A Feasibility Study:

Ductless Hydronic 

Distribution Systems with 

Fan Coil Delivery

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

Alliance for Residential Building Innovation (ARBI)

July 2012
NOTICE

This report was prepared as an account of work sponsored by an agency of the 

United States government. Neither the United States government nor any agency 

thereof, nor any of their employees, subcontractors, or affiliated partners makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the 

accuracy, completeness, or usefulness of any information, apparatus, product, or 

process disclosed, or represents that its use would not infringe privately owned rights. 

Reference herein to any specific commercial product, process, or service by trade 

name, trademark, manufacturer, or otherwise does not necessarily constitute or imply 

its endorsement, recommendation, or favoring by the United States government or 

any agency thereof. The views and opinions of authors expressed herein do not 

necessarily state or reflect those of the United States government or any agency 

thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
iii

A Feasibility Study: Ductless Hydronic Distribution Systems

with Fan Coil Delivery

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

D. Springer, B. Dakin, C. Backman 

Alliance for Residential Building Innovation (ARBI)

Davis Energy Group, Team Lead 

123 C Street 

Davis, California 95616 

NREL Technical Monitor: Cheryn Engebrecht 

Prepared under Subcontract No. KNDJ-0-40340-00

July 2012 
iv

[This page left blank]
v

Contents

Contents....................................................................................................................................................... v

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

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

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

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

1.2 Previous Research................................................................................................................3

1.3 Market Acceptability ...........................................................................................................4

2 Technology Description....................................................................................................................... 5

2.1 Ductless Distribution ...........................................................................................................5

2.2 Hot and Chilled Water Generation ......................................................................................6

2.3 Tradeoffs..............................................................................................................................7

3 Energy and Cost Savings Evaluation............................................................................................... 10

3.1 General Modeling Methodology........................................................................................10

3.2 TRNSYS Model Description .............................................................................................11

3.3 Modeling Results...............................................................................................................12

Analysis of Distribution System Energy Savings..............................................................12

Analysis of Zoning Impacts...............................................................................................14

3.4 Cost Estimates and Economic Evaluation .........................................................................15

4 Conclusions ........................................................................................................................................ 16

Appendix.................................................................................................................................................... 18

Load Calculations and Equipment Performance Assumptions................................................18

Ductless Hydronic System Assumptions.................................................................................19

Hydronic System Water Flow, Pipe, and Pump Sizing ...........................................................20

Air-to-Water Heat Pump Performance Maps ..........................................................................20

System Costs............................................................................................................................22

References................................................................................................................................................. 23
vi

List of Figures

Figure 1. Non-vented attic houses equipment and ducting ................................................................... 1

Figure 2. Duct chases in conditioned space............................................................................................ 2

Figure 3. Edwards’ valance is mounted high on the wall, uses no fans, and can provide heating 

and cooling............................................................................................................................................ 5

Figure 4. Airedale’s cassette is ceiling mounted and integrates fan, coil, and supply and return 

grilles in a single package................................................................................................................... 5

Figure 5. The Aqua Products Reverse Cycle Chiller combines a conventional heat pump with a 

heat exchanger and control package. ................................................................................................ 7

Figure 6. Daikin’s Altherma is an inverter-controlled packaged system that can provide water 

heating in addition to heating and cooling. ....................................................................................... 7

Figure 7. Alternate system configurations with storage......................................................................... 9

Figure 8. Ducted system floor plan (Base Case) ................................................................................... 10

Figure 9. Hydronic system floor plan...................................................................................................... 10

Figure 10. Estimated building loads for ducted and ductless homes................................................. 11

Figure 11. Indoor temperature profiles during heating for Sacramento house, unzoned case........ 15

Figure 12. Indoor temperature profiles during heating for Sacramento house, multi zoned case .. 15

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

List of Tables

Table 1. Hydronic Delivery Options .......................................................................................................... 6

Table 2. Building America House Characteristics for Each Climate Zone.......................................... 10

Table 3. TRNSYS Model System Characteristics................................................................................... 11

Table 4. TRNSYS Estimated Annual Site Heating and Cooling Energy Use for Selected Climate 

Zones ................................................................................................................................................... 12

Table 5. Tabulation of Estimated Distribution System Energy Savings ............................................. 13

Table 6. Energy Use Comparison Between a Single Zone and Multizone Hydronic Distribution for 

the Sacramento Climate..................................................................................................................... 14

Table 7. Cost Effectiveness Analysis...................................................................................................... 16

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

Definitions

AC

AFUE

AHRI

ARBI

Air conditioner

Annual fuel utilization efficiency

Air conditioning, Heating, and Refrigeration Institute

Alliance for Residential Building Innovation

CFA Conditioned Floor Area

DOE-2

EER

EIR

HSPF

HVAC

PEX

RCS

SEED

SEER

WCEC

The DOE-2 building energy simulation program

Energy efficiency ratio

Energy input ratio (used by DOE-2)

Heating seasonal performance factor

Heating, ventilation, and air conditioning

Cross-linked polyethylene

Reverse cycle chiller

Super Energy Efficiency Design

Seasonal energy efficiency ratio

Western Cooling Efficiency Center

SHGC

TRNSYS

Solar Heat Gain Coefficient

Transient System Simulation Tool
viii

Executive Summary

Hydronic distribution systems have a long history, and are widely used in commercial buildings. 

As with mini-split heat pumps, hydronic systems using small terminal fan coils have the 

potential to provide high-quality comfort to homes that have small heating and cooling loads, but 

at a lower cost and with greater flexibility. These “ductless hydronic” distribution systems

eliminate space requirements needed for ducting, eliminate duct thermal losses, and are easy to 

zone. Since heat pump chiller-heaters can be charged with refrigerant at the factory, hydronic 

systems also avoid refrigerant system faults such as incorrect charge and inadequate evacuation 

of lines that can lead to sub-standard performance. Other advantages include: reduction in the 

amount of energy required to deliver heating and cooling, reduced equipment cycling, and the 

potential for incorporating energy storage for peak load avoidance and demand response. 

Ductless hydronic systems are best suited for new homes, but with the relative ease of installing 

piping in attic spaces (for example under attic insulation), they are also suitable for existing 

homes that have old, leaky, and poorly insulated ducts. With a broader and more affordable

product offering from manufacturers, installation guidelines, and contractor training, this method 

of distribution has significant potential to meet the space conditioning needs of high performance 

homes.

The primary objectives of this study are to estimate potential energy savings relative to 

conventional ducted air distribution, and to identify equipment requirements, costs, and barriers.

The analysis focuses on ductless hydronic delivery systems that use water-to-air terminal units in 

each zone. This system type is applicable in all climate regions and can provide sensible and 

latent cooling and/or heating using a single hot and chilled water source such as a heat pump, or 

a separate water heater and chiller. 

Results of TRNSYS modeling indicate that annual heating and cooling energy use (site and 

source) can be reduced by up to 22% when substituting pipes, pump, small distributed fan coils, 

and a water-to-air heat pump for the ducts, air handler, indoor coil, and conventional air-to-air 

heat pump unit of similar rating as the air-to-water unit. Of the 22% savings, on average 36% is 

attributable to the lower energy required by pumps and small fans vs. air handler blowers, and 

64% to the reduced losses from pipes as compared to ducts. 

Using the modeled energy savings and estimated costs, hydronic distribution appears to be 

economically viable in three of the four climates evaluated. Other current field studies of air-towater heat pumps coupled to radiant floor systems are demonstrating heat pump performance 

that exceeds seasonal performance calculated using a standard heat pump performance map 

(German, 2011a, German, 2011b). 

Hydronic distribution systems can meet sensible and latent cooling needs in all climates. Major 

barriers to widespread application include the lack of low-cost, small, ceiling-mounted fan coils, 

and the limited availability and higher cost of air-to-water heat pumps. The latter could be 

overcome by the production of an add-on refrigerant-to-water conversion kit that could be used 

to adapt any heat pump to serve as a water chiller-heater. 

Development of accurate performance maps and additional data on field performance are areas 

that should be targeted by future research. 
1

1 Introduction

1.1 Background and Motivation

The Building America Space Conditioning and Analysis Methods Standing Technical 

Committee identified the following gaps and barriers that can be addressed by ductless hydronic 

systems:

• The need for development of low-cost space conditioning strategies for low load homes 

• The need to improve thermal efficiency of distribution systems

• The lack of availability of high efficiency, small capacity, cost-effective heating and 

cooling equipment

“Ductless hydronic” systems, which replace ducts with pipes, use small distributed fan coils that 

are sized to meet the loads of the zones or rooms in which they are located. Air conditioners, 

heat pumps, and particularly furnaces used with central forced air systems are often oversized 

when applied to high performance low-load buildings, resulting in cycling losses for singlespeed air conditioners and heat pumps and poor temperature control (ACCA, 1995). Oversized 

equipment has also been attributed to poor humidity control in humid climates (Hourahan, 2003).

Hydronic distribution offers several advantages over traditional forced air systems, including 

reduced surface area of conduits (pipes vs. ducts), elimination of the need for duct chases, 

substitution of pumps for higher energy use fans, and more efficient zoning (Siegenthaller 2010).

The current strategy to minimize duct losses in new homes is to move ductwork into conditioned 

space by either creating duct chases within the building enclosure or turning the attic into a nonvented semi-conditioned space.

Non-vented attics 

require that insulation 

is installed at the roof 

line rather than the 

attic floor. In addition 

to being more costly 

than conventional attic 

insulation because of 

the additional material 

and labor required, this 

approach increases the 

surface area of the 

enclosure, resulting in 

an increase in building 

heat transfer, all other

things being equal. 

Dedicated mechanical chases can present architectural challenges, since duct chases must be 

aesthetically integrated. Interior equipment consumes floor space and requires creation of 

Figure 1. Non-vented attic houses equipment and ducting
2 

dedicated mechanical spaces. Creation of duct 

chases requires specially fabricated trusses

and/or lowered ceilings. In order to create the air

barrier for the chase, the drywall installer must 

make two trips, once for the standard wall 

application and again to drywall around the 

chase, resulting in added labor and materials 

costs. 

Hydronic distribution eliminates the need to 

locate ducts in conditioned space. Hydronic 

piping, which is typically 1⁄2” to 1” in diameter, 

can easily be routed under attic insulation or 

through wall cavities within conditioned space. 

To eliminate the potential for condensation, piping must be insulated with a material that is 

relatively vapor impermeable, like closed-cell foam pipe insulation, even if it is buried under 

fiberglass insulation.

Installing multiple hydronic fan coils for cooling does mean that multiple condensate lines must 

be installed. Condensate drains must to be considered in the design and installation of these 

systems when they deliver cooling as well as heating. 

Hydronic systems can easily incorporate water storage, which can provide demand response 

capability and can be integrated with solar thermal systems. Unlike conventional single speed 

gas furnaces, capacity can be varied, allowing greater flexibility for extremely low load high 

performance buildings. Two other Building America projects underway (ARBI Task Order 2 

Projects 2.1 and 3.2) are investigating the potential for improving heat pump effective system 

EERs and COPs by moderating condenser/evaporator temperatures, which is possible with 

hydronic systems, particularly when thermal storage is provided. 

A method for estimating the efficiency of hydronic distribution systems is included in ASHRAE 

Standard 152 (Method of Test for Determining the Design and Seasonal Efficiency of 

Residential Distribution Systems). The standard is currently under review, and significant 

improvements to the chapter on hydronic distribution have been proposed. Once adopted, the 

revised standard may prove to be a useful tool for comparing the performance of forced air and 

hydronic distribution systems under design or seasonal conditions. 

In summary, problems that can be mitigated by the application of hydronic distribution include: 

• Thermal losses from ducting that is installed in non-conditioned space 

• Cost and difficulty of installing ducts in conditioned space 

• Restrictions in forced air distribution systems (undersized or restricted ducts) that result 

in reduced airflow, which particularly affects cooling system performance

• Limited availability of low capacity systems for buildings with low loads

Figure 2. Duct chases in conditioned space
3 

• Inefficient zoning of forced air systems 

• Excessive fan energy use by furnaces and heat pump air handlers 

• Distribution limitations and high cost of mini-split heat pumps. 

Other factors favoring use of hydronic distribution include: 

• The low cost of plastic piping 

• The relative ease of installation of piping through framing and under-attic insulation 

• The increasing availability of air-to-water heat pumps

• Increasing acceptance of combined hydronic systems and availability of high 

performance water heating equipment 

• The energy storage and demand response capability of hydronic systems 

• Avoidance of refrigerant system faults resulting from field installation and charging of 

refrigerant lines. 

Current barriers to hydronic distribution include: 

• Contractor unfamiliarity with the technology and implementation strategy

• The division of responsibility between HVAC and plumbing contractors and the 

additional communication that will be required between these trades.

• The requirements for condensate drains from each terminal unit. Condensate drains are 

not needed with radiant floor and ceiling distribution provided the chilled water delivery 

temperature is high enough to prevent condensation on radiant surfaces, or for heating 

only systems. 

• Availability of lower cost air-to-water heat pumps and small capacity fan coils 

• Increased potential for leaks and consequent water damage due to the additional use of 

piping 

• Inability of HERS and code compliance software to directly and accurately model 

hydronic systems. 

1.2 Previous Research

The first documented installation of a hydronic distribution system was in the Bank of England 

in 1790. The ASHRAE Systems and Equipment chapters on hydronic distribution and hydronic 

heating and cooling system design are supported by research that dates as far back as 1920, and 

design of these systems is well understood (ASHRAE 2008). With the resurgence of interest in 

radiant systems, a substantial amount of material has been written on radiant heating and cooling 

distribution in recent years (for example, Siegenthaller, 2011 and Olesen, 2008). Most of the 

literature is focused on radiant panel and baseboard heating. 

Less information is available on non-ducted systems using distributed fan coils. To develop 

information to aid determination of distribution efficiencies for ASHRAE Standard 152, 

Vineyard (2000) conducted a field test to measure distribution losses associated with a hydronic 

system operated in the cooling season. Installed in a 4,300 ft2 two-story home in Newark, New 
4 

Jersey, the system included eight variable speed fan coil units. Vineyard reported measured

delivery effectiveness values from 81.4% to 92.2% over a range of outdoor temperatures from 

field tests of systems using distributed fan coils1

. The distribution efficiency, which included 

thermal regain from the distribution losses, varied from 87.5% to 92.5%, meaning that only 

12.5% to 7.5% of the cooling effect that left the space conditioning equipment failed to reach the 

conditioned space. Results also showed that the indoor relative humidity was controlled within a 

range of 46%-52% RH, while the outdoor relative humidity varied from 25% RH to 100% RH. 

Putting the results from the single field test by Vineyard in perspective, measurements of five 

ducted systems in California homes by Siegel (2002) found delivery effectiveness values ranging 

from 76% to 91% and averaging 85%2. In these houses, which ranged from 1010 to 1670 ft2, the 

ducts were well insulated and measured leakage to outside averaged 6% of measured air handler

flow. 

Ongoing Building America research on two homes with radiant heating and cooling is shedding 

additional light on distribution efficiency for radiant floor systems. At one of the sites, the 

temperature difference across under-slab insulation and the thermal resistance of the insulation

were used to calculate an average distribution efficiency of 98.3% over one cooling season 

(German, 2011a). Both the underside and edge of the slab was insulated to R-10. Applying 

ASHRAE Standard 152 calculation methods, the same house with a typical forced-air system 

with ducts located in the attic would have a seasonal distribution efficiency of 65% with 15% 

leakage and 78% with 6% leakage.

1.3 Market Acceptability

Hydronic distribution is not a new concept, but residential systems that provide both heating and 

cooling delivery are rare. Heating-only hydronic distribution systems using baseboard convectors 

are common in New England states and radiant heating is being more widely applied in custom 

homes due to the low cost and durability of PEX tubing and the reputation for comfort that 

radiant heating has earned. Other systems that use hydronic distribution to some degree include 

commercial “four-pipe” hot/chilled water systems and combined hydronic systems, which are 

commonly used in multifamily buildings. The perception that cooling cannot be provided by 

hydronic systems, as well as HVAC contractor inexperience with “wet” systems (particularly in 

the western United States), contribute to the slow adoption of this technology. Field testing and 

energy modeling of systems will improve knowledge of the energy savings potential and 

commercial viability of these systems, and will help to identify the necessary subsequent steps 

needed for commercialization.

 

1

ASHRAE 152P defines delivery effectiveness as the ratio of thermal energy transferred to or from the conditioned 

space to the thermal energy transferred at the equipment/distribution system heat exchanger. Delivery effectiveness 

fails to fully represent the fraction of the supplied energy that reaches the conditioned space to satisfy the building 

load. Distribution efficiency is defined as the ratio between the energy consumption by the equipment if the 

distribution system had no losses and the energy consumed by the same equipment connected to the distribution 

system, and takes into account effects of thermal regain, and the impact, if any, of the distribution system on the 

equipment efficiency.

2 Siegel did not report distribution efficiency. Delivery effectiveness varies from distribution efficiency primarily by 

the regain factor, which is close to unity.
5

2 Technology Description

2.1 Ductless Distribution

Hydronic systems distribute heating and cooling using piping to convey hot or chilled water to 

either convective devices or radiant panels. Radiant systems employ traditional radiators, radiant 

floor slabs, or radiant wall and ceiling panels to deliver comfort using predominantly radiant heat

transfer. Convective systems include small, distributed fan coils, baseboard convectors, or 

valance systems. Fan coils (or air handlers) are commonly used with ducting, but can also be 

used to deliver air directly to spaces or through very short ducts that can be distributed within 

each zone and kept within the conditioned enclosure. Baseboard convectors and valance systems

rely on natural convection (no fans). Baseboards can only be used for heating but valance units 

(shown in Figure 3) include condensate drains and can deliver both heating and cooling. 

Self-contained ceiling-mounted fan coils similar to the Airedale brand shown in Figure 4

integrate the fan, coil, and supply/return grilles in a single ceiling-mounted unit. These units are 

similar to the cassettes used with mini-split heat pumps except that water, rather than refrigerant, 

is circulated as the heat transfer fluid. Airedale’s ceiling mounted cassette has a fan efficacy of 

0.16 W/cfm compared to the typical 0.58 W/cfm of furnaces, which can result in significant fan 

energy savings, especially if the cassettes are separately zoned. However, the market for these 

systems is still primarily commercial buildings.

 

 

Several manufacturers, such as Williams Comfort Products, First Company, and MagicAire offer 

small hot/chilled water fan coil products that can be installed in ceiling spaces or closets with 

little or no ducting. These are commonly seen in high-rise hotels.

Radiant heating systems are frequently used in custom homes and typically use PEX tubing, 

either embedded in the floor slab, or installed under, within, or over the subfloor. Radiant floor 

cooling is much more limited due to limited available cooling capacity and risk of condensation 

on the floor surface. If the floor surface temperature drops below the indoor dewpoint 

temperature, moisture from the air will condense and collect on the surface. 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 

Figure 3. Edwards’ valance is mounted 

high on the wall, uses no fans, and can 

provide heating and cooling

Figure 4. Airedale’s cassette is 

ceiling mounted and integrates 

fan, coil, and supply and return 

grilles in a single package
6

increase the risk of floor condensation as well as provide a better medium for mold growth, and 

vinyl flooring acts as a vapor barrier to trap the condensed moisture. Field tests conducted in dry 

climates have shown that it is possible to meet design cooling loads while keeping the floor 

surface temperature comfortably above the dewpoint temperature of indoor air (German, 2011a

and Springer, 2007).

Cooling can also be accomplished with some radiant ceiling products. For example, BEKA, a 

German manufacturer, offers mats made of micro-tubes that can be installed in plaster ceilings. 

Radiant ceiling panels provide certain benefits over radiant floor systems including increased 

capacity3

, reduced risk of condensation (if installed properly)4, and elimination of restrictions on 

floor coverings, but they are more costly to install than slab-on-grade systems. Table 1

summarizes available hydronic distribution delivery options.

Table 1. Hydronic Delivery Options

Delivery Options Load type Forced Air

“Pancake” fan coil or ceiling cassette Heating or Cooling Yes

Baseboard Convector Heating Only No

Wall radiators Heating Only No

Ceiling Panels Heating or Cooling No

Slab-on-Grade Radiant Floor Heating or Cooling5 No

Raised Floor Radiant Applications Heating Only No

Valence Convector Heating or Cooling No

2.2 Hot and Chilled Water Generation

Hot water can be easily sourced from water heaters or boilers. For radiant heating systems, it is 

typical to isolate the water in the distribution tubing from potable water using a heat exchanger 

due to concerns about contamination of water standing in tubes over long periods of time. For 

fan coils, potable water is commonly circulated directly through the heating coil using stainless 

steel or bronze fitted pumps. Ferrous components such as cast iron pumps and steel panel 

radiators must also be isolated to avoid corrosion resulting from oxygen that is present in open 

systems. Hot water can also be generated using water source heat pumps such as ground-coupled 

heat pumps, or air source (air-to-water) heat pumps. Electric resistance heaters are not generally 

used because of the high operating cost and large electrical demand.

Air-to-water heat pumps operate on the same mechanical principals as air-to-air split system heat 

pumps that have been on the market for several decades, and with proper installation and 

commissioning procedures, they can provide the same durable and reliable operation. Automatic 

defrost cycles allow the units to be operated at very low ambient temperatures, often below 0oF. 

 3 Radiant ceiling panels have a higher heat transfer coefficient in cooling mode than radiant floors (1.63 versus 1.08 

Btu/hr-ft2-F) (ASHRAE, 2008). 4 Because of higher cooling heat transfer coefficient and resultant cooling capacity per square foot of ceiling area, 

higher chilled water supply temperatures can be delivered through ceiling panels, reducing the risk of condensation.

5 Appropriate only in dry climates with exposed concrete or tile floors to prevent condensation moisture issues.
7 

Figure 6. Daikin’s Altherma is an invertercontrolled packaged system that can 

provide water heating in addition to 

heating and cooling.

Ground coupled water-to-water heat pumps that produce hot and chilled water have been 

available for many years. However, the high cost of installing the ground heat exchangers limits

their market acceptability in many areas of the United States. 

While large commercial chillers are widespread, there are few on the market that are sized for 

residential or small commercial loads. Currently, there are only three heat pump products on the 

U.S. market that produce hot and chilled water. The Aqua Products “Reverse Cycle Chiller” 

(RCS) pairs a conventional heat pump with a refrigerant-to-water heat exchanger to generate hot 

and chilled water (see Figure 5). Multiaqua produces a variety of chillers and chiller-heaters for 

the residential and commercial sectors. The Daikin Altherma (Figure 6) is a newcomer to the 

U.S. market and makes domestic hot water in addition to hot and chilled water for heating and 

cooling. Unico, known for their high velocity duct systems, produces a "UniChiller" selfcontained air-to-water heat pump. LG makes a product similar to the Altherma, but they have not 

yet released it to the U.S. market. These systems produce water that is in the range between 40°F 

and 125°F. 

Carrier produced a coaxial heat exchanger (09WQ) in the 1980s that could be used to convert 

their heat pumps into water chiller-heaters. Others have used brazed plate heat exchangers to 

convert condensing units to water chillers (Smith, 2003 and Springer, 2007). The conversion can 

be accomplished by any reasonably skilled HVAC practitioner. 

Local codes can prove to be an obstacle to manufacturers of residential chillers and air-to-water 

heat pumps because there is no U.S. standard for rating their performance. Daikin found it 

necessary to appeal to the California Energy Commission to obtain a heating performance rating 

better than the minimum DOE standard value of 7.7 HSPF. Based on European test results they 

were granted an HSPF of 11.0. (CEC 2012). 

2.3 Tradeoffs

Significant energy savings stemming from the replacement of hydronic heating and cooling 

distribution systems are expected, mostly resulting from tradeoffs between fan and pump energy 

use and between duct losses and pipe losses. Additional savings may also derive from zoning and 

improved part load performance, depending on the design and application. 

Figure 5. The Aqua Products Reverse 

Cycle Chiller combines a conventional 

heat pump with a heat exchanger and 

control package.
8 

Heating and Cooling Plant Options. Integrated systems with inverter driven heat pumps like 

Daikin’s Altherma tend to be much more expensive than built-up systems that use conventional 

heat pumps and add on controls and components, like the Aqua Products RCS. Preliminary data 

suggests Altherma heating performance is much higher than for standard heat pumps; however, 

rated cooling performance is not exceptional. Their best application may be for all-electric 

houses because of their ability to efficiently heat domestic hot water. Their variable speed 

capability also favors systems that have zoning and use fan coils instead of radiant slabs for 

distribution. Where low cost is a high priority, built-up systems similar to Aqua Products' RCS 

will be a better choice, and they can be ordered with heat pumps of various efficiencies.

Distribution System Tradeoffs. A radiant distribution system can be installed for under $1.30 

per ft2 (RS Means 2011, assuming 75% coverage). Addition of insulation below the slab and at 

the edges adds to the cost, but also improves year-round performance of the building. In climates 

requiring cooling, radiant floor distribution cannot be used with vinyl, carpet, cork, wood, or 

other non-ceramic floor coverings due to risk of floor condensation and damage to flooring 

materials, though limited use of carpeting and throw rugs may be acceptable. Slab-on-grade 

radiant floor systems provide optimal comfort, and because their high mass allows them to store 

energy and shift loads, they can reduce operating costs where utility time-of-use rates are in 

place. Preliminary research results also show that the floor heating and cooling used in buildings 

with tight enclosures allows systems to be operated at times that produce higher heat pump 

performance (German, 2011b). For example, heating can be operated during the daytime when 

outdoor temperatures are higher, and cooling during the nighttime when outdoor temperatures 

are lower.

Houses on raised floor foundations and wood-framed floors of multistory buildings are best 

served by forced air delivery using fan coils with no or very short ducts serving individual spaces 

when cooling is required. Fan coil options include cassettes (as in Figure 4), or small 

conventional ceiling or closet-mounted fan coils. Small hydronic cassettes have efficiency and 

space advantages but are not manufactured or commonly used in the United States, and tend to 

cost more than conventional fan coils. An opportunity exists for the manufacture of low capacity, 

low cost ceiling mounted fan coils that are designed specifically for the residential market. Fan 

coil motor energy use should be considered when selecting equipment for forced air systems. 

Fan motor energy use can far exceed pump energy use, but fan coils that use efficient brushless 

permanent magnet motors are available and will improve the overall efficiency of the system.

The use of baseboard and valence systems may be limited by their lower aesthetic appeal. 

Baseboard heaters are designed for heating only and are typically designed to be used with 

supply water temperatures over 140°F. 

System Design Tradeoffs. Inverter-driven heat pumps (or variable capacity water heaters in 

heating only systems) may be installed without storage because they can vary capacity and are 

less likely to short cycle when loads are low during mild conditions or when not all zones are 

calling for heating or cooling. When slab-on-grade radiant floor distribution is used, single speed 

heat pumps and chillers may be installed without water storage, provided that the smallest zone 

is able to absorb sufficient energy to keep the heat pump from short-cycling. Otherwise, radiant 

and convective systems should include sufficient storage volume to allow the system to operate 

for at least 5 minutes under low load conditions. Storage tanks may either be installed in parallel 
9

or in series as shown in Figure 7. The parallel configuration may require larger storage capacity 

but is preferred when used with fan coils because of the faster response time. However, storage 

losses will be greater with this configuration because the tank must be always maintained at the 

required heating or cooling temperature. The series configuration only requires a single pump 

and simplifies controls6.

Figure 7. Alternate system configurations with storage

 6 This information is general and there are many other considerations. References such as Siegenthaller (2010) 

should be consulted for detailed information on design requirements of hydronic systems.
10

3 Energy and Cost Savings Evaluation

3.1 General Modeling Methodology

To estimate performance and potential energy savings, TRNSYS Version 17 was used to model

a typical house with a ducted system (Figure 8) and alternatively with a hydronic distribution

system (Figure 9). A model of a 2,400 ft2 single story, slab-on-grade house was developed using 

characteristics of the Building America Benchmark home. Window area is approximately equal 

in all orientations. Heating and cooling set points of 72°F and 76 °F were used, respectively.

Various simulation time steps were investigated and a 5-minute time step was ultimately used 

providing a compromise between simulation time and accuracy.

The house was simulated in four cities representing three climate regions: Hot-Dry, Hot-Humid, 

and Cold. The standard 2010 Building America simulation protocol assumptions were applied as 

listed in Table 2, corresponding to the climate zones modeled (DOE, 2010).

Table 2. Building America House Characteristics for Each Climate Zone

Climate Region Hot-Dry Hot-Humid Cold Hot-Dry

City Sacramento, CA Houston, TX Denver, CO Phoenix, AZ

Wall Type R-13 R-13 R-13+57 R-13

Window U-Value/SHGC 0.40/0.30 0.40/0.30 0.35/0.35 0.40/0.30

Ceiling Insulation 30 30 38 30

Slab Insulation/depth 0 0 R-10 for 2 ft 0

Figure 8 and Figure 9 show the heat pump system duct layout for the conventional design and 

the piping and fan coil layout for the hydronic distribution design, respectively. Sizing of heat 

pumps, ducts, and pipes was completed using ACCA manual J and D.

 7 R-13+5 means R-13 cavity insulation with R-5 continuous insulating sheathing on the exterior

Figure 8. Ducted system floor plan 

(Base Case)

Figure 9. Hydronic system floor plan
11

Figure 10 shows the calculated heating and cooling design loads for each case. The elimination 

of attic ducts resulted in an average reduction of heating loads of 23% and reduced cooling loads 

on average by 35%. Additional sizing information is provided in the Appendix.

Figure 10. Estimated building loads for ducted and ductless homes

3.2 TRNSYS Model Description 

The base case TRNSYS model incorporated a ducted air-to-air heat pump rated at 13 SEER and 

7.7 HSPF (11 EER cooling and 3.28 COP heating at rated conditions). The model assumed a 

centrally located air handler with a DX heating and cooling coil and a single zone thermostat 

located in the Great Room. Ducts, located in the vented attic, were assumed to have R-8 

insulation and a leakage rate of 15% of cooling airflow (see Table 3).

Table 3. TRNSYS Model System Characteristics

Base Case Hydronic

System Description Central ducted split-system 

air-source heat pump with air 

handler in attic

Air-source heat pump outdoor 

unit connected to refrigerantto-water heat exchanger

Distribution System R-8 ducts located in vented 

attic. Duct leakage = 15% of 

rated air flow

Hydronic distribution to small 

fan coils. Minimal ducting in 

conditioned space.

Heat Pump Efficiency 11 EER / 3.28 COP 11 EER / 3.28 COP

Fan Airflow 0.028 cfm/Btuh 0.024 cfm/Btuh

Fan Power W/cfm 0.365 W/cfm 0.365 W/cfm. 

The TRNSYS model for the air-to-air heat pump used the DOE-2 coefficient approach for heat 

pump performance, which determines off-rated power and capacity using a bi-quadratic 

equation. Base case heat pump coefficients were taken from the option editor in BEopt v1.1, 
12

which also provided the parameters for fan flow (0.028 cfm/Btuh) and fan power (0.365 W/cfm).

For the ductless hydronic case, an air-to-water heat pump having the same rated efficiency 

values as the air-to-air heat pump was applied in the model. However, the TRNSYS model for an 

air-to-water heat pump requires use of a performance map for off-rated conditions in a unique 

format distinct from the DOE-2 coefficient approach, for which the only available data is that 

based on manufacturer specifications. Given that the only performance data available for air-towater heat pumps at the time was for the inverter driven Daikin Altherma, this data was used to 

develop the air-to-water heat pump performance map. Further details regarding the performance 

parameters are provided in the Appendix. 

Performance characteristics of the fan coils were based on three MagicAire fan coils serving the 

living/kitchen area, master bedroom, and guest bedrooms. Each fan coil was scheduled to deliver 

between 200 to 500 cfm, depending on zone loads, at 0.365 W/cfm. See the Appendix for 

background on these assumptions. All terminal units were operated from a single thermostat 

located in the living room. Additional simulations were completed to investigate the impact of 

having each fan coil controlled by a thermostat located in the respective zone.

3.3 Modeling Results

Analysis of Distribution System Energy Savings

Table 4 lists TRNSYS predictions of energy use by end-use for the two cases for four climate 

regions. The substantial energy savings (averaging 55%) are due to both improved distribution 

efficiency of the ductless hydronic case, and to a large extent, improved part load efficiency of 

the heat pump. Savings in heating mode were slightly higher than for cooling.

Table 4. TRNSYS Estimated Annual Site Heating and Cooling Energy Use for Selected Climate 

Zones

Sacramento Houston Phoenix Denver

Energy Use 

(kWh/yr)

Base 

Case

Ductless 

Hydronic

Base 

Case

Ductless 

Hydronic

Base 

Case

Ductless 

Hydronic

Base 

Case

Ductless 

Hydronic

Heat Pump

Heating 7,825 3,241 3,965 1,789 3,095 1,254 13,098 6,089

Heat Pump

Cooling 1,104 522 3,808 1,877 7,450 3,102 512 276

Fan and Pump 1,470 609 1,367 769 2,625 973 1,260 746

Total 10,398 4,373 9,140 4,435 13,171 5,329 14,870 7,112

As previously described, the air-to-water heat pump was modeled using part load curves from 

the Altherma, which modulates capacity based on load, and the air-to-air heat pump used the 

generic DOE-2 heat pump curves. Even though the air-to-water heat pump performance was 

calibrated to provide similar heating and cooling EIRs as the air-to-air heat pump at rated 

conditions, the off-rated part load performance of the air-to-water system produced unexpectedly 

high energy savings, which were a substantial contributor to total savings.

For the purposes of this study, the critical outcome is the reduction in distribution system energy 

use, which includes duct or piping losses and fan and pump energy use, but not necessarily 

differences in heat pump performance. Since the heat pump models used in the conventional and 
13 

hydronic systems were based on different sources of information and different performance 

curves, the modeling results may include biases that decrease the significance of the predicted 

heat pump energy savings. In order to separately identify distribution savings, the seasonal 

efficiencies (effective COP and EER) of the standard heat pump system were calculated by 

dividing annual delivered heating and cooling energy by the annual heating and cooling electric 

use (excluding indoor fan) that were computed by TRNSYS for the base case. These efficiencies 

were then multiplied by the delivered energy for the ductless hydronic case from TRNSYS to 

develop the estimated outdoor unit electric use for the hydronic system. Finally, fan and pump 

energy were added to the seasonal energy use to obtain total annual energy use for each 

distribution system type to obtain the results presented in Table 5. Annual site electric savings 

range from a low of 16% for Houston to a high of 27% for Sacramento and Denver. On average, 

36% of the savings is attributable to the lower energy required by pumps and small fans vs. heat 

pump air handler blowers, and 64% to the reduced load imposed by pipe vs. duct losses. 

Table 5. Tabulation of Estimated Distribution System Energy Savings

Seasonal 

Performance

Heat Pump Energy 

Delivered (MBtu/yr)

Heat Pump Energy 

Use (kWh/yr)

Fan & Pump 

Energy Use 

(kWh/yr)

Total 

Energy Use 

(kWh/yr)

Energy 

Savings 

COP EER Heating Cooling Heating Cooling (kWh/yr)

Sacramento

Air-to-air 1.57 6.43 40.56 7.90 7,574 1,229 1,470 10,273

Hydronic 1.57 6.43 30.47 7.68 5,690 1,195 609 7,494 2,778

Houston

Air-to-air 1.49 7.22 19.46 30.60 3,838 4,238 1,367 9,444

Hydronic 1.49 7.22 15.98 29.28 3,153 4,057 769 7,978 1,466

Phoenix

Air-to-air 1.54 5.29 15.75 43.88 2,996 8,293 2,625 13,915

Hydronic 1.54 5.29 12.27 41.37 2,333 7,819 973 11,125 2,789

Denver

Air-to-air 1.51 7.64 65.49 4.35 12,679 570 1,260 14,509

Hydronic 1.51 7.64 48.12 4.50 9,316 589 746 10,651 3,857

This approach assumes that the effectiveness of the refrigerant-to-air indoor coil coupled with 

the standard heat pump is equivalent to the effectiveness of the refrigerant-to-water coil coupled

to the multiple water-to-air coils. With the exception of data obtained during the summer of 2011 

under a separate Building America project located in Tucson, there is no known laboratory or 

field test data to support this assumption. Monitoring data from the Tucson site show that the 

Aqua Products system using a 13 SEER heat pump yielded an average seasonal EER of 10.7, 

including pump and limited fan energy (German, 2011a). Thus, the assumed EERs for the 

hydronic heat pump used in the calculation of annual energy savings in Table 5 may be 

conservative. 

Field data from the Tucson house and another house in Chico, California, that ARBI is currently 

monitoring will shed more light on the performance of air-to-water heat pumps. The data will 

contribute to development of realistic performance maps for the two systems that can be used for 

future evaluations.
14 

Analysis of Zoning Impacts

Adding zoning to hydronic systems does not introduce the same difficulties as zoning with 

forced air systems. Compared to air systems, hydronic systems are relatively immune to the 

problem of increased fluid velocity and pressure when fewer than all zones are operating. For 

example, the increased pressure in piping does not cause the noise or leakage problems 

experienced with duct systems. Pumps are available that throttle flow in response to increasing 

pressure, and heat exchanger penalties are reduced because of the higher heat capacity of water.

To explore the potential benefits of zoning from an energy use standpoint, additional TRNSYS 

modeling was completed to determine whether zoning would further improve distribution 

efficiency. The house model shown in Figure 9 was modified to include thermostats in each of 

three zones (Living, Master Bedroom, and Guest Bedrooms) to control zone valves. Identical 

temperature schedules were used in all zones. A variable speed pump was modeled such that the 

pump would maintain equal pressures to meet the flow requirements of each respective zone.

Results, shown for the Sacramento climate in Table 6, indicate that introducing zoning decreases 

fan and pump energy use but increases heat pump and total energy use. Similar results were seen 

for the other four climate zones. This outcome is surprising considering the favorable (Altherma) 

part load performance map incorporated in the model. 

Table 6. Energy Use Comparison Between a Single Zone and Multizone Hydronic Distribution for 

the Sacramento Climate

System Type 

Annual Heat 

Pump Heating 

(kWh/yr)

Annual Heat 

Pump Cooling 

(kWh/yr)

Annual 

Pump 

(kWh/yr)

Annual 

Fan 

(kWh/yr)

Total 

(kWh/yr)

Ductless-Single Zone 3,241 522 158 451 4,373

Ductless-MultiZones 3,554 538 126 347 4,566

Savings -313 -16 32 104 -193

% Savings -10% -3% 20% 23% -4%

Upon further investigation, it was noted that the entering water temperature during heating was 

on average 7°F warmer for the multizone case. This effect is expected because when fewer zones 

are calling for heat, a smaller load is imposed on the system and the circulation loop returns 

water at a higher temperature to the heat pump than if all zones were being called 

simultaneously. This results in greater thermal lift. It appears that the increased thermal lift 

combined with lower part load efficiency trumps increased heating load resulting from 

overheating of the bedroom zones. Also, the integrity of the thermal enclosure and mass of the 

slab may have diminished the value of distributing heat only to those zones that require it.

TRNSYS results do suggest that zoning can provide improved comfort. The temperature graphs 

below are for a typical heating day in Sacramento. The average outdoor temperature over the 24-

hour period was 40 °F with a low of 34°F. Figure 11 plots the indoor temperature profiles with 

the system modeled as a single zone. The bedrooms and master suite rise above the 72oF heating 

setpoint by 4°F during each heating call. Figure 12, which plots temperatures in a zonecontrolled system, shows much less dramatic temperature swings. During the same 24-hour 

period the bedroom only called four times for heating in the zoned case but six times with the 
15 

single zone case. Further research is needed to determine whether the zoning penalty seen in the 

TRNSYS results is supported by field data. The improved temperature control in a zoned house, 

as well as the opportunity to modify thermostat settings in unoccupied zones, should yield 

energy savings.

Figure 11. Indoor temperature profiles during heating for Sacramento house, unzoned case

Figure 12. Indoor temperature profiles during heating for Sacramento house, multi zoned case

3.4 Cost Estimates and Economic Evaluation

To evaluate the economics of ductless hydronic vs. forced air distribution, costs were compiled 

for the primary components of each system type that was modeled (see the Appendix for details). 

An incremental cost of $1200 was estimated to account for the refrigerant-to-water heat 

exchanger and controls added to a conventional heat pump (comparable to Aqua Products). The 

incremental costs were amortized over 30 years using a 6% interest rate. The utility savings are 

based on the estimated energy use in Table 4 and assume a flat rate for electricity of $0.116 per 

kWh (based on an approximate national average at the time of the study). Installation labor is 

included in duct and pipe costs but not in the cost of other components. 

70

72

74

76

Zone Temperature 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

(°F)

Hour of the Day

Bedrooms

Master Suite

Living/Kitchen

70

72

74

76

Zone Temperature 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

(°F)

Hour of the Day

Bedrooms

Master Suite

Living
16 

Table 7. Cost Effectiveness Analysis8

The majority of the incremental cost (53%) is attributed to the air terminal units which, for 

purposes of this cost analysis, were assumed to be small horizontal fan coils costing about $795 

each (based on a quote from MagicAire). The Airdale ceiling cassettes cost about $2000 each. 

Given the components involved (small fan with efficient motor, small coil, condensate pan, 

housing, and diffuser), it is likely that the mature market cost for such a product would be 

considerably lower. Not counted in the savings is the elimination of the cost for HERS tests of 

duct tightness and verified refrigerant charge. 

4 Conclusions

Hydronic distribution is a viable alternative to forced air distribution that increases construction 

costs but can have very favorable economic value, and eliminates the architectural challenges 

and cost of locating ducts in conditioned space. Separate challenges may exist in identifying 

space for distributed fan coils and running multiple condensate drains in units used for cooling. 

Properly designed hydronic systems can also serve very small heating loads. 

The use of thermal storage such as through water storage tanks or radiant slab distribution allow 

the heat pump to operate under more favorable full load conditions producing additional energy 

savings. Preliminary results from research completed under another Building America technical 

report (German, 2011b) showed energy savings and efficiency improvements through precooling 

the building slab in the summer by running the heat pump between 1:00 a.m.-6:00 a.m. and 

letting the building coast during the day, shifting air conditioning operation to periods when it is 

cooler outside. Systems that incorporate thermal storage (water storage or radiant floors) provide 

added value in locations that have time-of-use rates.

TRNSYS simulations predicted 16%-27% energy savings in the four climate zones evaluated 

relative to traditional ducted systems, using conservative performance parameters for the air-towater heat pump. The highest savings are associated with heating dominated climates. The 

model also indicated an improvement in distribution efficiency averaging 16%. TRNSYS 

predicted energy savings of 55% when the hydronic system is coupled to an inverter driven 

Altherma heat pump, but the cost of this equipment is considerably higher (about $8,000 - 

 

8 Installed costs are not total installed costs but only consider components that differ between the two 

system types.

Base Case Ductless

Sacramento $ 2,242 $ 4,481 $ (2,238) $ 322 $ 161 

Houston $ 2,242 $ 4,667 $ (2,424) $ 170 $ (4)

Phoenix $ 2,242 $ 4,667 $ (2,424) $ 324 $ 149 

Denver $ 2,242 $ 4,587 $ (2,344) $ 447 $ 279 

Climate Zone Estimated Installed Capital 

Cost 

Savings

Annual 

Utility Cost 

Savings

Annual 

Average 

Cash 
17

$9,000 incremental). The Altherma can also provide efficient heat pump water heating, the cost 

advantage of which is not evaluated in this report.

Two field monitoring projects that incorporate radiant floor distribution are underway. 

Additional field studies are needed to evaluate forced air distribution with no or minimal ducting 

to validate the TRNSYS data. Field monitoring data is also needed to develop realistic 

performance maps for air-to-water heat pumps that are based on entering water temperature, 

outdoor temperature, and part load for radiant and forced air terminal distribution systems.

The predominant barriers to widespread commercialization are the cost and availability of air-towater heat pumps (as well as ground-coupled water-to-water heat pumps) and particularly small 

forced air terminal units. There are no technological barriers to the development of either, and 

increased market demand would prompt the introduction of new and lower cost products. 

Stakeholders such as manufacturers of heat pumps, fan coils, hydronic components, PEX tubing, 

and the home building industry should be informed of this opportunity. A minor barrier is the 

differentiation of services between plumbing and mechanical contractors, who would be required 

to work together in ways that may be unfamiliar in certain regions of the United States.
18

Appendix 

Load Calculations and Equipment Performance Assumptions

Loads and airflow requirements were completed for each of the four climates using the Manual J 

and D components of Right-Suite Universal. The base case assumes ducts in a vented attic with 

moderate leakage (≈20%). Table A-1 lists results of these calculations and other assumptions 

used in the base case.

Table A-1. Base Case Load Calculations, Airflow, and Fan Power Assumptions

Base Case with ducts in attic Sacramento Houston Phoenix Denver

Heating Load (Btuh) 39,917 42,767 37,922 47,112

Sensible Cooling Load (Btuh) 32,887 33,612 51,178 22,891

Latent Cooling Load (Btuh) 920 6,793 0 0

Total Airflow (cfm) 1,596 1,597 2,622 1,025

Fan Power(W) 581 581 954 373

Living Flow (CFM) 263 263 435 188

Living Duct Diameter (in) 10 10 14 10

Kitchen (cfm) 403 403 660 248

Kitchen Duct Diameter (in) 10 10 14 9

Bedrooms (cfm) 489 489 803 308

Bedroom Duct Diameter (in) 10 10 12 10

Master Bed (cfm) 441 442 724 281

Master Duct Diameter (in) 12 12 16 10

To develop the air-to-air heat pump model, the equipment capacity, heating and cooling EIR, and

DOE-2 performance coefficients were entered into TRNSYS. The DOE-2 coefficients for a 

SEER 13, 7.7 HSPF heat pump were taken from BEopt option editor. Equipment performance 

parameters used are listed in Table A-2. 

Table A-2: Base Case Heat Pump Performance Parameters

Equipment Type HP

Nominal SEER Value 13

EER 11.07

Nominal Sens. Heat Ratio 0.726

Nominal Cooling EIR 0.2718

Nominal HSPF 7.7

Nominal COP 3.28

Nominal Heating EIR 0.3202

For the ductless hydronic case, the building loads were recalculated assuming no duct losses

(piping losses are accounted for in the TRNSYS model). The modified loads were used to size 

the terminal units for each zone. 
19

Ductless Hydronic System Assumptions

The ductless hydronic system modeled had three zones with MagicAire type units serving the 

Kitchen/Living, Master Suite, and Bedrooms. Even though each zone had an exclusive fan 

delivery system, the operation was controlled by a single thermostat, located in the living room. 

The fan power was assumed constant during operation. Table A-3 lists system loads and 

specifications.

Table A-3. Ductless Hydronic System Loads and Specifications

Sacramento Houston Phoenix Denver

Heating Load (Btuh) 32,000 33,404 25,343 38,895

HP rated heating Power (W) 2,859 2,985 2,265 3,475

Sensible Cooling Load (Btuh) 21,724 22,099 29,359 16,434

Latent Cooling Load (Btuh) 706 4190 0 0

HP rated Cooling Power (W) 2,040 2,390 2,670 1,494

Total Hydronic Pump Flow (gpm) 5.32 6.54 6.58 5.94

3 Zone Scenario

Living/Kitchen

MagicAire System CHA04 CHA06 CHA06 CHA06

Airflow 400 500 500 400

% air delivered to Kitchen vs. Living 61% 61% 61% 61%

Fan Power (W) (0.365 Watts/cfm) 146 182.5 182.5 146

Hydronic Flow 2.08 2.6 2.73 2.6

% of Total Hydronic Flow 39% 40% 41% 44%

Pipe Diameter 3/4 3/4 3/4 3/4

Bedrooms 

MagicAire System CHA02 CHA04 CHA04 CHA04

Airflow 265 400 400 300

Fan Power (W) (0.365 Watts/cfm) 97 146 146 110

Hydronic Flow 1.9 2.04 1.95 1.9

% of Total Hydronic Flow 36% 31% 30% 32%

Pipe Diameter 3/4 3/4 3/4 3/4

Master Suite

MagicAire System CHA02 CHA04 CHA04 CHA02

Airflow 200 300 300 265

Fan Power (W) (0.365 Watts/cfm) 73 109.5 109.5 96.725

Hydronic Flow 1.34 1.9 1.9 1.44

% of Total Hydronic Flow 25% 29% 29% 24%

Pipe Diameter 1/2 3/4 3/4 1/2
20

Hydronic System Water Flow, Pipe, and Pump Sizing

Design flow rates were based on a 15oF delta-T for heating and a 10oF delta-T for cooling. The 

hydronic piping was assumed to be cross-linked polyethylene (PEX). The Hazen-Williams 

method was used to calculate friction loss, and pipes were sized to limit total friction head loss to 

4.5 feet of water per 100’ of pipe at design flow rates. Pipe lengths were approximated from the 

plan and based on the storage tank located inside the garage and the heat pump being located 

outside the garage near the storage tank (see Figure 9). The pump was assumed to draw 45 Watts 

rated at a rated flow of 6.5 gpm. The equation below describes how fractional power 







 P 

PRated

varies as a function of fractional mass flow 







 m 

mRated according to a 2nd order polynomial. The 

coefficients were derived using performance data from a Grundfos Alpha pump.

P

PRated

= 









 ˉ0.0099 + ˉ0.0317 







 m 

mRated

+ 1.06722 







 m 

mRated

2

∙

Air-to-Water Heat Pump Performance Maps

Data provided by Daikin for the Altherma heat pump were used to develop the performance 

maps shown in Figures A-1 through A-4.

Figure A-1. Fractional heating capacity of Daikin Altherma as a function of entering water 

temperature for various outdoor temperature conditions
21

Figure A-2. Fractional heating power of Daikin Altherma as a function of entering water 

temperature for various outdoor temperature conditions

Figure A-3. Fractional cooling capacity of Daikin Altherma as a function of entering water 

temperature for various outdoor temperature conditions
22

Figure A-4. Fractional cooling power of Daikin Altherma as a function of entering water 

temperature for various outdoor temperature conditions

System Costs

Detailed system costing is described in Table A-4 and Table A-5.

Table A-4. Base Case System Costs (all climates)

Table A-5: Hydronic System Costs

HP Air Handler & coil $872

Diffusers $49

Return Grilles $19

Ducts $1,302

TOTAL $2,242

Sacramento Houston Phoenix Denver

Water-Air HP (incremental) $ 1,200 $ 1,200 $ 1,200 $ 1,200

Living Zone Fan Coil $ 795 $ 848 $ 848 $ 848

Bedrooms Fan Coil $ 742 $ 795 $ 795 $ 795

Master Suite Fan Coil $ 742 $ 795 $ 795 $ 742

Piping $ 276 $ 303 $ 303 $ 276

Condensate Drain (incremental) $ 94 $ 94 $ 94 $ 94

Manifolds $ 53 $ 53 $ 53 $ 53

Diffusers $ 23 $ 23 $ 23 $ 23

Return Grille $ 5 $ 5 $ 5 $ 5

Pumps, heat pump & zone $ 371 $ 371 $ 371 $ 371

Air separator $ 103 $ 103 $ 103 $ 103

Expansion tank $ 29 $ 29 $ 29 $ 29

Switching relay $ 48 $ 48 $ 48 $ 48

Total $ 4,481 $ 4,667 $ 4,667 $ 4,587
23 

References 

ACCA Manual S. (1995). Manual S: Residential Equipment Selection. Air Conditioning 

Contractors of America. Arlington, VA.

ASHRAE Standard 152. (2004). Method of Test for Determining the Design and Seasonal 

Efficiencies of Residential Thermal Distribution Systems. American Society of Heating, 

Refrigeration, and Air Conditioning Engineers. Atlanta, GA. 

ASHRAE Handbook – Systems and Equipment. (2008). Chapter 6 (Panel Heating and Cooling), 

Chapter 12 (Hydronic Heating and Cooling), and Chapter 35 (Hydronic Distribution and 

Radiators). American Society of Heating, Refrigeration, and Air Conditioning Engineers. 

Atlanta, GA.

Bean, R., Olesen, B.W., Kim, K.W. (2010). “History of Radiant Heating & Cooling Systems – 

Part 2.” ASHRAE Journal. February 2010. 

DOE. (2010). “Building America House Simulation Protocols” DOE/GO-102010-3141: Building 

Technologies Program. U.S. Department of Energy 

CEC (2012). “Final Evaluation Report - Proposed Compliance Option for Altherma Air-to-Water 

Source Heat Pump for the Residential Efficiency Standards.” March 2012. California Energy 

Commission report number CEC‐400‐2011‐010‐SF. 

German, A. et al. (2011a). “Technical Report: S.E.E.D. Home.” Building America draft project 

report completed under the Alliance for Residential Building Innovation (ARBI). To be available 

electronically at http://www.osti.gov/bridge. 2012. 

German, A. et al. (2011b). “Technical Report: Air-to-Water Heat Pumps with Mixed Mode 

Delivery.” Building America draft project report completed under the Alliance for Residential 

Building Innovation (ARBI). To be available electronically at http://www.osti.gov/bridge. 2012. 

Hourahan, G.C., (2003). “Sizing and Selecting Equipment for Proper Humidity Control.” ARI 

Magazine. Spring 2003. pp 24-34. 

Olesen, B.W. (2008). “Radiant Floor Cooling Systems.” ASHRAE Journal, September 2008. pp 

16-22. 

Siegal, J.A., McWilliams, J., Walker, I.S. (2002). “Comparison Between Predicted Duct 

Effectiveness from Proposed ASHRAE Standard 152P and Measured Field Data for Residential 

Forced Air Cooling Systems.” LBNL-50008. Lawrence Berkeley National Laboratory. Be 

Siegenthaller, J. (2011). Modern Hydronic Heating, 3rd Edition. Delmar Publishers, Albany NY. 

Siegenthaller, J. (2010) “State of the Art Hydronics: Where Are We Headed?” ACI Energy Star 

Summit, Denver, CO. 
24

Smith, V; Baskin, E. (2003). Residential Hydronic Radiant Cooling and Heating Assessment. 

California Energy Commission. P-500-03-096-A14.

Springer, D., et al. (2007). Comparative Performance of Four Prototype Mechanical Systems in 

a Desert Climate. ASHRAE Transactions Vol. 114, Part 2. 

Vineyard, E.A., Linkous, R.L. (2000). “Field Evaluation of a Residential Hydronic Distribution 

System in the Cooling Mode.” ASHRAE Transactions vol. 106, pt. 2.
DOE/GO-102012-3629 ▪ July 2012

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.