the Chico house, heat delivered to the DHW storage tank by the HP and heat energy supplied 

from the storage tank are measured. Losses from the hot water recirculation system are treated as 

part of the DHW load. Electrical energy use from both the HP and the backup electric element is 

monitored and used to calculate seasonal water heating efficiency. A performance map of 

capacity and COP relative to delivered water and outside air temperatures is to be developed. 

For DHW delivered to the house, the following equation will be used. 

Equation 16: Qdelivered = FWD * (TWHO – TWCS) * 8.33 (Btu)

where

FWD = DHW flow (gallons/monitored time period) 

TWHO = DHW supply temperature (°F)

TWCS = cold water supply temperature (°F.) 

The status of the DHW three-way valve, SVDW, is monitored to determine when the unit is in 

water heating mode. Energy consumption of the Altherma and backup element is converted to 

Btu using Equation 17. Total system efficiency, Equation 18, is calculated using total energy 

delivered to the house and all the energy inputs into the system. Recirculation pump energy is 

calculated from the current status meter, SPC, and one-time measurement of pump power draw. 

Heating delivered from the Altherma to the storage tank can also be quantified with Equation 1. 

Equation 17: Edhw = (EHP + EWH + ERPUMP) * 3,412 (Btu)

where

EHP = HP energy (kWh)

EWH = electric use of water heater (kWh)

EPUMP = recirculation pump energy (kWh). 

Equation 18: Total System Efficiency = Qdelivered / Edhw

3.4.3 Indoor Comfort and Dew Point

Indoor air temperature and RH are continuously monitored. Temperature sensors are placed 

below the slab insulation, above the slab insulation, and at the surface of the floor to estimate 

heat losses and gains across the slab and identify the dew point temperature of the slab. 

The dew point of the air in each zone is calculated using interior temperature and RH

measurements and Equation 19 and Equation 20. The dew point temperature is then compared to 

the temperature of the zone floor surface to determine if and how often condensation is occurring 

at the floor during cooling. 
25

Equation 19: α = ln (Pw * 0.01450377)

Equation 20: Tdp = 100.45 + 33.193 * α + 2.319 * α2 + 0.17074 * α 3 + 1.2063 * (Pw * 0.01450377) ^(0.1984)

Heat loss or gain through the bottom of the slab and the slab perimeter is estimated according to 

Equation 21 and Equation 22, respectively. The distribution efficiency of the radiant slab is 

calculated in Equation 23. In cooling mode QHP_radiant floor will be negative and it is expected that 

QCond will be positive.

Equation 21: QCond_under = (A_slab * |TSF3 – TSF2| / R) * t (Btu)

Equation 22: QCond_perim = (P_slab * |OAT – TSF2| / R) * t (Btu)

where

R = thermal resistance of insulation (h*ft2

*°F/Btu)

A_slab = total slab area (ft2

)

P_slab = exposed (insulated) area of slab perimeter (ft2

)

TSF3 = temperature at underside of underslab insulation (°F)

TSF2 = temperature of concrete immediately above underslab insulation (°F)

OAT = outdoor air temperature (°F)

t = time (hour).

Equation 23: ηDist = (QHP_radiant floor + (QCond_gain_under + QCond_gain_perim))/ QHP_radiant floor

3.4.4 Additional Data

Outdoor temperature and RH are monitored on site. Local weather data including insolation and 

wind speed are obtained through online sources (e.g., National Weather Service) to qualify the 

results.

3.5 Modeling Methodology

An AWHP with MM delivery was modeled in TRNSYS v.17. Monitoring data from the S.E.E.D. 

house were used to calibrate the AWHP model in order to evaluate the feasibility and 

applicability of such a system in various climates compared to a ducted forced-air system. The 

calibration process was conducted as described below. Of primary importance was calibration of 

the mechanical system operation. While calibration of the S.E.E.D. house was only a secondary

objective, it was important to calibrate certain components to ensure that the delivery system, 

primarily the interaction between the slab and the house, was correct. The calibrated model was 

then utilized to evaluate heating and cooling energy use in four climates via both MM and 

radiant-only delivery. The energy impact of using a precooling control strategy compared to a 

standard thermostat control was also evaluated. The base case developed in TRNSYS assumes a 

minimum efficiency air-to-air HP with ducted distribution located within the attic. Because this 

AWHP strategy is designed for high performance, low-load homes, all scenarios were evaluated 

using a high performance, low-load home with the floor plan of the S.E.E.D. house and similar 

insulation levels.

3.5.1 Calibration Process

3.5.1.1 Heat Pump Calibration

The TRNSYS AWHP model Type 941 was utilized to calibrate performance against monitored 

data. Type 941 takes a user-specified input file that maps HP power and capacity based on 

entering air and water temperatures. This performance map was generated using a linear 

regression tool that looked at full-load monitoring data for the S.E.E.D. house over the 2011 
26

summer. Data from the Cana house were not used because the variable capacity function of the 

compressor made it difficult to normalize data and develop accurate performance maps, and also 

because the Type 941 model is only for a single-speed HP. See Appendix A for a description of 

the linear regression method. The circulation pump was defined as follows according to actual 

operating specifications (Table 9).

Table 9. HP Flow Parameters

Parameter Value

Flow Rate 5.6-gpm, one-speed , 150-W pump

The HP model was validated by controlling entering water temperature, outdoor air temperature, 

and the water flow rate by using monitoring data as inputs to the model, while observing the 

leaving water temperature and HP power. The results of the analysis showed that over the 

monitoring period (April 2011–August 2012) the temperature differential across the heat 

exchanger was within 1.6% of the observed data during heating and 9.7% during cooling. The 

modeled power was on average 8% different in heating operation and 1% different in cooling 

operation. Samples of heating and cooling events are graphed in Figure 10 and show the leaving 

water temperature (LWT) and power (PWR) for given outdoor air temperature (OAT) and 

entering water temperature (EWT).

Figure 10. HP calibration step showing alignment between model and monitoring data for a 

heating event (left) and cooling event (right)

The temperature variation between modeled and observed at the beginning of the heating event is 

partially due to the data being truncated to only include full-load events over a 15-min period. In 

reality the HP did have some part-load run time that “warmed up” the LWT. This cannot be 

captured in the model on a 15-min simulation time step. Smaller time steps were used in later 

analysis to better evaluate the impacts of part-load performance.

0

1

2

60

65

70

75

80

85

90

95

100

105

110

3:50 PM 4:26 PM 5:02 PM 5:38 PM 6:14 PM 6:50 PM

HP Power (kW/15min)

Temperature (F)

OAT EWT

Data LWT Model LWT

Data HP Pwr Model HP Pwr

0

1

2

40

45

50

55

60

65

70

75

80

85

90

2:00 AM 2:57 AM 3:55 AM 4:52 AM 5:50 AM 6:48 AM

HP Power (kW/15min)

Temperature (F)

OAT EWT

Data LWT Model LWT

Data HP Pwr Model HP Pwr
27

3.5.1.2 Fan Coil Calibration

The coil is located within conditioned space and is set to operate whenever there is a call for 

cooling. The fan coil bypass fraction was modified in order to achieve similar water and air 

temperature splits as was observed during the monitored operation. Table 10 lists the parameters 

that define the validated model.

Table 10. Fan Coil Parameters

Parameter Value

Fan Flow Rate 831 cfm

Fan Power 0.25 kW

3.5.1.3 House and Slab Calibration

A single-zone Type-56 model house in TRNSYS was constructed to approximate the 

construction characteristics of the S.E.E.D. house. Operating characteristics such as occupancy 

and internal loads from the monitored period were used in the initial calibration. The house 

model was calibrated by modifying radiant floor characteristics, internal gains (latent and 

sensible), and house capacitance. Ultimately these variables were chosen to minimize the 

deviations between modeled and observed interior and slab exiting loop temperatures. The 

monitored flow rate and inlet temperature were used as inputs to drive heating and cooling 

events for the radiant floor model, and actual meteorological year (AMY) data, from August 

2011 through July 2012, were used in the model.

While details of the radiant floor design are known, pipe spacing and depth of pipes in the slab 

were varied to produce slab temperature responses similar to those seen in monitored data. Table 

11 lists the initial and post calibration specifications.

Table 11. Slab and House Characteristics

Parameter Initial Specification Postcalibration 

Specification

Slab Depth 8 in. 8 in.

Radiant Floor Spacing 12 in. on-center, tubes 4 in.

below slab surface

15 in. on-center, tubes 5 in.

below slab surface

House Capacity 5,986 kJ/°K 8,986 kJ/°K

In Figure 11 the slab exiting water temperature during a series of heating and cooling events is 

shown. During this calibration step EWT, OAT, and water flow rate remain controlled by inputs 

from monitoring data. This comparison was used to calibrate modeled energy delivered to the 

slab with monitored data. Throughout the monitoring period, occupancy, thermostat set points, 

window operation, energy recovery ventilator (ERV) operation, and cooling mode (radiant floor 

versus fan coil delivery) varied. The slab calibration focused on specific monitoring periods that 

were identified to have only radiant floor delivery with documented interior set points.
28

Figure 11. Heating event (left) and cooling (right) temperature comparison during flow events

The house capacitance was adjusted to minimize the deviations between interior air temperatures 

and slab loop exiting water temperatures both when the house was “floating” and during radiant 

space conditioning events. Floating implies that the heating and cooling systems are inactive and 

the house simply responds to exterior conditions and internal loads. Initially, the house in 

TRNSYS responded quicker to changes in environmental conditions than reflected in monitoring 

data. An increase in capacitance of the house airnode (to 8,986 kJ/K) dampened and delayed the 

house’s response, resulting in a model that produced similar daily temperature change and 

coincident peak temperature times as those monitored. Table 11 lists slab and house 

characteristics before and after this calibration step.

Figure 12 shows monitored and modeled house interior temperature over 15-min simulation time

steps. The green line indicates when cooling events occur and it can be seen that the modeled 

interior temperature of the house responds similarly to the monitored data during these events.

0

1

2

3

4

5

40

50

60

70

80

90

100

110

120

2/17 12 AM 2/18 12 AM 2/19 12 AM 2/20 12 AM Temperature (F) Power(kBtu/h)

SlabIn_Temp SlabOut_Temp_Monitored

SlabOut_Temp_Model HP Power

0

1

2

3

4

5

20

30

40

50

60

70

80

90

100

9/28 12 AM 9/29 12 AM 9/30 12 AM 10/1 12 AM Power (kBtu/h) Temperature (F)

SlabIn_Temp SlabOut_Temp_Monitored

SlabOut_Temp_Model HP Power
29

Figure 12. Interior temperature comparison

At this point in the calibration process the controlled variables (EWT, OAT, and flow rate) are 

no longer driven by monitoring data and the model is allowed to operate based on the imposed 

loads and the thermostat set point. Heating and cooling performance were compared on a 

seasonal basis, using AMY data in the TRNSYS model. Heating and cooling events were defined 

as steady-state operation of the HP over a minimum time period of ~45 min. The response of the 

slab over the course of these events was evaluated and compared for the model and monitoring 

data. Figure 13 demonstrates similar changes in slab temperature over an event for a given 

amount of delivered energy. The modeled data exhibit a greater number of events delivering 

higher cooling energy than the monitoring data, largely a result of HP cycling in the monitored 

data. If the HP ran for 2 h, then stopped for 15 min and ran again for 2 h more, this registered as 

two small events with correspondingly smaller cooling energy delivered and slab temperature 

change. Long run times were observed in both the model and the monitored data during radiantonly delivery on very hot days due to the slab’s slow response. However, HP run times in the 

TRNSYS model were longer than in the monitoring data on these hot days. This may suggest 

that the house internal loads assumed in the model are greater than those in actuality. Detailed 

information on occupant loads and load scheduling was very difficult to ascertain. As part of the 

calibration process, the internal loads in the model were adjusted to provide the best match to the 

monitored slab response.

0

1

70

72

74

76

78

80

82

84

86

88

90

10000 10200 10400 10600 10800 11000 11200

Temperature (F)

Full Load Events

Simulation Timestep

Indoor Temp - Monitored

Indoor Temp - Model

Full Load Events
30

Figure 13. Energy input required to change slab temperature for heating (left) and cooling (right)

3.5.2 Climate Zone Evaluation

Using the calibrated model, the four cities shown in Table 12 were selected as representative of 

the major climate zones where the AWHP MM strategy may be appropriate. 

A high performance, low-load home was utilized for this analysis with the properties presented 

in Table 13. This house is based on the S.E.E.D. house built for TRNSYS. HP equipment sizing 

was adjusted by climate based on Air Conditioning Contractors of America Manual J load 

calculations for each climate zone (CZ) (ACCA 2006). Design heating and cooling loads are 

shown in Table 14.

Table 12. Evaluated Cities and Climate Zones

IECC and BA CZ Representative City

CZ 2 Hot-Dry Tucson

CZ 3 Hot-Dry Sacramento

CZ 5 Cold Denver

CZ 2 Hot-Humid Houston

0

2

4

6

8

10

12

Heating Slab Temperature Change 

(°F/Event)

Heating Energy Delivered (kBtu/Event)

Modeled Data

Monitored Data

-

2 

4 

6 

8 

10 

Cooling Slab Temperature Change 

(°F / Event)

Cooling Energy Delivered (kBtu/event)

Modeled Data

Monitored Data
31

Table 13. House Characteristics as Modeled in TRNSYS

Specification

Conditioned Floor Area 1,935 ft2

Walls R-23 equivalent (U = 0.25 W/m2-K)

Roof R-49 (U = 0.114 W/m2-K)

White (0.2 absorptivity)

Infiltration 5.2 ACH50 (0.0003 specific leakage area@ 50 Pa)

Ventilation 50 cfm, per ASHRAE 62.2

Internal Gains

Occupants: 2 @ 13,008 Btu/day (57% sensible)

Lights: 100% compact fluorescent: 7,572 Btu/day

(100% sensible)

Miscellaneous electric load /appliance load: 28,475 

Btu/day (84% latent)

Total: 49,055 Btu/daya a Total daily internal gain was calculated as 20,000 + 15*CFA (Btu/day) from the CEC Title-24 2008 Alternative 

Calculation Methodology (ACM) (CEC 2010). Breakdown was taken from a combination of the ACM and the 

Building America House Simulation Protocols (Hendron and Engebrecht 2010).

Table 14. Design Heating and Cooling Loads (Btu/h)

Tucson Denver Houston Sacramento

Heating Design Load 13,449 25,340 14,890 13,993

Cooling Design Load 16,471 13,777 14,735 14,913

The base case system for this analysis was an air-to-air HP with forced-air delivery. The duct 

system was assumed to be located in a vented unconditioned attic. The duct leakage was 

assumed to be 6% of total air flow. HP cooling and heating efficiencies are 11 EER and 2.8 

COP, respectively, and are based on the BA benchmark HP. Two proposed alternative HVAC 

systems were evaluated using an AWHP and a circulation pump. The first alternative uses 

radiant floor distribution only. The second uses the MM distribution design and therefore 

includes the hydronic fan coil downstream of the HP and before the radiant floor. Both 

alternative system designs were run with and without external dehumidification. Table 15 lists 

the equipment specifications used in this analysis.

Table 15. Equipment Assumptions Used in the TRNSYS Model

Equipment Specification Value Unit

Hydronic Fan Coil Fan efficacy 0.33 W/cfm

Circulation Pump Pump flow

Pump power

5.6

150

Gpm

W

AWHPa Rated cooling EER

Rated heating COP

10.4

3.1

Btu/Wh

W/W

Dehumidifier Efficiency rating 1.2 L/kWh a Rated conditions were selected to match AHRI conditions for air-to-air HP as best as possible. 

Cooling: 95°F outdoor air dry bulb and 75°F entering evaporator water temperature. 

Heating: 45°F outdoor air dry bulb and 85°F entering condenser water temperature.
32

Two control strategies were investigated in this study: 1) constant heating/cooling set point 

control and 2) C&C. In the first case, fixed thermostat set points of 71°F for heating and 76°F 

for cooling were assumed. The C&C strategy utilized the same heating set point (71°F) but 

during the cooling season, the thermostat was set to precool the house overnight. The thermostat 

was set back to 73°F at night (12 a.m. to 6 a.m.) and up to 78°F during the day (6 a.m. to 12

a.m.). While both heating and cooling were available anytime throughout the year, C&C 

operation was restricted to May 1 through October 31. 
33

4 Results

This section presents monitoring results from the two test houses, as well as TRNSYS modeling

results. Results from the commissioning process are presented in Appendix B.

4.1 Monitoring Results and Discussion

This section reports on monitored performance of the AWHP systems at both test houses. All 

presented results are based on full-load data. Full load for all analyses in this report is defined as 

whenever the HP was running for a minimum of 13 min in steady-state operation. Unless 

otherwise noted, calculated efficiencies include HP compressor and pumping energy. Where 

appropriate, fan energy is also included for MM cooling operation.

4.1.1 Heating Performance

Figure 14 shows calculated full load heating COP of the HP (outdoor unit + circulation pump) at 

the S.E.E.D. house compared to both OAT and EWT. Average seasonal COP over the 

monitoring period of the 2011–2012 heating season was 3.26.

For reference, a comparison is made to the published engineering data for the standard (air-toair) Ruud HP (data points are referenced to entering air temperature [EAT] instead of EWT). In

the curve versus OAT an average entering air temperature of 74°F was selected to represent 

typical air-to-air HP operation. The manufacturer data shown represent a best-case scenario for 

supply air systems based on research that has shown field performance of HPs to be much lower 

than expected from unit ratings (Proctor et al. 2011). There is substantial spread in the 

monitoring data, which may be explained by the variation in water temperatures in the supply 

loop due to load variations and zoning (there are three zones). The data presented are relative 

to each dependent variable; for example, the data relative to OAT are not normalized to a 

constant EAT.
34

Figure 14. Calculated full load COP of the S.E.E.D. house HP in space heating versus outdoor 

dry-bulb temperature and EWT and compared to manufacturer-rated specifications 

(n = 1,241)

Figure 15 shows calculated full load heating COP at the Cana house compared to both outdoor 

air and LWT.

8 The chart also includes curves representing the Altherma engineering 

specifications at various operating conditions. Field performance of the Altherma tracks 

laboratory performance quite well with respect to OATs. Monitoring data show a strong 

correlation between efficiency and outdoor air and little correlation between efficiency and load 

conditions (LWT). This is partially a factor of the narrower band of temperature conditions on 

the load side as well as the variable capacity of the inverter-driven compressor. The Altherma is 

an inverter-driven compressor that adjusts compressor frequency and ultimately output capacity 

based on load. If the water temperature is overshooting the set point the compressor will throttle 

down, and likewise if it is falling short it will ramp up. This allows the system to better supply 

the building load while improving occupant comfort and maintaining efficiencies by reducing the 

temperature lift and reducing cycling. While the Altherma has the capability for outdoor air reset 

in heating mode, this functionality was not enabled for this data monitoring period. The 

Altherma specifications are generated for high-speed operation only, and do not reflect 

 8 Note that Altherma engineering tables are related to LWT when performance is ultimately dependent on EWT.

0

1

2

3

4

5

6

20 30 40 50 60 70 80 90 100 110 120

COP

Temperature (F)

Calculated COP vs. OAT

Calculated COP vs. Ent. Water Temp

Rated HP Specs vs. OAT @ EAT = 74F

Rated HP Specs vs. EAT @ OAT =56F

AverageEWT = 92

AverageOAT = 56

AverageSeasonal COP = 3.26
35

performance at various compressor frequencies. If compressor frequency could have been 

captured within the monitoring data, the information may have led to a more defined trend with 

respect to LWT and improved correlation with the Altherma specifications.

Figure 15. Calculated full load COP of the Cana house HP in heating mode versus outdoor drybulb temperature and LWT and compared to manufacturer-rated specifications (n = 1383)

4.1.2 Domestic Hot Water Performance

Figure 16 shows heat pump COPs at the Cana house for DHW operation that range from 

approximately 1 to 3. These COPs are considerably below manufacturer-rated data with 

significant variation in the data set (see Altherma specs in figure). Poor heat transfer from the 

HP to the storage tank was observed and is thought to be the cause of lower performance. 

Temperature differentials of only 2°F were seen on average across the DHW heat exchanger coil, 

resulting in the low heating capacities. As a result, the HP was not able to meet the storage tank 

set point of 130°F, and the auxiliary electric resistance heat was activated to satisfy water heating 

demand. ARBI worked with the Altherma representative and the installer to identify the cause of 

the low performance, and several program (operational) adjustments have been made to the 

Altherma unit based on initial monitoring results (including lowering the tank set point). The 

4-ton Altherma unit is rated at a water flow rate of 10.6 gpm. Monitoring data showed that the 

0

1

2

3

4

5

6

0 20 40 60 80 100 120 140

COP

Temperature (o

F)

Calculated Space Heating COP vs. OAT Calculated Space Heating COP vs LWT

Altherma Specs vs. OAT @ LWT = 95F Altherma Specs vs. LWT @ OAT = 45F

AverageLWT = 94

AverageOAT = 44

AverageSeasonal COP = 4.18
36

average flow rate through the storage tank was 9.8 gpm. With just 15.6 ft2 of surface area,

9 the 

DHW tank heat exchanger is undersized compared to other indirect water heaters. It is 

hypothesized that this combination of high flow rate and small effective area is the primary cause 

of reduced efficiencies. Because of the low delta-T through the heat exchanger, the inverterdriven compressor ramps down its capacity. Over the 13- month monitoring period, data showed 

that the HP supplied 76% of the energy to the DHW tank, just slightly lower than the 78% 

estimate from the Altherma engineering manual for a tank set point temperature of 120°F. 

Seasonal total system COP10 over 1 year (November 2011 – October 2012) was 1.05. This low 

value was a result of electric resistance heat operation, poor HP efficiencies, and storage losses. 

Figure 16. Calculated full load COP of the Cana house HP in DHW mode versus outdoor dry-bulb 

temperature and LWT and compared to manufacturer-rated specifications (n = 1607)

4.1.3 Cooling Performance

Heat pump EER using water side energy delivery and total electricity input including condenser 

and pump energy use, relative to operating conditions during full load operation, were reviewed 

to identify performance and operating efficiency. Figure 17 and Figure 18 show calculated full 

load EER at the S.E.E.D. house and Cana house, respectively, compared to both OAT and EWT 

 9 See http://www.daikinac.com/content/residential/whole-house/daikin-altherma/faq-2/ 10 Recovery load divided by electrical input to both the HP and the backup electric resistance heater

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40 60 80 100 120 140

COP

Temperature (oF)

Calculated DHW COP vs. OAT Calculated DHW COP vs. LWT

Altherma Specs vs. OAT @ LWT = 122F Altherma Specs vs. LWT @ OAT = 59F

AverageLWT = 126 AverageOAT = 60
37

for the S.E.E.D. system and LWT for the Cana system.

11 The chart also includes curves 

representing the manufacturer’s engineering specifications at various operating conditions. 

The S.E.E.D. house data illustrate a strong correlation between efficiency and OAT. However, as 

was observed in the space heating data, performance is not as dependent on load-side conditions

(EWT). Supply water temperatures within the hydronic loop continuously decreased throughout 

cooling events. This resulted in reduced efficiencies over periods of prolonged cooling operation 

due to lower return water temperatures to the HP. In addition, latent cooling was delayed until 

the loop reached the indoor air dew point temperature. This partially explains the spread in the 

data for a given OAT.

A comparison to published engineering data for the Ruud HP is also shown in Figure 17. 

Manufacturer’s data assume that the HP is air-to-air and paired with a standard air handler and 

all data points are referenced to entering air wet-bulb temperature (EWBT) instead of EWT. In 

the curve of rated performance versus OAT an average entering air wet-bulb temperature of 60°F 

and dry-bulb temperature of 78°F were selected to represent typical air-to-air HP operation in a 

dry climate.

Because of cooler evenings and the effect of nighttime ventilation cooling at the Cana house, the 

Cana cooling hours of operation were significantly reduced as compared to the S.E.E.D. house. 

Perhaps due to the small sample size it is difficult to make a comparison to the Altherma 

engineering specifications. However, on average efficiencies are close or higher than rated 

values.

Results from a 2009 California study show measured equipment efficiencies well below 

manufacturer-rated efficiencies (Proctor et al. 2011). The study evaluated 80 residential singlefamily and multifamily units built and occupied in 2007. Efficiency and capacity measurements 

were taken on ten of these systems before and after refrigerant and airflow repairs were made as 

appropriate. Sensible EERs (corrected to standard AHRI conditioned) ranged from 4 to 7 prior to 

repairs. After repairs, efficiency increase was on average 24% with normalized sensible EERs 

ranging from 5.5 to 8.5. 

 11 The Altherma engineering tables present LWT when performance is ultimately dependent on EWT.
38

Figure 17. Calculated full load EER of the S.E.E.D. house HP (condenser + pump) in space cooling 

versus outdoor dry-bulb temperature and LWT and compared to manufacturer-rated 

specifications (n = 8,208)

0

2

4

6

8

10

12

14

16

18

20

40 50 60 70 80 90 100 110 120

Heat Pump EER

Temperature (°F)

Calculated EER vs. OAT

Calculated OAT vs. EWT

Rated HP Specs vs. OAT @ EWBT = 60F

Rated HP Specs vs. EWBT @ OAT = 83F

Average EWT = 60

Average OAT = 83

Average Seasonal EER = 11.2
39

Figure 18. Calculated full-load EER of the Cana house HP (condenser + pump) in space cooling 

versus outdoor dry-bulb temperature and LWT and compared to manufacturer-rated 

specifications (n = 337)

4.1.4 Nighttime Precooling Strategy

The C&C precooling strategy was tested at the S.E.E.D. house to evaluate its effectiveness in 

reducing cooling energy use by improving HP efficiencies and shifting load from on to off peak.

Figure 19 shows a comparison of daily average EER12 and daily HVAC energy use as a function 

of maximum daily OAT for the two control strategies operating in radiant floor delivery mode 

(no fan coil).

Shifting C&C air-conditioner operation to cool nighttime periods results in higher average 

efficiencies than with constant set point operation and a much less performance degradation 

impact with outdoor temperature. Daily C&C energy use is clearly lower than constant set point

operation, with an average 27% savings at a daily maximum outdoor temperature of 90°F and up 

to 40% savings at maximum temperatures greater than 100°F. However, below a certain OAT,

C&C operation can result in HVAC energy use that may have been avoided by conventional 

 12 Calculated as the average of all full-load operating points during a 24-h daytime period. Average temperatures 

within the living space under all scenarios remain relatively constant at close to 76°F. However, results are 

normalized to a daily internal average temperature of 76°F.

0

2

4

6

8

10

12

14

16

18

20

50 60 70 80 90 100 110 120

EER

Temperature (°F)

Calculated EER vs. OAT

Calculated EER vs. Lvg. Water Temp

Altherma Specs vs. OAT @ LWT = 59F

Altherma Specs vs. LWT @ OAT = 95F

59

Average OAT =

Average LWT =

92

Average Seasonal EER = 10.8
40

thermostat operation. Overcooling during milder conditions could be avoided by including a 

control strategy that monitors weather conditions and alters the nighttime setback based on the 

anticipated cooling load for the next day.

Figure 19. Daily average EER and energy use comparison for C&C (n = 82) versus constant set

point (n = 30) operating strategies in radiant floor delivery mode

Figure 20 compares indoor and outdoor temperature and HP power for the constant set point 

mode of operation and the C&C mode. The shape of the HP electrical demand profile during the 

course of the day shows the performance sensitivity to temperature with a roughly 30% increase 

in cooling demand from the nighttime minimum to the daytime maximum. Also, running the HP

for extended periods of time to satisfy the reduced temperature setback reduces cycling, thus 

increasing steady-state operation and ultimately system efficiency. As a result of the long 

evening run time and the associated low nighttime cooling loads, the HP EWTs are often lower 

than during peak daytime operation. EWTs gradually decline over the run period, increasing the 

thermal lift on the HP and lowering efficiency slightly. However, any reduction in efficiency due 

to this effect is overshadowed by the more favorable OATs and reduced cycling. This result 

corresponds well with the efficiency curves presented in Figure 17 and Figure 18, which 

demonstrate that performance is most influenced by source conditions. 

0

10

20

30

40

50

60

70

80

0

2

4

6

8

10

12

14

16

60 70 80 90 100 110 120

Daily Energy Use (kWh)

EER (Btu/Wh)

Maximum Daily Outdoor Air Temperature (oF)

C&C Daily EER

Constant Setpoint Daily EER

C&C Daily Energy (kWh)

Constant Setpoint Daily Energy (kWh)
41

Figure 20. Interior temperature and HP operation for two hot days comparing C&C versus 

constant set point operating strategies in radiant floor delivery mode

4.1.5 Distribution System Performance

The total distribution efficiency of radiant floor delivery was calculated based on estimated heat 

losses and gains between the slab and ground and the exposed slab perimeter and ambient air 

(see Equation 21 and Equation 23). Pipe losses from pipes outside of the slab and conditioned 

space were not considered in this evaluation. Slab temperature was also assumed to be uniform 

throughout the slab. Figure 21 shows the radiant slab distribution efficiency in both heating and 

cooling for the two test houses over the monitoring periods. Data points represent calculated

efficiencies based on 15-min data intervals during full-load operation.

Heat losses and gains through the slab at the Cana house are driven by the ground temperature 

given the larger area for heat exchange and reduced underslab insulation (R-5) as compared to 

slab edge conditions (R-8). During the summertime there were many instances when ground 

temperatures were lower than that of the slab, which actually contributed to slab cooling, albeit 

negligibly since temperature differentials across the underslab insulation were minimal. Because 

of cool ground temperatures and minimal compressor cooling operation, average seasonal 

cooling distribution efficiency at the Cana house was greater than 100%

0

0.5

1

1.5

2

2.5

0

20

40

60

80

100

120

12:00:00 AM 12:00:00 PM 12:00:00 AM 12:00:00 PM 12:00:00 AMPower (kW) Temperature (°F)

OAT-C&C OAT-Constant Setpoint

Indoor Air Temp-C&C Indoor Air Temp-Constant Setpoint

HP Power-C&C HP Power-Constant Setpoint
42

Average seasonal distribution efficiencies for both the Cana and S.E.E.D. test houses are 

presented in Table 16 and compared to ducted air supply systems. With the exception of the 

Cana house during cooling operation, the measured distribution efficiencies are roughly 

equivalent to a ducted distribution system with ductwork located inside conditioned space.

Distribution efficiencies are improved by almost 50% from the benchmark case with ductwork in 

the attic at a 15% leakage rate.

Figure 21. Radiant floor distribution efficiency versus outdoor dry-bulb temperature for S.E.E.D 

house (left) and Cana house (right)

Table 16. Distribution Efficiency Comparison

Case Distribution Efficiency

Monitoring Data

Heating Cooling

S.E.E.D. Cana S.E.E.D. Cana

96% 96% 97% 108%

Model 

Resultsa

Benchmark forced-air

unit

(15% leakage)

68% 61%

Benchmark w/tight 

ducts (6% leakage) 78% 74%

Benchmark w/ducts 

in conditioned space

94% 94%

a Seasonal distribution efficiencies are calculated according to ASHRAE Standard 152 (ASHRAE, 2004) assuming a 

2,400-ft2 house in Sacramento.

Performance of the AWHP was also compared using radiant-floor-only distribution and MM

distribution. This comparison is presented in Figure 22, which demonstrates clear energy savings 

from removal of the fan coil. MM operation provides latent cooling ability and the potential for 

improved compressor efficiency by raising entering HP water temperatures and reducing thermal 

lift, but parasitic energy use from the fan in MM resulted in higher daily energy use for cooling 

relative to floor cooling delivery alone.

80%

85%

90%

95%

100%

105%

110%

115%

120%

20 30 40 50 60 70 80 90 100 110 120

Distribution Efficiency

Outdoor Air Temperature (oF)

S.E.E.D. Test House Cooling Season

Heating Season

80%

85%

90%

95%

100%

105%

110%

115%

120%

20 30 40 50 60 70 80 90 100 110 120

Distribution Efficiency

Outdoor Air Temperature (oF)

Cooling Season Cana Test House

Heating Season
43

Figure 22. Daily average HP (condenser + pump + fan) energy use comparison for radiant floor 

delivery (n = 82) versus MM delivery (n = 71) modes in C&C operation

4.1.6 Latent Cooling, Dehumidification, and Condensation

To evaluate the potential and risk for condensation on the floor, the temperature of the slab 

surface was monitored inside the living room. The dew point temperature of the air inside the 

zone is calculated and compared to the slab surface temperature (see Equation 19 and Equation 

20). All cooling operation at the Cana house was with the fan coil active; therefore, ARBI was 

unable to test conditions with radiant delivery only. However, during cooling operation the 

indoor air dew point remained on average 17°F above the floor temperature with a minimum 

difference of 8°F, and interior RH remained within the ASHRAE comfort range.

13 These data 

suggest that in a very dry climate such as Chico floor condensation in a radiant floor cooling 

application should not be a concern as long as internal latent loads from cooking and showers are 

adequately removed with intermittent ventilation. High latent loads could potentially present a 

concern if not properly addressed. While not presented in this report, radiant floor cooling has 

been successfully demonstrated in other California research projects within hot-dry climates

(CEC 2003).

 13 ASHRAE Standard 55-2010: Thermal Environmental Conditions for Human Occupancy allows a wider range 

from about 25%– to 80% (ASHRAE 2010).
44

In climates with increased levels of humidity during the summertime, such as in Tucson, which 

experiences a summer monsoon season, some level of dehumidification, primarily for occupant 

comfort, is required, even if for only a short period of the cooling season. The occupant of the 

S.E.E.D. house commented that comfort was compromised during heavy rains due to the 

sensation of high interior RH. Figure 23 compares indoor and outdoor conditions (temperature 

and RH), dew point temperature and floor surface temperature during a week in the monsoon 

season with outdoor RH exceeding 80%. During this period the fan coil was operational and the 

floor surface temperature approached to within 5°F above the dew point temperature of the 

indoor air. Indoor RH exceeded 60% on one day. The condensate measurement indicated 

negligible amounts of condensate off the evaporator coil except for during the monsoon season. 

The correlation between dehumidification and outdoor moisture is shown in Figure 24. During 

the nonrainy season the system operated in radiant-floor-only mode without any comfort or 

condensation concerns. 

Figure 23. Evaluation of S.E.E.D. house radiant slab condensation potential

during 2012 monsoon season (MM operation)

-100

-80

-60

-40

-20

0

20

40

60

80

100

25

35

45

55

65

75

85

95

105

115

125

7/15 - 00:00 7/17 - 00:00 7/19 - 00:00 7/21 - 00:00 7/23 - 00:00 7/25 - 00:00 Temperature (

Relative Humidity (%)

°F)

Floor Surface Indoor Temp - Living

Indoor Air DewPoint Temp Outdoor Temp

Indoor RH Outdoor RH
45

Figure 24. Monitored S.E.E.D. house evaporator coil condensate flow (MM operation)

4.2 TRNSYS Modeling Results

The validated AWHP TRNSYS model was used to project annual energy use for the two 

operating modes: radiant-only and MM, and the two control strategies: constant set point and 

C&C. The strategies were applied to the validated S.E.E.D. house model while modifying the 

internal gains and ventilation to reflect BA benchmark assumptions. Comparisons have been 

made to a base case with an air-to-air HP and forced-air delivery with ductwork located in the 

attic (6% leakage). An alternative case is also evaluated, which is identical to the base case 

except duct losses are eliminated by moving all ductwork inside the conditioned envelope. Four 

climates were evaluated as described in Table 12 using Typical Meteorological Year 3 weather 

files.

The Tucson climate was evaluated to assess the effect of a stand-alone dehumidifier (DH) on 

energy use and interior comfort (Table 17 and Figure 25). The stand-alone dehumidifier was 

controlled such that interior RH never exceeded 60%. As was discussed previously in the 

monitoring data evaluation, there were comfort concerns during the summer with radiant-only 

operation. The model validates this with floor condensation occurring when the radiant-only 

mode was applied without dehumidification. While the MM strategy did not have floor 

condensation errors, the internal RH did exceed 60% and fell outside the ASHRAE comfort 

-50

0

50

100

150

200

0

20

40

60

80

100

120

7/15/2012 7/25/2012 8/4/2012 8/14/2012 8/24/2012

Daily Condensate Flow (oz)

Daily Average Temperature (°F) & RH

Outdoor Air Temperature

Outdoor Air Relative Humidity

Evap Coil Condensate Flow
46

regime (see Figure 25) according to ASHRAE Standard 55-2010 (ASHRAE 2010). This comfort 

regime is defined by a range of temperature and humidity at which 80% of sedentary or slightly 

active people would find the environment comfortable. The ranges are defined by clothing level, 

where CLO 1 is equivalent to a winter business suit and is commonly used to represent winter 

comfort ranges, and CLO 0.5 is equivalent to short sleeves and trousers, representing summer 

comfort. Data in Figure 25 reference ASHRAE comfort range CLO 0.5.

Table 17. TRNSYS Model Predictions for Tucson, AZ.

Mode Control Supp.

DH

Condensation 

Errors

HP 

Heating 

(kWh)

HP 

Cooling 

(kWh)

Pump 

(kWh)

Fan 

(kWh)

DH 

(kWh)

Total 

HVAC 

(kWh)

Base 

Case: 

Ducts in 

Attic

Set

point Yes No 205 3,155 0 895 69 4,324

BC + 

Ducts in 

Cond. 

Space

Set

point Yes No 198 3,094 0 881 28 4,201

RadiantOnly

Set

point Yes No 277 3,312 334 0 287 4,210

RadiantOnlySet

point No Yesa 277 3,126 316 0 0 3,719

RadiantOnly C&C Yes No 288 2,812 302 0 209 3,611

RadiantOnly C&C No Yesb 288 2,693 291 0 0 3,272

Mixed 

Mode

Set

point Yes No 246 3,074 281 434 159 4,194

Mixed 

Mode

Set

point No No 246 3,009 273 422 0 3,950

Mixed 

Mode C&C Yes No 253 2,581 270 415 108 3,626

Mixed 

Mode C&C No No 253 2,531 266 407 0 3,457

a The TRNSYS model predicted 28 h in which condensation formed on the slab surface. b The TRNSYS model predicted 3 h in which condensation formed on the slab surface.
47

Figure 25. Tucson modeled indoor conditions during cooling season

Because dehumidification was found to be necessary in Tucson under certain operational 

strategies, a stand-alone DH was included in all subsequent simulations, except for climates 

where it was deemed unnecessary. A DH was applied to the model in both Tucson and Houston. 

Decoupling the two functions of sensible and latent cooling avoids overcooling in humid 

climates and can expand the feasibility of radiant cooling to a wider range of climates, although 

it does add first cost. Due to minimal latent loads in Sacramento and Denver and provided that 

both climates do not commonly employ dedicated humidification systems, a DH was not 

included in the simulations for these climates. 

Results for all evaluated cases in each climate zone are presented in Table 18 through Table 23.

Table 18 presents energy consumption and savings model results for Tucson. The results show 

AWHP savings of 3% of annual HVAC energy compared to the base case air-to-air HP with 

ducts in the attic for the set point control strategy and similar energy consumption compared to 

the base case with ducts in conditioned space. However, 16% annual savings over the base case

are achieved by combining the AWHP and the C&C control strategy. C&C seasonal cooling 

savings (May–October) are 17% for the MM case and 18% for the radiant-only case.

Ashrae Standard 55 

CLO 0.5

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

50 60 70 80 90 100

Humidity Ratio (lb

w/lbda)

Dry Bulb Temperature (°F)
48

There are some noted differences in these results compared to the monitoring of the S.E.E.D. 

house. TRNSYS results do not demonstrate any energy advantage with the set point strategy 

from removal of the fan coil and operating in radiant-only delivery mode, while the monitoring 

data show clear savings. It is expected this principally has to do with longer run times on very 

hot days in the TRNSYS model with radiant-only delivery, as was discussed in the calibration 

results presented above. On very hot days the HP in TRNSYS is projected to operate the entire 

day. While long run times were also observed in the monitoring data on hot days, they were not 

seen to the extent that they were in the TRNSYS model. Additionally, daily energy savings for 

C&C compared to set point strategy on hot days were not as high as observed in monitoring 

data14 and energy use did not trend with daily maximum temperature as well as in the monitoring 

data (Figure 19). This was more apparent with radiant-only delivery and may also indicate 

reduced effectiveness of the slab delivery model.

Table 18. Tucson TRNSYS Results Comparison

Mode Control 

Strategy

Energy Use 

(kWh)

% 

Savings 

Versus

Base Case

C&C 

Savings 

Versus

Set Point HP Pump Fan DH Total

Base Case: Ducts 

in Attic Set point 3,360 0 895 69 4,324 – –

BC + Ducts in 

Cond. Space Set point 3,292 0 881 28 4,201 3% –

Radiant-Only Set point 3,589 334 0 287 4,210 3% –

Radiant-Only C&C 3,100 302 0 209 3,611 16% 14%

MM Set point 3,320 281 434 159 4,194 3% –

MM C&C 2,833 270 415 108 3,626 16% 14%

Table 19 presents results for Sacramento. Similar to Tucson, Sacramento is also in the hot-dry 

climate zone but has fewer cooling degree days, drier summers, and greater summer diurnal 

temperature swings. Because humidity is not a significant issue in Sacramento, results are 

presented for the case without a DH.15 Percent savings in this climate are much higher than in 

Tucson with AWHP savings of 21%–23% over the base case (set point strategy). This increase 

can be partially attributed to the elimination of the DH and associated energy consumption. It is 

expected that the higher heating loads are also a factor with the results indicating that the radiant 

distribution effectiveness is higher for heating than for cooling.

Since the Sacramento cooling season is shorter than in Tucson and the nights are much cooler, 

there were no observed savings for the C&C strategy as previously defined in this report. An 

alternative precooling strategy was evaluated that moved the setback period to 4 a.m.–6 a.m. to 

take advantage of nighttime building losses due to cool temperatures and shortened the season to 

June through September. In the model minimal cooling is observed in May and October. With 

this strategy, C&C annual savings over the base case are 2%–4%. Seasonal C&C cooling savings 

 14 As reported earlier, monitoring at the S.E.E.D. house demonstrated 27% savings from precooling operation at a 

daily maximum outdoor temperature of 90°F and up to 40% savings at maximum temperature greater than 100°F

15 A comfort evaluation was conducted and the simulation run with a stand-alone DH and results are presented in 

Appendix D.
49

(June–September) are even higher than in Tucson at 21% for the MM case and 43% for the 

radiant-only case.

Table 19. Sacramento TRNSYS Results Comparison

Mode Control 

Strategy

Energy Use 

(kWh)

% 

Savings 

Versus

Base Case

C&C 

Savings 

Versus

Set Point HP Pump Fan Total

Base Case: 

Ducts in Attic Set point 2,946 0 627 3,573 – –

BC + Ducts in 

Cond. Space Set point 2,799 0 596 3,395 5% –

Radiant-Only Set point 2,606 214 0 2,820 21% –

Radiant-Only C&C 3,213 277 0 3,489 2% –24%

MM Set point 2,507 203 23 2,733 23% –

MM C&C 2,674 224 49 2,946 18% –8%

Table 20. Sacramento TRNSYS Results Comparison With C&C Operation 

June–September From 4:00 a.m. to 6:00 a.m.

Mode Control 

Strategy

Energy Use 

(kWh) % Savings 

Versus

Base Case

C&C 

Savings 

Versus Set

Point HP Pump Fan Total

Radiant-Only C&C 2,493 205 0 2,698 24% 4%

MM C&C 2,457 202 22 2,682 25% 2%

Table 21 shows results for the heating-dominated climate of Denver. The low cooling load 

requires almost no cooling energy use and therefore this study did not analyze the C&C control 

strategy. Similar to Sacramento, because of negligible latent cooling loads, a DH was not 

included in this simulation (see Appendix D for results with a stand-alone DH). In Denver and 

other cold climates, radiant delivery with an AWHP is an effective strategy that saves 31% of 

annual HVAC energy. The higher distribution effectiveness in heating is more evident in the 

cold climate of Denver where percent savings are almost 50% higher than in Sacramento.

Table 21. Denver TRNSYS Results Comparison

Mode Control 

Strategy

Energy Use 

(kWh)

% Savings 

Versus Base 

HP Pump Fan Total Case

Base Case: Ducts in Attic Set point 7,439 0 1,502 8,941 –

BC + Ducts in Cond. Space Set point 7,136 0 1,441 8,577 4%

Radiant-Only Set point 5,787 398 0 6,184 31%

MM Set point 5,759 395 10 6,164 31%

This study also evaluated the feasibility of this strategy for hot-humid climates. Table 22

presents simulation results for Houston with a stand-alone DH similar to that used in the Tucson 

model. Due to high latent cooling loads in Houston, DH energy use is almost 50% of total 
50

HVAC energy use for the AWHP cases. TRNYSYS modeling indicates that there is no savings 

potential for AWHPs with radiant or MM delivery in hot-humid climates. C&C operation is not 

evaluated for Houston since no savings were observed for the AWHP.

It was also of interest to evaluate the case in which dehumidification is accomplished in such a 

way that the system heat is not rejected back into the house. Results are presented in Table 23.

16

In this case the DH system also contributes substantially to sensible cooling load and represents 

the majority of total HVAC energy use for all cases. While total energy use for the AWHP 

strategies is reduced for this scenario compared to the previous scenarios, positive savings over 

the base case are still not achieved.

Table 22. Houston TRNSYS Results Comparison

(Stand-Alone DH, DH Waste Heat to Conditioned Space)

Mode Control

Strategy

Energy Use 

(kWh)

% Savings 

Versus

HP Pump Fan DH Total Base Case

Base Case: Ducts in Attic Set point 3,182 0 820 1,669 5,671 –

BC + Ducts in Cond. Space Set point 3,049 0 787 1,711 5,547 2%

Radiant-Only Set point 4,118 378 0 3,394 7,890 –39%

MM Set point 3,370 298 359 2,608 6,635 –17%

Table 23. Houston TRNSYS Results Comparison

(DH Waste Heat Rejected to the Outside)

Mode Control 

Strategy

Energy Use 

(kWh)

% Savings 

Versus

HP Pump Fan DH Total Base Case

Radiant-Only Set point 2,035 165 0 4,153 6,354 –12%

MM Set point 1,659 132 15 4,209 6,015 –6%

Results for the four climate zones are summarized in Table 24 including total HVAC energy 

savings and utility cost savings. All results are presented for the radiant-only delivery mode. 

The greatest annual savings are seen in Denver, a cold climate, with an estimated $310 per year 

utility bill savings over the base case and $269 compared to ducts within conditioned space. 

Savings for C&C operation are greatest for Sacramento,

17 with 24% HVAC energy savings and 

$99 utility bill savings. Note that time-of-use rates may significantly increase utility bill savings 

for the C&C strategy.

 16 This could be accomplished by using a chilled water coil similar to that in the MM scenario, which is decoupled 

from the primary cooling delivery system.

17 C&C results for Sacramento are presented for the alternative precooling strategy with a setback from 4 a.m.–6 

a.m. June through September.
51

Table 24. Climate Zone Annual HVAC Energy and Utility Cost Savingsa

Climate Zone

AWHP vs. Base 

Case

AWHP Versus BC + 

Ducts in Cond. Space

AWHP + C&C 

Versus Base Case

kWh 

Savings

Utility 

Savings

kWh 

Savings

Utility 

Savings

kWh 

Savings

Utility 

Savings

Tucson (Hot-Dry) 3% $13 0% $0 16% $80

Sacramento (Hot-Dry) 21% $85 17% $65 24% $99

Denver (Cold) 31% $310 28% $269 n/a n/a

Houston (Hot-Humid) 0% $0 0% $0 0% $0 a Utility costs are based on a national average cost of $0.1126/kWh.

4.3 Cost Effectiveness

Table 25 shows results of a cost-effectiveness analysis for this strategy in the three climates that 

demonstrated energy savings. This analysis assumes that the incremental cost of the energy 

efficiency measures will be financed at an interest rate of 4.5% and a loan term of 30 years. The 

mature market system incremental cost of $6,400 as presented in Table 4 is used. Annualized 

utility savings are estimated using an escalation rate for electricity of 4% and a real discount rate 

of 3%. 

Denver is the only climate with a positive annual cash flow at $108 annually. The break-even 

point, which results in a neutral cash flow, is achieved with first-year utility savings of $217, or 

about 1,925 kWh of annual electricity savings. For the AWHP strategy to be cost effective in 

Sacramento, provided $85 annual savings, the total incremental cost must come down to about 

$2,500. This may be achievable if the costs of the radiant floor are removed from the mature 

market incremental cost estimate, assuming that radiant distribution is desirable from a comfort 

perspective.

Table 25. Cost-Effectiveness Evaluation

Climate Zone

AWHP Versus Base Case

First-Year 

Utility Savings 

Annualized 

Utility Savings

Average Annual 

Cash Flow

Tucson (Hot-Dry) $13 $15 ($235)

Sacramento (Hot-Dry) $85 $98 ($152)

Denver (Cold) $310 $358 $108
52

5 Conclusions and Recommendations

AWHPs with radiant or MM delivery are an effective and efficient means of providing space 

heating and cooling in residential buildings in certain climates. This strategy presents a viable 

alternative to locating ductwork in conditioned space, which may not be feasible in all homes 

due to architectural challenges, while providing the comfort, thermal storage, improved 

distribution, and reduced noise benefits of radiant slab delivery. TRNSYS modeling estimates up 

to 31% HVAC energy savings compared to a standard HP with tight ducts located in the attic

and up to 28% compared to the same base case with ducts located within conditioned space. 

Current system costs are high; however, there is justification to anticipate lower incremental 

costs as this strategy gains wide market acceptance. Cost reductions can be expected with

increased contractor familiarity and reductions in manufactured equipment costs from volume 

production. Further research focused on development of packaged AWHPs as well as packaged 

controls for zoned systems is necessary. This will drive cost reductions and simplified 

installation procedures, in addition to ensuring consistent levels of quality and gaining market 

acceptance from contractors and installers.

The following are conclusions to the research questions posed in this study.

1. What are the average effective heating COPs? What is the efficiency of the integrated 

water heating/space heating system in heating mode (Altherma) and how does it compare 

to manufacturer specifications?

Seasonal space heating COPs over the full monitoring period of 3.26 and 4.18 were observed at 

the S.E.E.D. house and Cana house, respectively. Measured heating performance of the Altherma 

HP was very comparable to the manufacturer’s specifications. However, water heating COPs

with the Altherma were much lower than expected (COP = 1.63). It is anticipated that this is a 

result of poor heat transfer between the HP supply loop and the storage tank. The seasonal water 

heating system COP of 1.05 was also lower than expected due to regular operation of the electric 

resistance back-up heater to satisfy water heating demand.

2. What are the average effective cooling EERs, and can the dramatic improvement in 

performance relative to typical forced-air-only systems seen in previous testing be 

replicated?

Seasonal EER over the monitoring period in space cooling was 11.2 and 10.8 at the S.E.E.D. 

house and Cana house, respectively. This is a substantial improvement over measured 

performance in the field of residential air conditioners with ducted air delivery of 5.5 to 8.5 EER 

(Proctor et al. 2011). Performance was most dependent on outdoor air conditions with less than 

expected sensitivity between efficiency and EWT on the load side. Data were not able to confirm 

expected performance improvements of the hydronic system due to reduced thermal lift from 

high supply temperatures in cooling and lower supply temperatures in heating. 

3. How effective is nighttime precooling in improving HP efficiencies and reducing cooling 

energy use?

Substantial cooling energy savings can be achieved from a precooling operating strategy that 

shifts air conditioner daytime operation to cooler nighttime hours and utilizes the house thermal 

mass to ride out most peak afternoon cooling events. Monitoring results from the S.E.E.D. house 
53

show 27% savings from precooling operation at a daily maximum outdoor temperature of 90°F 

and up to 40% savings at maximum temperature greater than 100°F. TRNSYS modeling 

estimates up to 18% seasonal cooling energy savings from precooling in the Tucson climate and 

up to 43% cooling savings in Sacramento with an alternative precooling strategy more 

appropriate to that climate. Seasonal percent savings can be lower than daily savings on hot days 

due to overcooling on milder days. An optimized solution could minimize this by employing a

“smart” precool strategy that monitors weather conditions and changes the precooling set point 

accordingly. A precooling strategy is recommended in homes with available thermal mass (i.e,. 

exposed slab) for energy storage. Primary advantages result from more efficient operation during 

cool nighttime temperatures and increased efficiency due to reduced cycling. Daytime airconditioner operation is delayed or eliminated, reducing peak load demand and utility bills for 

those customers on time-of-use electricity rates. The success of this strategy relies on occupants 

being comfortable with cooler interior temperatures during the night and early mornings, and the 

compressor being located such that nighttime operation does not affect occupants sleeping. To be 

effective, a minimum nighttime setback of 5°F is recommended. 

4. How does the distribution efficiency of the MM system compare to that of a typical 

forced-air delivery system with ducts in unconditioned space?

Measured distribution efficiencies of the radiant floor distribution averaged 96%. This is 

approximately equivalent to a ducted distribution system with ductwork located inside 

conditioned space (94%) but is much higher than the 76% estimated for typical attic-located tight 

ducts (<=6% air leakage). Ductwork distribution efficiencies can commonly be much lower still, 

with 61% estimated for attic ducts with 15% leakage. In certain climates, underfloor slab losses 

may be eliminated in the cooling season due to cool ground temperatures. In cooling-dominated 

climates with temperate ground temperatures it may be possible to eliminate slab insulation

without significantly affecting heating season performance.

5. Is the fan coil and latent cooling it provides necessary for dehumidification and to prevent 

floor condensation in a hot-dry climate, or can the forced-air delivery be eliminated 

completely?

Some form of dehumidification is required in all but the driest climates. Monitoring and 

modeling results from the dry Central Valley of California indicate that neither floor 

condensation nor interior comfort is a concern with radiant only cooling distribution. In other dry 

climates, such as Tucson, some dehumidification is required during the monsoon season, which 

can be accomplished with the MM distribution strategy. A control strategy to optimize 

performance could incorporate a humidistat control on the fan coil to switch from floor cooling 

to MM cooling only during periods of rising indoor moisture conditions.

6. Can TRNSYS reliably predict performance of this HVAC strategy?

Calibration of the AWHP was able to achieve results very close to that from monitored data. The 

calibrated model was able to match monitoring data over the period of April 2011–August 2012, 

with temperature differentials across the heat exchange within 1.6% of the observed data during 

heating season and within 9.7% during the cooling season. HP power was on average 8% 

different in heating operation and 1% different in cooling operation. The TRNSYS AWHP

module can be used with confidence to evaluate additional climates and applications.
54

There is less confidence in the radiant slab delivery component of the model and future work 

could contribute to improved calibration. Detailed information on occupant loads and load 

scheduling was very difficult to ascertain. As part of the calibration process, the internal loads in 

the model were adjusted to provide the best match to the monitored slab response. Long run 

times were observed in both the model and the monitored data during radiant-only delivery on 

very hot days due to the slab’s slow response. However, HP run times in the TRNSYS model 

were longer than in the monitoring data on these hot days. The model delivery system and 

interaction between the slab and the house may have reduced accuracy or these results may 

suggest that the house internal loads assumed in the model are greater than those in actuality.

7. In what climate zones is this strategy applicable?

The calibrated TRNSYS model was used to predict energy savings for air-to-water systems in 

four locations for radiant-only and MM operating modes and using both fixed set point and C&C

night setback control strategies. Long-term monitoring confirms TRNSYS modeling results that 

AWHPs with radiant delivery are an effective, energy saving strategy for hot-dry climates with 

negligible latent cooling loads. Modeling indicates that when whole-house dehumidification is 

properly addressed, such as via a stand-alone DH, radiant cooling is a viable strategy in dry 

climates with seasonal humidity loads. TRNSYS modeling estimates 3%–21% annual HVAC 

energy savings in a hot-dry climate compared to a standard HP with tight ducts located in the 

attic, with the higher savings in milder hot-dry climates. Savings may be as high as 24% with a 

precooling strategy. In cold climates, estimated annual HVAC savings are even higher at 31% 

due to high distribution effectiveness during radiant heating mode.

Due to high latent loads, the TRNSYS model found that radiant cooling is not appropriate in 

humid climates. A stand-alone DH was necessary to avoid significant operation outside the 

ASHRAE Standard 55-2010 comfort zone (ASHRAE 2010) and to prevent floor condensation in 

the Houston climate. It is possible that a strategy that combines radiant cooling and a decoupled 

dehumidification strategy could be applicable in mixed-humid climates. One possible solution 

would be to operate the MM scenario in such a way that chilled water could be delivered just to 

the fan coil and bypass the radiant floor when latent loads are high. This single system could 

serve both the sensible and latent cooling loads and may be more viable. However, this would 

only be appropriate for climates that do not have dehumidification needs during the heating 

season. Field testing is necessary to test future research ideas in hot-humid climates.

Based on a cost benefit analysis over a 30-year mortgage using current projections for mature 

market costs, the strategy was found to be cost effective only in cold climates and not in hot-dry

climates. System incremental costs would have to come down by a factor of three to make the 

strategy cost effective in a hot-dry climate, or utility rates would have to triple. Combining the 

AWHP with a precooling strategy can greatly increase cost effectiveness, especially if time-ofuse utility rates are in use. There are various benefits provided by the AWHP radiant system that 

are not factored into a cost analysis. These include increased occupant comfort, improved 

thermal distribution, noise reductions, and peak load reduction. In addition, as electricity prices 

increase, this may move the technology to be cost effective for additional climates.
55

6 References

ACCA. (2006). Residential Load Calculation, 8th ed.- Manual J. Air Conditioning Contractors 

of America. Arlington, VA: ACCA.

ASHRAE. (2004). ANSI/ASHRAE Standard 152-2004: Method of Test for Determining the 

Design and Seasonal Efficiencies of Residential Thermal Distributions Systems. Atlanta, GA: 

ASHRAE.

ASHRAE. (2010). ANSI/ASHRAE Standard 55-2010: Thermal Environment Conditions for 

Human Occupancy. Atlanta, GA: ASHRAE.

CEC. (2003). Final Report Compilation for Residential Hydronic Radiant Cooling and Heating 

Assessment. Prepared by Architectural Energy Corporation for the CEC. P-500-03-096-A14. 

Accessed June 5, 2013: www.energy.ca.gov/2003publications/CEC-500-2003-096/CEC-500-

2003-096-A14.PDF.

CEC. (2010). 2008 Building Energy Efficiency Standards – Residential Alternative Calculation 

Method (ACM) Approval Manual. CEC-400-2008-002-CMF. Sacramento, CA: CEC.

CEC. (2012). Final Evaluation Report—Proposed Compliance Option for: Altherma Air-ToWater Source Heat Pump for the Residential Energy Efficiency Standards. Sacramento, CA: 

CEC. Accessed March 5, 2013: www.energy.ca.gov/2011publications/CEC-400-2011-010/CEC400-2011-010-SF.pdf.

EIA. (2009). “Table CE3.1 Household Site End-Use Consumption in the U.S., Totals and 

Averages.” 2009 Residential Energy Consumption Survey. Washington, DC: EIA. Accessed 

February 8, 2013:

www.eia.gov/consumption/residential/data/2009/index.cfm?view=consumption#end-use.

German, A.; Dakin, B.; Backman, C.; Weitzel, E.; Springer, D. (2012). Super Energy Efficient 

Design (S.E.E.D.) Home Evaluation. DOE/GO-102012-3701. Work performed by the Alliance 

for Residential Building Innovation, Davis, CA. Golden, CO: National Renewable Energy 

Laboratory. Accessed April 20, 2013:

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/seed_home_eval.pdf.

Hendron, R.; Engebrecht, C. (2010). Building America House Simulation Protocols. NREL/TP550-49246. Golden, CO: National Renewable Energy Laboratory. Accessed February 4, 2013: 

www.nrel.gov/docs/fy11osti/49246.pdf.

Proctor, J.; Chitwood, R.; Wilcox, B. (2011). “Efficiency Characteristics and Opportunities of 

New California Homes.” Sacramento, CA: CEC. 

RSMeans Building Construction Cost Data 2011. (2010) 69th edition. R. S. Means Company, 

Incorporated.

Springer, D.; Eastment, M.; Dakin, B. (2008). “Comparative Performance of Four Prototype 

Mechanical Systems in a Desert Climate.” ASHRAE Transactions (114:2).
56

Springer, D.; Dakin, B.; Backman, C. (2012). A Feasibility Study: Ductless Hydronic 

Distribution Systems with Fan Coil Delivery. DOE/GO-102012-3629. Work performed by the 

Alliance for Residential Building Innovation, Davis, CA. Golden, CO: National Renewable 

Energy Laboratory. Accessed June 5, 2013: http://apps1.eere.energy.gov/buildings/publications/ 

pdfs/building_america/ductless_hydronic_dist.pdf.
57

Appendix A: Analysis Details

A multiple linear regression analysis was conducted to develop two predicted relationships 

between HP performance and outdoor dry-bulb temperature and EWT. The two performance 

parameters that were used were condenser power and capacity. The DOE2 simulation approach,

which assumes a bi-quadratic relationship between the variables of interest, was used. The biquadratic equation is of the following form:

c1+c2*OAT+c3*OAT2+c4*TWHE+c5*TWHE2+c6*OAT*TWHE

cn are constants that are determined through the regression, OAT is outdoor air temperature, and 

TWHE is HP EWT.

The function linest in Excel was used to estimate the coefficients of the curve based on full-load 

monitoring data. Full-load data for this analysis is quantified as any 15-min period during which 

the HP is operating greater than 92% of the time and had been operating greater than 92% of the 

time during the previous 15-min period

Statistical outputs from this analysis are presented in Table 26 and Table 27. The F probability 

distribution was evaluated to test the hypothesis that the observed relationship between the 

dependent and independent variables occurred by chance and resulted in a 0% chance in all 

cases. 

Table 26. Statistical Results from Multiple Linear Regression of Condenser

Power and Capacity in Space Cooling in Relation to OAT and LWT

Statistic Result—Power Result—Capacity

n 3,607 3,607

r

2 98.1% 87.4%

Standard Error 0.0298 785

95% Confidence Interval (CI) (–0.058, 0.058) (–1,539, 1,539)

99% CI (–0.084, 0.084) (–2,204, 2,204)

Table 27. Statistical Results from Multiple Linear Regression of Condenser

Power and Capacity in Space Heating in Relation to OAT and LWT

Statistic Result—Power Result—Capacity

n 1,105 1,105

r

2 98.8% 95.8%

Standard Error 0.0201 793

95% CI (–0.039, 0.039) (–1,553, 1,553)

99% CI (–0.057, 0.057) (–2,225, 2,225)
58

Appendix B: Short-Term Testing and Commissioning Results

S.E.E.D.

Major commissioning tasks were conducted over 2 days in April 2011. The focus of the effort 

was to verify correct operation of the mechanical systems (specifically the HP), verify correct 

operation of the monitoring equipment including sensors and communications, and take one-time 

measurements of pertinent data points.

Integrating the zoning, the hydronic and air systems, and the HP in both heating and cooling 

required some creative on-site revisions to the original design as well as troubleshooting. 

Although efforts were made to use off-the-shelf components, this was not possible in all 

instances. A custom control box was constructed to communicate between the zone thermostats, 

the zone valve controller, the HP, and the fan coil. The Taco brand zone controller is designed 

for heating-only systems and is not capable of controlling both heating and cooling. A residential 

zone control may have worked, but the defrost control may not have been compatible with the 

AWHP equipment. A relay was installed to open all floor zone valves when a defrost signal is 

received from the HP, allowing the HP to absorb heat from the entire slab instead of from a 

single zone.

The Aqua Products controls require that a heating call be received from a single zone to activate 

the reversing valve in heating. The living room zone was selected as the master zone and has to 

call for heating for either of the other two zones to receive heating. For cooling, the controls 

were able to be set up such that a call for cooling from any zone would initiate HP operation. 

There was some difficulty wiring the pump into the Aqua Product controls. It was expected that 

if the pump relay output was wired directly to the unit it would operate whenever there was a call 

for heating or cooling; however, at startup it was found that the pump was not operating at all. A 

second relay was installed between the zone controller and the HP. A control schematic can be 

found in Appendix C.

A zone bypass was installed on the radiant floor system to maintain a minimum flow rate 

through the HP when only one zone is calling. The monitoring equipment was used to verify 

both pump flow and power under different operating modes. Both flow and power remain 

relatively constant regardless of the number of zones calling.

One-time HERS tests, including duct and building envelope leakage testing were completed by 

HERS raters contracted by the local utility (Tucson Electric Power). ARBI also conducted 

testing to verify fan coil airflow, fan coil power, and pump power. 

Cooling operation began with using only the floor for delivery (air handler turned off). When the 

air handler was later enabled to test MM delivery performance, the builder discovered that the 

condensate pan was slanted, so that the condensate often pooled in the pan. This water then 

reevaporated into the supply airstream resulting in the reintroduction of humidity to the space. 

The builder fixed the problem by leveling the pan, which facilitated proper drainage, and the 

monitored indoor RH decreased. Indoor RH never exceeded 60% during this period, and the 

occupants did not express any discomfort.
59

The following is the verification checklist used during commissioning with select results from 

the process.

System/Sensor Commissioning

Preparation and Base Load Measurement

 Shut off breakers for water heater element, refrigerator, microwave, range, washing 

machine, and PV system.

Run hot water tap (bathtub) to deplete solar storage (did not do).

 Shut off the ERV.

 Unplug solar pump.

 Unplug recirc pump.

 Wait 5 min while base load is being measured.

Verify Hydronic System Operation

 Install flow meters and recharge system.

 Review control wiring and verify transformer in zone control is disconnected.

 Disconnect HP compressor from contactor.

 Unplug HP from switched outlet and plug into live outlet.

 Set all thermostats to cool and power up HP.

 Verify pump operation.

 Set up logger to read flow and set logger spans correctly.

 Manually open Zone 1 (guest bedrooms) and wait 5 min while pump power and flow are 

recorded.

 Manually open Zone 2 (living) and wait 5 min.

 Manually open Zone 3 (master bedroom) and wait 5 min.

 Adjust bypass valve to ensure a minimum flow of 5 gpm in all zone-calling scenarios.

Table 28 documents the measured flow rates during the radiant system for various zone calling 

scenarious.

Table 28. Hydronic System Flow Rate With Various Zones Calling

Zones Calling Flow Rate

(gpm)

All Open 6.21

Zone 1 Only 5.72

Zone 2 Only 5.69

Zone 3 Only 5.69

Zone 1 and 2 Only 5.74

All Closed 5.65
60

DHW Pump Power Tests

Solar Pump (could not test – leak in solar collector)

Plug in the solar pump.

Wait 5 min (after pump starts running).

Unplug and turn off water.

Recirc Pump

 Plug in recirc pump and set to “manual on.”

 Wait 5 min while pump power is recorded.

Air Handler Airflow Tests

To check supply/return temperature calibration:

 Turn on switch at fan coil.

 Unplug the pump from live outlet.

 Close coil bypass valve (noting original position).

 Activate living room thermostat in cooling mode (verify fan operation).

 Wait 5 min while supply/return sensors are checked.

To measure airflow:

 Plug the pump into the switched outlet.

 Set living room thermostat to heat and raise temperature until it turns on the HP.

 Manually close the zone valve for the living room.

 Wait 15 min or until supply/return temperature difference stabilizes.

Restore System

 Set living room thermostat to 78°F (cooling mode). 

 Reset the position of the coil bypass valve.

 Turn on breakers that were turned off in Step 1, Preparation.

 Plug in the solar pump.

 Turn on the ERV.

 Set recirc pump to “timer.”

HERS Tests

Duct Tightness Test

 Test total duct leakage: 24 supply/22 return: 46 cfm total leakage

Blower Door 

 Test building leakage with blower door (CFM50 < 1935): 750 CFM50

One-Time Measurements

 Verify fan coil airflow: 831 cfm

 Measure solar pump power: 137 W

 Measure HP circulation pump power: 150 W

 Measure DHW recirculation pump power: 38 W
61

Cana

Commissioning of the Cana house was conducted over several visits to ensure that all issues 

were resolved and the Altherma system was functioning properly. An initial visit was scheduled 

with the Daikin Altherma representative, who verified operation and programmed the HP based 

on the design criteria. During the first visit, several basic installation mistakes were identified 

and brought to the attention of the builder and HVAC contractor. They included the following:

• Incorrect zone valve models were installed. Normally open valves were specified, but 

normally closed valves were installed.

• Zone dampers were wired incorrectly, resulting in incorrect operation.

• HP went off on a “7H” error, indicating insufficient water flow. The sound of air in the 

lines could be heard.

• Duct leakage was just below 6% of design airflow.

The contractor returned and corrected the above items, including purging of the lines to remove 

air. ARBI returned to the site to complete the commissioning process. The commissioning 

process included the following procedures:

1. Test duct leakage.

2. Verify water heating mode and operation.

3. Verify systems in heating and cooling modes.

4. Verify adequate pump flow in each mode of operation.

5. Verify adequate flow through each floor circuit.

6. Calibrate airflow measurements relative to fan power and supply plenum pressure.

The data logging equipment was used to assist in commissioning and readings from handheld 

equipment were taken to calibrate and verify correct operation of all monitoring sensors. A 

blower door test was also performed to measure building envelope leakage at 50 Pa. 

At the time of commissioning, the HP was operating correctly in all modes. Once data collection 

began, it was noticed that the HP was not operating during calls for cooling or water heating. The 

HVAC contractor returned several times to reset the HP and flush the loop, but the heat pump 

continued to turn off after short periods of operation. On September 21, 2011, when ARBI was 

on site for a walk-through with the homeowner and the Altherma representative, the HP was 

again disabled due to a flow error. The filter screen on the Altherma unit was found to be almost 

completely plugged with plumber’s dope (see Figure B-1). The HVAC contractor cleaned out the 

filter screen, reflushed each circuit individually, cleaned the screen one more time, and then bled 

air out of the loop. The HP was then restarted and it has been operating reliably since.
62

Figure 26. Plugged Altherma filter screen

HERS Tests

Duct Tightness Test

 Test with outside air and relief louvers in normally closed position to evaluate leakage 

from dampers.

 Retest with outside and relief louvers sealed: 92 cfm

Blower Door

 Test now and evaluate need for test after ceiling insulation is installed (SLA < 3.5, 

CFM50 < 2995): 2637 CFM50

System/Sensor Commissioning

 Verify NightBreeze control settings.

Verify Water Heating Mode and Sensors

 Run down water temperature (as needed) to initiate a water heating call (heat pump and 

resistance) and leave tap open, start HP.

 Verify change in three-way valve position.

 Measure/verify: FWD: _4.2__ gpm FWS_11_gpm EWH_4.8__kW

 Time to fill cup: _10.5____ s for _3.5____ cups

Verify Systems in DHW Mode

 Verify HP operation.

 Verify mode status:

TWHL__132.3_____ TWHO__100____ FWS______

Verify Systems in Cooling Mode

 Set zones 1 and 2 to cooling. 

 Wait 15 min and measure temperatures and power:

TWHS__66.3____ TWHL__52.7____ TWFS_56.7____ TWCS__75.8___
63

EFAN __42____ EHP__________

 Test individual zones:

Zone 1 calling: FWS__6.0_____ gpm (> 4.5) Verify SZD2 = 1

EFAN__56____

Zone 2 calling: FWS___5.6____ gpm (>4.5) Verify SZD1 = 1

EFAN__56____

Verify Vent Cooling Mode and Status Sensor

 Set manual fan to “outside air.”

 Verify airflow at relief (verified damper operation).

 Verify SDMP status and signal: 0 = Recirc 1 = OA

Flow Balance Check

 While system is operating in heating or cooling mode, check flow at each circuit. Adjust 

and balance if necessary and double check system flow (FWD).

Table 29 presents results from the flow balance check process.

Table 29. Hydronic System Flow Balance Test Results

Design

Heating 

(gpm)

Design 

Cooling

(gpm)

Measured

(gpm)

MANIFOLD 1 – MASTER BEDROOM CLOSET

Loop 1: M Bath / Closet 0.28 0.34 0.75

Loop 2: M Bedroom 0.33 0.40 0.75

MANIFOLD 2 – CLOSET AT END OF HALL

Loop 3: Hallway 0.32 0.39 0.5

Loop 4: Bed 2 0.21 0.26 0.5

Loop 5: Office 0.17 0.21 0.75

Loop 6: Office 0.16 0.20 0.75

Loop 7: Gallery 0.4 0.47 0.5

Loop 8: Gallery 0.4 0.47 0.5

Loop 9: Gallery 0.4 0.47 0.5

MANIFOLD 3 – CABINET OFF KITCHEN NEAR PANTRY

Loop 10: Kitchen / Laundry 0.47 0.58 0.75

Loop 11: Kitchen / Pantry 0.53 0.65 0.9

Airflow Calibration and Balance

 Install True-Flow grid.

 Install pitot tube in plenum.

 Set manual fan to “on” and verify pressure sensor operation.

 Measure system airflow at various settings to correlate with supply plenum pressure 

(PAS) and fan power (EFAN). (See Table 30)
64

Table 30. NightBreeze Airflow Test Results

NightBreeze 

Airflow 

Setting (cfm)

Measured 

Airflow 

Reading (cfm)

PAS (Pa) EFAN (W)

200 N/A 2.7 23

500 540 5.7 45

1,000 860 9.3 113

1,500 1,350 18.0 360

1,800 1,610 32.0 608

Plenum Pressure Normal (Pan) = 9.3 Pa w/ TrueFlow (PaTF) = 8.9 Pa

 Measure room-by-room airflows in cooling mode. (SeeTable 31)

Table 31. Room-by-Room Airflow Test Results

Room

Measured Airflow 

Reading

(cfm)

Master Bed 130

Master Bath 95

Bath 2 23

Bed 2 132

Office 143

Gallery-1 (NW) 169

Gallery-2 (SW) 146

Gallery-3 (NE) 97

Gallery-4 (SE) 97

Kitchen-1 (N) 211

Kitchen-2 (S) 121

Laundry 71

Pantry 65

ZV1 closed = living zone only

ZV2 closed = sleeping zone only
65

Appendix C: S.E.E.D. House Mechanical Systems Control

The control diagram and associated description for the HP and zone control are shown in Figure 

27.

Figure 27. Mechanical system controls schematic

ZONE VALVES

1

TACO ZVC403

ZONE VALVE CONTROL

END

SWITCH N/O COM N/C 1 2 3 4

ZONE 1

1 2 3 4

ZONE 2

1 2 3 4

ZONE 3

T T

ZONE 1

T T

ZONE 2

T T

ZONE 3

R C

2 3

H

M

L

C

FAN

MOTOR

BLACK

BLUE

RED

FAN COIL

WHITE

DESCRIPTION OF OPERATION

Z1 Z2 Z3 R/ES ES C

HEAT PUMP

OMROM MY3-AC24(S)

3PDT 24VAC RELAYS

INTERFACE BOX WITH

H

E

A

T

C

O

O

L

C Y1

W1

R C

AQUA PRODUCTS RCS

R W2 Y B/O R W Y R W Y

ZONE1

GREAT ROOM

ZONE 2

M. BEDROOM

ZONE 3

W. BEDROOMS

THERMOSTATS

PUMP RELAY

PUMP

120 V

N

L1

240V

POWER

L1

L2

HEATING: A heating call from the Zone 1 

master thermostat activates the interface 

relay, causing the heat pump to operate in 

heating mode. On a call for heating from

any thermostat, the "W" signal passes

through the interface relay to the zone control.

The zone control opens the corresponding

zone valve. When the zone valve is open the

zone control end switch signals the "Y/W"

input to the heat pump, causing it to run in

heating mode. The zone control relay enables

the fan coil to operate (if the manual switch

is in the on position).

COOLING: The Zone 1 master thermostat

is set to "Cool" causing the heat pump to

operate in cooling mode. On a call for

cooling from any thermostat, the "Y" signal

passes through the interface relay to the

zone control. The zone control opens the

corresponding zone valve. When the zone

valve is open the zone control end switch

signals the "Y/W" input to the heat pump,

causing it to run in cooling mode. The zone

control relay simultaneously starts the

pump and activates the fan coil relay,

causing it to run the fan. The manual switch

can be used to disable the fan coil.

RELAY

ZONE CONTROL

C R Y/W CL HT DEF

Y W R Y W R

THERMOSTATS A/H

W2 Y B/0 R W C

ZONE 1 ZONE 2 ZONE 3 AUX HT

R1

R2

DEFROST

6

2 4

5

1 3

R

C

W1

W2

G

6

5

4

3

2

1

W2

W1

C

CBC2

TB2

1

2

3

4

L1

FAN

L2

240 VAC

MANUAL

FAN SWITCH

DEFROST: On receiving a

defrost signal from the heat

pump the interface control

will open all zone valves,

allowing the heat pump to

obtain defrost cycle heat

from the entire slab.

RED

BRN

ORN

VIO

BLK

BLK

YEL

RED

VIO

ORN

BRN

L2

L1

CBC1 BLK

RED

RED

BLK

RED

RED

STRIP HEAT SECTION

BRN

ORN

NOTE: MEASURE VOLTAGE

BETWEEN 'C' LEG OF FAN COIL

TRANSFORMER & 'C' LEG AT

INTERFACE BOX BEFORE

CONNECTING TO TB2 TO

INSURE THEY ARE IN PHASE.

REVERSE 24V WIRES AS NEEDED.

R

C

EMERGENCY HEAT: On a

2nd stage heat call from the

Zone 1 thermostat (only), the

interface box signals the

fan coil relay to activate

strip heat and run the fan 

at medium speed.

Set Zone 1 thermostat

changeover switch to 'B'.

NOTE: DISCONNECT

TRANSFORMER FROM

TERMINAL.

S.E.E.D HOUSE MECHANICAL SYSTEM CONTROLS 4/7/11
66

Appendix D: Additional TRNSYS Model Results

Following are additional TRNSYS results not reported in the main body of the report.

Table 29 presents TRNSYS results for Sacramento with a stand-alone DH. None of these 

strategies produced condensation warnings during floor cooling. The Sacramento radiant-only 

indoor conditions are graphed in Figure 28 for each time step of the case without external 

dehumidification. There was some operation above the ASHRAE 55-2010 comfort standard of 

0.012 humidity ratio (ASHRAE 2010) but given ARBI’s experience in this region, DHs would 

not be needed with a well-designed radiant cooling system.

Table 32. Sacramento TRNSYS Results Comparison with Dehumidification Control

Mode Control 

Strategy

Energy Use (kWh) % Savings 

Versus Base 

Case HP Pump Fan DH Total

Base Case: Ducts in Attic Set point 2,927 0 623 79 3,630

BC + Ducts in Cond. Space Set point 2,774 0 591 79 3,443 5%

Radiant-Only Set point 2,587 212 0 117 2,916 20%

MM Set point 2,521 204 26 71 2,823 22%
67

Figure 28. Sacramento indoor conditions over the course of the year as compared to ASHRAE 55-

2010 comfort

Table 30 presents TRNSYS results for Denver with a stand-alone DH. DH operation is very 

minimal.

Table 33. Denver TRNSYS Results Comparison With Dehumidification Control

Mode Control 

Strategy

Energy Use (kWh) % Savings 

Versus Base 

Case HP Pump Fan DH Total

Base Case: Ducts in Attic Set point 7,426 1,500 41 8,967 –

BC + Ducts in Cond. Space Set point 7,116 1,438 41 8,595 4%

Radiant Only Set point 5,796 399 35 6,230 31%

MM Set point 5,737 393 9 36 6,175 31%

Ashrae Standard 55 

CLO 0.5 to 1.0

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

50 60 70 80 90 100

Humidity Ratio (lbw/lbda)

Dry Bulb Temperature (°F)
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