Combined Space and 

Water Heating: Next Steps 

to Improved Performance

B. Schoenbauer, D. Bohac, and P. Huelman

NorthernSTAR

July 2016
ii

NOTICE

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iii

Combined Space and Water Heating:

Next Steps to Improved Performance

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: 

B. Schoenbauer, D. Bohac, and P. Huelman

NorthernSTAR Building America Partnership

2004 Folwell Avenue

Minneapolis, MN 55108

NREL Technical Monitor: Stacy Rothgeb

Prepared under Subcontract No. KNDJ-0-40338-04

July 2016
iv

The work presented in this report does not represent 

performance of any product relative to regulated 

minimum efficiency requirements.

The laboratory and/or field sites used for this work are 

not certified rating test facilities. The conditions and 

methods under which products were characterized for 

this work differ from standard rating conditions, as 

described.

Because the methods and conditions differ, the reported 

results are not comparable to rated product performance 

and should only be used to estimate performance under 

the measured conditions.
v

Contents

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

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

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

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

1 Problem Statement............................................................................................................................... 1

1.1 Introduction..........................................................................................................................1

1.2 Background..........................................................................................................................1

1.3 Relevance to Building America’s Goals..............................................................................2

1.4 Cost-Effectiveness ...............................................................................................................3

1.5 Trade-Offs and Other Benefits.............................................................................................3

2 Experiment ............................................................................................................................................ 4

2.1 Research Questions..............................................................................................................4

2.2 Methodology........................................................................................................................4

3 Measurements....................................................................................................................................... 7

3.1 Equipment............................................................................................................................8

4 Analysis ............................................................................................................................................... 11

4.1 Laboratory Tests ................................................................................................................11

4.2 Field Tests..........................................................................................................................15

4.2.1 Set Point Reduction................................................................................................15

4.2.2 Domestic Hot Water Priority .................................................................................16

5 Results and Discussion ..................................................................................................................... 17

5.1 Laboratory Tests ................................................................................................................17

5.1.1 Tankless Water Heaters .........................................................................................21

5.1.2 Boilers....................................................................................................................23

5.1.3 Short Cycling .........................................................................................................24

5.1.4 Additional Benefits................................................................................................27

5.2 Field Tests..........................................................................................................................28

5.2.1 Set Point Reduction................................................................................................28

5.2.2 Domestic Hot Water Priority .................................................................................29

6 Conclusions ........................................................................................................................................ 35

References................................................................................................................................................. 37

Appendix: Solution Center Content........................................................................................................ 38
vi

List of Figures

Figure 1. Photos of test laboratory with combi system installations.................................................... 9

Figure 2. Combi system space-heating operating capacity: Eternal hybrid water heater ................ 13

Figure 3. Daily outside air temperature to space-heating load relationships for three homes ........ 14

Figure 4. Seasonal DHW use profiles ..................................................................................................... 15

Figure 5. Daily space-heating load by outdoor air temperature for a field site .................................. 16

Figure 6. Steady-state space-heating efficiency for the Vertex SWH.................................................. 17

Figure 7. Combined (space and DHW) efficiencies for both control strategies................................. 19

Figure 8. Space-heating efficiency versus return water temperature for the Rinnai TWH................ 22

Figure 9. TWH space-heating input versus output relationship .......................................................... 22

Figure 10. Water heater and boiler-based combi system plumbing configurations.......................... 23

Figure 11. Steady-state space-heating test results: Navien boiler...................................................... 24

Figure 12. Cycling operation of an SWH-based fixed input combi system using a Polaris water 

heater................................................................................................................................................... 25

Figure 13. Cycling performance for combi systems with modulating burners (Phoenix water 

heater).................................................................................................................................................. 25

Figure 14. Impacts of short space-heating events on combi efficiency ............................................. 26

Figure 15. Conditioned space temperature during peak heating season for days with high and 

low DHW use (Site 1037).................................................................................................................... 30

Figure 16. Simultaneous DHW and space-heating draws with a TWH-based combi system ........... 31

Figure 17. A small DHW draw interrupted by a space-heating draw with a TWH-based combi 

system ................................................................................................................................................. 32

Figure 18. Simultaneous DHW and space heating draws on a SWH-based combi system .............. 33

Figure 19. DHW priority operation on a combi boiler system .............................................................. 34

Unless otherwise noted, all figures were created by NorthernSTAR.

List of Tables

Table 1. Steady-State Laboratory Tests.................................................................................................... 7

Table 2. Cyclical Laboratory Tests............................................................................................................ 8

Table 3. Laboratory Instrumentation Equipment..................................................................................... 9

Table 4. Laboratory-Tested Combi Equipment ........................................................................................ 9

Table 5. Field Monitoring Test Sites........................................................................................................ 10

Table 6. Eternal Water Heater-Based Combi System Performance Bins for Operation at 5 GPM 

and 900 CFM........................................................................................................................................ 12

Table 7. Space-Heating Characteristics for Binned Analysis............................................................... 13

Table 8. Steady-State Space Heating Laboratory Results for the Vertex SWH .................................. 18

Table 9. Energy Use and Savings from Control Strategies .................................................................. 20

Table 10. Impacts of Short Space-Heating Events on Combi Efficiency ............................................ 26

Table 11. Eternal Water Heater Based Combi System Performance Bins for Operation at 5 GPM 

and 900 CFM........................................................................................................................................ 27

Table 12. Field Monitoring Sites and Combi System Equipment......................................................... 28

Table 13. Efficiency and Energy Use Improvements from Set Point Reduction ................................ 29

Table 14. Steady-State Space Heating Laboratory Results for SWHs (2.5 GPM and 900 CFM)........ 29

Unless otherwise noted, all tables were created by NorthernSTAR.
vii

Definitions

AFUE Annual fuel utilization efficiency

AHU Air handling unit

BEoptTM Building Energy Optimization software

CFM Cubic feet per minute

DHW Domestic hot water

GPD Gallons per day

GPM Gallons per minute

ND Natural draft

SWH Storage water heater

TWH Tankless water heater
viii

Executive Summary

A combined space- and water-heating (combi) system uses a high-efficiency direct-vent burner 

that eliminates safety issues associated with natural draft appliances. Past research with these 

systems shows that using condensing water heaters or boilers with hydronic air handling units

can provide both space and water heating with efficiencies of 90% or higher.

1 Improved controls 

have the potential to reduce complexity and improve upon the measured performance.

This project demonstrates that controls can significantly benefit these first-generation systems. 

Laboratory tests and daily load/performance models showed that the set point temperature reset 

control produced a 2.1%–4.3% (20–40 therms/year) savings for storage and hybrid water heater 

combi systems operated in moderate-load homes. The full modulation control showed additional 

savings over set point control (in high-load homes almost doubling the savings: 4%–5% over the 

no-control case). At the time of installation the reset control can be implemented for $200–$400,

which would provide paybacks of 6–25 years for low-load houses and 3–15 years for high-load 

houses. Full modulation implementation costs would be similar to the outdoor reset and would 

provide paybacks of 5-1⁄2–20 years for low-load houses and 2-1⁄2–10 years for high-load houses.

Field monitoring of five homes was conducted to measure the energy savings impact of set point 

reduction only. The field testing and analysis showed that a reduction in set point temperature 

from 140°–130°F resulted in an average reduction in return water temperature of 4°F. These 

homes experienced savings of 1%–4% (an average of 2.5%) on their annual energy bills with the 

reduced set point.

Although the control strategies provided energy cost savings, the greatest benefit may be a 

simpler commissioning process that more reliably results in high-efficiency operation. For 

instance, previous research by the U.S. Department of Energy’s Building America research team 

NorthernSTAR has shown that improper commissioning or inaccurate characterization of the 

design heating load can reduce combi system efficiency from approximately 90% to 75%. These 

control strategies would allow the system to automatically reduce capacity to the most efficient

operating point and still meet the load. For example, the study showed that design loads were 

typically overestimated and had additional safety factors applied. This approach resulted in 

optimized design loads that were much higher than the actual design loads; therefore, the return 

water temperature increased and the efficiency decreased. If the same systems were installed 

with a set point reset or fully modulating control, the system would operate at a lower-capacity 

stage; water temperature would decrease and efficiency would increase. The control system 

would thus reduce the number of design calculations and the amount of time that would be 

required to properly install and commission a combi system.

 1 Similar to the results reported for this research, efficiencies reported for combi systems in these previous research 

studies are not comparable to regulated product ratings and should be used only to estimate performance under the 

measured conditions.
1

1 Problem Statement 

1.1 Introduction

Better insulation and tighter envelopes reduce space-heating loads for new and existing homes.

In many cases a single heating plant can provide space heating and domestic hot water (DHW).

The systems studied are called combination (combi) systems and use high-efficiency direct-vent 

burners that eliminate the safety issues associated with natural draft (ND) appliances.

Single heating plants that supply both forced-air space heating and water heating have been in 

use for many years. Bohac et al. (1991) installed and monitored combis in small commercial and 

multifamily buildings in 1989. These systems used ND storage water heaters (SWHs) as the 

heating plants. The 1-1⁄2 years of monitored operation demonstrated that these systems could be 

reliably installed, perform without failure, and save energy. The combi systems in this project 

had annual fuel utilization efficiency (AFUE) ratings of at least 78% and replaced ND water 

heaters with efficiencies of about 50% and furnaces with AFUEs around 60%. The study found 

an average energy savings of 24%.

The U.S. Department of Energy’s Building America research team NorthernSTAR has used 

combi systems to replace natural gas forced-air distribution system furnaces and tank type water 

heaters. The combi systems consisted of a water heater or boiler heating plant teamed with a 

hydronic air handling unit (AHU) that included an AHU, a water coil, and a water pump to 

circulate water between the heating plant and the coil. The combi water heater or boiler had a 

separate circuit for DHW. Various options for DHW priority, DHW tempering, and heating plant 

temperature set point control were considered.

Previous projects characterized the installed performance of combi systems that used several 

types of condensing heating plants (Schoenbauer et al. 2014). Although the primary objective 

was to measure installed performance of combi systems, analyses were conducted before and 

after installation to estimate energy savings. Combi systems reduced natural gas consumption for 

space and water heating by 19%. This project also monitored the operational parameters and 

delivery capacities of the system. These measurements allowed for the identification of several 

potential control improvements. A discussion and evaluation of those opportunities are presented 

here.

1.2 Background

Historically, mechanical contractors have custom engineered and pieced together combi systems 

in the field. They focused on assembling functional systems and often paid little attention to 

efficiency and optimization. As high-efficiency condensing water heaters and boilers gain a 

larger share of the residential market, there is greater potential to use these systems to improve 

the efficiency of providing space heating and DHW.

Twenty years ago a combi system that used a noncondensing water heater could provide energy 

savings when it replaced an older low-efficiency furnace and water heater. Currently, in many 

parts of the country furnaces with 90%+ AFUE ratings are standard. Thomas et al. (2011) used 

laboratory testing to demonstrate that when combi systems replace mechanical equipment in 

modern homes those systems must use condensing heating plants to achieve similar or improved

energy performance.
2

Schoenbauer et al. (2014) conducted a 200-home implementation project and a 20-home detailed 

monitoring project. This project collected valuable information about combi system performance 

and occupant comfort. These authors showed that properly configured combi systems typically 

operate at an annual efficiency of 85%–92% and result in 19% average savings over ND water 

heaters and noncondensing furnaces. Careful monitoring also highlighted potential areas for 

improvement that were analyzed by this project.

1.3 Relevance to Building America’s Goals

Combi systems have the potential to significantly reduce home energy use. A properly installed 

combi system can provide approximately 20% natural gas savings compared to an 80% AFUE 

furnace and a 60%2 energy factor water heater. Replacing an ND water heater with a direct-vent 

heating plant also allows the home to be more airtight without causing combustion safety issues. 

Such replacement also eliminates air infiltration through the combustion makeup air duct. These 

two features can further improve the energy performance of a home.

Combi systems are feasible for typical houses in most climate regions. In colder climates the 

application may not be possible in very large homes with poorly insulated and leaky envelopes.

Some currently available equipment can meet space-heating design loads up to 70,000 Btu/h 

while still operating in the condensing mode. Higher-capacity hydronic coils than those that are 

traditionally available would allow current heating plans to meet even higher space-heating 

loads.

Combi systems can provide high-efficiency space and water heating at a lower cost than two 

high-efficiency appliances (furnace and water heater). Previously, each installation required 

customized contractor designs, equipment selection, and commissioning to ensure the proper 

installation, sizing, and operation to achieve efficient performance. The installation and 

optimization process used by NorthernSTAR for the previous field project was summarized in the 

final report. Future work will develop this experience into a full measure guideline for design, 

installation, optimization, and verification of combi system performance. 

In addition to improving the efficiency and energy savings of combi systems, this project 

addresses the installation and design difficulties with combi systems. Improved control can 

reduce the necessity of having a system designed and manually adjusted to narrowly defined and

optimized parameters. Combi system efficiency is largely a factor of the water temperature 

returning to the heating plant from the AHU and burner cycling characteristics. Lower return 

temperatures and longer cycles produce higher system efficiency. All current AHUs have a 

constant airflow rate and a constant water circulation flow rate for the heating mode. To achieve 

the best performance, these flow rates must be adjusted to meet the house design heating load 

and minimize the return water temperature. Variable air and water flow rate control would allow 

a single AHU to provide more efficient performance over a wider range of heating loads and 

should eliminate time-consuming manual adjustments to the water flow rate. The controls 

improve efficiency by allowing the system to operate at a lower average return water temperature 

and longer cycles.

 2 For a gas-fired water heater with a rated storage volume of 40 gal.
3

1.4 Cost-Effectiveness

Installing a high-efficiency combi system may cost less than installing a similarly efficient

separate furnace and water heater. In a retrofit application in Minnesota, a homeowner can 

expect to pay approximately $4,250 for a high-efficiency (90%–98% AFUE) furnace and $5,300 

for a high-efficiency (0.80–0.95 energy factor) SWH. As the number of high-efficiency water 

heater installations increased over the past few years, the installed cost dropped dramatically. In 

the Minneapolis area, contractors with limited combi-systems experience currently bid a highefficiency system for $8,200—on average with a typical range of $6,500–$10,000.

1.5 Trade-Offs and Other Benefits

Combi systems have several secondary benefits. The system replaces a separate furnace and 

water heater with a single boiler or water heater. This reduces the number of gas lines and 

exhaust vents from two to one and often reduces the equipment footprint. A single highefficiency burner also has combustion safety and venting benefits. The high-efficiency combi 

heating plants have power-vent or sealed combustion burners that eliminate combustion spillage 

concerns for tight houses.
4

2 Experiment 

2.1 Research Questions

The test laboratory and field research addressed the following research questions:

• What is the effect of improved control methods on combi system energy efficiency? 

• What are the cost and comfort impacts of several improved control strategies?

• Can improved control strategies limit the amount of site-specific design and 

commissioning necessary for high-performance combi systems?

• Do combi systems have capacity issues that can be solved through improved controls?

2.2 Methodology

This project builds on past research that shows combi systems that use condensing water heaters 

or boilers with hydronic AHUs can provide space and water heating with efficiencies of 90% or 

higher. Condensing combi systems are still relatively new and complex; they often require onsite engineering and optimization to achieve desired performance. Improved controls have the 

potential to reduce some of this installation and operational complexity and improve upon the 

measured performance. The control measures evaluated in the laboratory include temperature set 

point control (outdoor reset and/or turndown after heating season), space-heating modulation 

(both water flow and airflow) and DHW priority. The benefits to installation, sizing, and 

optimization were characterized for all three control schemes. Set point control and spaceheating modulation were analyzed for efficiency and energy-saving improvements through 

laboratory and field testing. 

Set point reset. The supply water set point temperature reset by outdoor air temperature control 

method has been widely used for commercial and residential boilers. This control method has 

several potential benefits for combi systems as well:

• Reducing the set point temperature in the winter and shoulder seasons increases the 

system’s thermal efficiency. Laboratory tests at a range of water temperature set points 

were used to characterize the space-heating efficiency and output impacts. 

• Lower set points reduce the standby losses of combi systems with internal water storage 

capacity.

• The reset control increases system runtimes. As the reset control reduces the set point 

temperature the system output is also reduced; this mirrors the reduction in space-heating 

load as outdoor temperatures rise. Set point temperature reductions should not reduce the 

system output capacity below the space-heating load of the home.

Flow rate modulation. Airflow and water flow rate modulation can also improve the 

performance and efficiency of combi systems. With current equipment airflow and water flow 

can only be varied manually and typically remain fixed throughout the product’s lifetime. Full 

modulation or multiple step modulation allows the combi system to operate at an optimized 

output rate that depends on demand. This approach has several potential benefits:
5

• Lower output rates increase runtimes, which reduces the impacts of short cycling and 

improves comfort.

• Laboratory testing indicates that lower output rates (achieved by reducing temperature, 

water flow rate, or airflow rate) may yield lower return water temperatures, thus

improving efficiency.

• Multiple operating capacities can enable downsizing equipment or using the same 

equipment in higher load homes. Large air coils are required to achieve high efficiency at 

peak loads. However, if a combi system with a smaller coil capacity and flow modulation 

is optimized for less than peak load, the water circulation and airflow rates can be ramped 

up only when necessary to meet the peak loads. The system efficiency will be lower 

during peak periods, but it will be higher for the more moderate loads that occur for the 

greater portion of the heating season. 

• This process can also reduce some of the site-specific optimization and engineering.

Modulation control allows the combi system to change the output based on the home’s 

need, so the contractor does not need to optimize the system during installation.

Cyclical and steady-state laboratory test results were used to evaluate and optimize these control 

methods. Cyclical tests quantify the impact of short cycling and determine the cycle length 

necessary to avoid the efficiency impacts. Steady-state testing at various water flows and 

airflows was used to map performance over a range of operating conditions.

Both space-heating control strategies were designed to match the daily space-heating load on the 

system. For the analysis, the outdoor air temperature was used to determine the daily spaceheating load, which was used to assign each day to the highest-efficiency operating stage with an 

output capacity higher than the daily load plus a safety factor. This strategy can be implemented 

in several ways. For the set point reset, the design load can be used to set the combi set point 

water temperature with respect to the outdoor air temperature, similar to the strategy used for 

condensing boiler applications. Space-heating runtime can also be used to increase the set point 

temperature. If the space-heating system runs longer than a specified time (20 minutes, for 

example) without meeting the thermostat setting, the set point temperature will increase. These 

control implementation methods can be used for the flow modulation control as well, using 

algorithms developed based on the operation of a specific system.

DHW priority. The third control strategy considered was DHW priority. This control prevents 

any space heating when DHW is active. Field data from Schoenbauer et al. (2013) were used to 

analyze this control method. The potential benefits were assessed in three key areas.

1. What is the impact on the conditioned temperature from locking out space heating? 

NorthernSTAR’s previous study had five sites with boiler systems that had DHW priority 

installed. Data from these homes were analyzed to determine the temperature variance 

from DHW priority. Data from all homes were used to determine the frequency of long 

consistent DHW use that would likely impact the temperature in the conditioned space.

2. Can DHW priority improve DHW supply water temperature consistency during 

simultaneous events? Water temperatures in homes with and without DHW priority were 

compared when calls for space heat were initiated during active DHW draws.
6

3. DHW priority allows for systems to be sized for independent space and DHW loads 

instead of the sum of the two loads. Combi systems sized to meet the combined space and 

DHW loads typically operate at capacities much lower than the combined design 

capacity. Sizing systems for the two loads separately results in smaller capacity systems,

which decreases cycling frequency and allows for better optimization.
7

3 Measurements

The laboratory testing part of the project characterized the performance of currently available 

components and developed recommendations for optimized combi system controls. A series of 

tests on the fully assembled systems was used to evaluate the control methods discussed 

previously.

• Steady state at a range of parameters (Table 1):

o Space heating flow rate: 1–5 gallons per minute (GPM)

o Airflow rate: 500–1000 cubic feet per minute (CFM)

o Heating plant set point temperature 120°–140°F

• Cyclical performance at typical operating parameters (Table 2):

o Various part-load conditions

 AFUE and Canadian Standard P.9 on/off cycle lengths

 On/off cycles lengths determined from field data from previous studies.

Table 1. Steady-State Laboratory Tests

Flow Rate 

(GPM)

Airflow 

(CFM) Tset

Return Air 

Temperature

(°F)

Ambient 

Temperature

(°F)

Test 1 2.5 900 140 68 70

Test 2 2.5 900 135 68 70

Test 3 2.5 900 130 68 70

Test 4 2.5 900 125 68 70

Test 5 2.5 900 120 68 70

Test 6 5 900 130 68 70

Test 7 4 900 130 68 70

Test 8 3 900 130 68 70

Test 9 2 900 130 68 70

Test 10 1 900 130 68 70

Test 11 2.5 800 130 68 70

Test 12 2.5 700 130 68 70

Test 13 2.5 600 130 68 70

Test 14 2.5 500 130 68 70

Extra 1 0.5 500 120 68 70

Extra 2 1 500 120 68 70

Extra 3 2 500 120 68 70
8

Table 2. Cyclical Laboratory Tests

Cycle Pattern

Test 1 Test 2 Test 3 Test 4 Test 5 Test 6

ON 1 min 5 min 10 min 10 min 30 min 45 min

OFF 10 min 10 min 10 min 50 min 30 min 15 min

ON 1 min 5 min 10 min 10 min 30 min 45 min

OFF 10 min 10 min 10 min 50 min 30 min 15 min

ON 1 min 5 min 10 min 10 min 30 min 45 min

OFF 10 min 10 min 10 min 50 min 30 min 15 min

ON 1 min 5 min 10 min

OFF 10 min 10 min 10 min

ON 1 min 5 min 10 min

OFF 10 min 10 min 10 min

ON 1 min 5 min 10 min

OFF 10 min 10 min 10 min

Note: Cyclical tests were conducted at 2.5 GPM, 900 CFM, and 130°F.

Field testing was used to supplement the laboratory testing where applicable. This project 

monitored five homes with combi systems and monitoring equipment installed for previous 

studies. Set point temperature modulation was tested in these homes. The impact on system 

efficiency and capacity was analyzed for a one-time reduction in set point temperature. An 

analysis of the data prior to and after the set point reduction determined the potential impact on 

outdoor reset temperature controls.

3.1 Equipment

The test laboratory was equipped with a comprehensive and accurate monitoring system utilizing 

high-precision instruments (Table 3). A Campbell Scientific model CR-3000 data logger was 

programmed to measure instrument outputs and record processed data at specified intervals. A 

propagation of errors method using the uncertainties of individual instruments for typical 

operating conditions was used to estimate an uncertainty of 2% for the calculated hot water 

energy output and an uncertainty of 2.5% for the system efficiency. In the laboratory, three 

identical sets of monitoring systems were used for tests on seven different heating plants using a 

single AHU (Table 4). Figure 1 includes photos of some of the systems installed in the 

laboratory.
9

Table 3. Laboratory Instrumentation Equipment

Measurement Sensor Type Resolution Precision Range

Water Volume 

Flow Rate

Nutating disk flow 

meter 198.4 pulses/gal 2% of reading 0.5–25 GPM*

Natural Gas

Volume

Diaphragm meter, 

with pulse output 40 pulses/ft3 0.3% of reading 0–250 CFM

Water 

Temperatures

Matched pair of 

immersion RTDs 0.002°F at 140°F 1/10 DIN: 0.03°F at 32°F –148° to 752°F

Electric

Energy Watt transducer 0.02 Watts 0.2% of reading 0–1,000 watts

Air 

Temperature Thermocouple array 0.03°F at 140°F Greater of 1.8°F and 

0.75% of reading –454° to 725°F

* The meter measures flow rates lower than 0.5 GPM, but the precision decreases for flow rates outside the specified 

range.

Table 4. Laboratory-Tested Combi Equipment

Equipment Type Manufacturer Model

Condensing SWH AO Smith Vertex

Condensing SWH American Polaris

Condensing SWH HTP Phoenix

Condensing TWH* Rinnai 98Lsi

Hybrid Condensing TWH Grand Hall Eternal

Condensing Boiler Navien Combi Boiler

Condensing Boiler Rinnai Q175C

Hydronic AHU Enerzone XAH 70VS-X13-4R-PT

* Tankless water heater

Figure 1. Photos of test laboratory with combi system installations
10

The combi system field monitoring package included the same energy input and energy output 

instruments as those used in the test laboratory (Table 3). The impact of the control change on 

energy use was computed from a comparison of the input versus output regressions analysis. 

This is described in the following section. Table 5 describes the combi equipment and loads of 

the five test sites.

Table 5. Field Monitoring Test Sites

Site 

Number

Combi Appliance Calc Load*

(Btu/h)

Number of 

Showers

Number 

Heating Plant AHU of People

1027 Polaris Enerzone 38,774 2 4

1031 Polaris Enerzone 38,260 2 1

1049 Eternal Enerzone 36,277 2 5

1056 Phoenix Enerzone 26,112 1 1

1070 Eternal Enerzone 29,111 1 1

Note: Calc Load was determined from the field measured data
11

4 Analysis

4.1 Laboratory Tests

Laboratory testing used test data from a previous NorthernSTAR project (Schoenbauer et al. 

2012) and new test data. New data consisted of a series of steady-state and cyclical tests (Table 1

and Table 2). The system energy input, output, and efficiency were computed for each test from 

direct measurements of water flow rates, water temperatures, and gas use. The laboratory test 

data were used with results from the NorthernSTAR field monitoring project (Schoenbauer et al. 

2014) to compute the seasonal system efficiency and annual energy use for four control 

strategies. In general, empirical field data were used to create seasonally dependent space- and 

water-heating load models. Laboratory data were used to calculate the system performance and 

energy use for the model loads over a full year.

Daily average method. Daily average load and system performance relationships were used to 

estimate the annual energy use and efficiency for combi systems using four control methods. The 

first two strategies used a single space-heating operating point (fixed airflow rate, water flow 

rate, and set point temperature) for a 140°F set point temperature (SP 140); for the second the set 

point was optimized for the design space-heating condition (optimized SP). These were

compared to combi performance with a set point temperature reset control (SP reset) and a full 

modulating control that varied the set point temperature, water flow, and airflow rates (full 

modulation). A four-step process was used to estimate the annual efficiency of each control 

operation:

1. Characterize the combi system capacity and efficiency.

2. Calculate the daily space-heating energy use based on the outside air temperature and 

heating load relationship and the combi system performance.

3. Calculate the daily DHW energy use based on the DHW use and the temperature set 

point required for space heating.

4. Sum the annual energy use and compute annual efficiencies. 

A more detailed description of each step follows:

Characterize the system operation. Return water temperature had the greatest impact on 

system efficiency (Schoenbauer et al. 2012). The characteristics of the AHU, both design (coil 

size and effectiveness) and operation (flow rates and water temperatures), determined the return 

water temperature. One high-performance AHU was selected for use in this study. The AHU was 

tested previously in the laboratory and used with every field installation. The large coil size and 

good component design led to a very high heat transfer effectiveness. Using an AHU with a 

smaller output (i.e., smaller water temperature drop) or choosing nonoptimized flow rates would 

have resulted in lower system efficiencies.

For the previous field monitoring project the combi systems were installed with a constant set 

point temperature of 140°F, constant hydronic AHU airflow rate, and constant water flow rate 

adjusted to achieve a return water temperature no higher than 105°F (typically 2–3 GPM). The 

operating characteristics for the base case of this analysis were selected to mimic that setup: 

combi heating plant set point = 140°F, airflow rate = 900 CFM, and water flow rate = 2.5 GPM. 
12

The second case also used constant heating plant set point temperature, airflow rate (900 CFM), 

and water flow rate (2.5 GPM). The difference was that the heating plant set point temperature 

was optimized for the design space-heating load. The set point temperature was specified to be 

the lowest value (and highest efficiency) that would provide the required space-heating output 

for outdoor air design temperature conditions. 

The other two strategies used variable control methods. The supply water set point reset control 

method allowed the combi system to operate at various set point temperatures. The combi 

systems were optimized for the design load. The space-heating performance conditions were 

determined by changing only the set point temperature. Table 6 shows the bins for an AHU

water flow rate of 5 GPM and airflow rate of 900 CFM for the Eternal water heater.

Table 6. Eternal Water Heater-Based Combi System

Performance Bins for Operation at 5 GPM and 900 CFM

Space-Heating 

Output

(Btu/h)

Supply Water 

Temperature

(°F)

Return Water 

Temperature

(°F)

System 

Efficiency

(%)

68,810 140 113 86.6

63,437 135 110 90.2

59.625 130 106 91.8

55,032 125 103 92.3

50,439 120 100 92.8

The full modulation control method allowed the combi system to operate at different airflow and 

water flow rates as well as water set point temperatures. A matrix of heating system performance 

was created to represent the expected range of combi system operating conditions. Laboratory 

tests were used to determine the combi system output capacity and efficiency for each condition 

or set of operating parameters (set point temperature, water flow rate, and airflow rate), which 

determined the output capacity and the system efficiency. Varying all three parameters increased 

the flexibility of the system by allowing for a wider range of AHU capacities and increased 

heating plant efficiency.

Figure 2 shows the operating conditions for the hybrid water heater-based combi system. With 

the SP 140 control method, the system had only one operating condition over the full range of 

operation. The reset SP and full modulation control strategies provided multiple operating 

conditions with increased efficiency as the operating capacity was reduced. For both control 

methods the analysis methodology used a fixed number of conditions to reduce the number of 

laboratory test conditions. In practice, set point temperature, water flow rate, and airflow rate 

could be continuously variable. This variability would increase the flexibility of the system, 

improving the fit between system capacity and daily load. However, the fixed conditions selected 

for this analysis provide a good representation of performance, because the change in efficiency 

is only 0.5% for 5°F set point changes from 130° to 120°F and 1.6% from 135° to 130°F.
13

Figure 2. Combi system space-heating operating capacity: Eternal hybrid water heater

Calculate space-heating use. The daily space-heating input was calculated by matching the 

combi system performance stages (Step 1) to the daily space-heating load. The home’s spaceheating load was computed from a linear relationship with outside temperature. Results from a 

utility bill study were used to determine space-heating loads for three characteristic categories of 

homes in a heating-dominated climate (Minneapolis, Minnesota). This study was used solely to 

select heating load profiles representative in Minnesota. Utility billing data were analyzed for 63 

combi system installations and the results were used to determine the design loads. Low-, 

medium-, and high-load homes were defined. The design heating load, design outdoor 

temperature, and balance point temperature define the relationship between the outside 

temperature and the space-heating load (Table 7 and Table 4). The homes with design loads 

lower than 30,000 Btu/h (the 15th percentile) were used to define the low-load category. Those 

had an average design load of 25,000 Btu/h. The homes with loads higher than 40,000 (the 80th 

percentile) had an average design load of 49,300, which was used as the high-load case. Sixtyfive percent of the homes analyzed had design loads of 30,000–40,000 Btu/h. The average load 

of 30,900 Btu/h for these homes was used as the medium load. The average balance point 

temperature for all the homes (63°F) was used for all three load levels, and the outdoor air design 

load temperature was set equal to the value specified for Minneapolis, Minnesota (–18°F).

Table 7. Space-Heating Characteristics for Binned Analysis

House Category

Design SpaceHeating Load

(Btu/h)

Design Outdoor 

Air Temperature 

(°F)

Balance Point 

Temperature

(°F)

Low Load 25,000 –18 63

Medium Load 34,900 –18 63

High Load 49,300 –18 63
14

Figure 3. Daily outside air temperature to space-heating load relationships for three homes

Each day was matched to a combi system stage as defined in Step 1. For each day the spaceheating load was multiplied by 1.20 to account for setback recovery and a safety factor. The 

operating mode that provided the highest system efficiency while meeting this need was assigned 

to the day. The combi system supply water set point was required to be at least 120°F as a 

minimum value for DHW use. Laboratory cycling and steady-state test data were used to 

determine the total daily output energy and efficiency for each day.

Calculate DHW use. The DHW load was treated separately from the space-heating load. Daily 

DHW use in each home has significant variance, typically ranging from 0 to twice the daily 

average. Seasonal DHW use profiles were developed in the previous field study to show average 

use and seasonality impacts (Schoenbauer et al. 2014). The profiles from 19 homes were divided 

into high (80 gallons per day [GPD] average), medium (35 GPD average), and low (12 GPD

average) use (Figure 4). The median use home of each group was selected for the analysis. The 

DHW energy use was estimated using field data collected from a high-efficiency water heating 

field monitoring project (Schoenbauer et al. 2010) and the standby testing previously conducted 

on combi systems in the laboratory (Schoenbauer et al. 2012).

Sum the annual use. Annual performance was determined by summing the total energy output 

(space heating and DHW) and input for each day. The annual efficiency was computed from the 

ratio of the annual energy output to input.
15

Figure 4. Seasonal DHW use profiles

4.2 Field Tests

4.2.1 Set Point Reduction

The field test part of the project compared the efficiencies of combi systems as they were 

installed in the previous field study to those of the same systems with a reduced set point 

temperature. The annual energy use and efficiencies were computed from the sum of the 

modeled energy load and use for each day in the year (Schoenbauer et al. 2012). Regressions of 

monitored daily average data established a linear balance point relationship between the spaceheating load and the outside temperature (Figure 5). The seasonal variation in the inlet water 

temperature and the relationship between the inlet water temperature and DHW load were used 

to determine the DHW load for each day of the year (Figure 4). 

These load curves were used with the outside air temperature from the Minneapolis/St. Paul 

Typical Meteorological Year 3 weather data set to calculate the space- and water-heating loads

for each day. Regressions of the monitored daily average data established the linear relationship 

between combi system energy input and output. The loads were assumed to be equal to the sum 

of the system energy output. Loads and input to output relationships were split into groups: 

DHW only and space heating and DHW combined. The previous study found that for days with 

both DHW and space-heating loads, the input/output relationship was not significantly affected 

by the fraction of the load that was due to DHW or space heating. Consequently, the sum of the 

DHW and space-heating loads was used with the daily linear input output relationships to 

determine the daily energy use. Daily energy use and loads were summed to determine annual 

use, consumption, and efficiency.
16

Figure 5. Daily space-heating load by outdoor air temperature for a field site

After monitoring for the previous project was complete, the water temperature set point was 

reduced for five of the test sites. Data collected at these sites were used to create new system 

energy input/output performance relationships; these were applied to the same space-heating and 

DHW load profiles. The new efficiency and energy use data were compared to the system as 

operated during the previous project to determine the savings potential of set point temperature 

reduction.

4.2.2 Domestic Hot Water Priority

DHW priority controls the operation of the combi system during times when the system has 

simultaneous DHW and space-heating loads. This control strategy gives priority to DHW loads.

For example, if hot water is being used for a shower when the house thermostat calls for heat, a

flow control valve in the combi system prevents (or for some systems limits) the space-heating 

flow rate system until the DHW flow is no longer sensed. Additionally, if a space-heating call is 

being met and a hot water fixture is turned on, the flow controllers stop (or reduce) flow to the 

hydronic AHU and prioritize the DHW.

The DHW priority control was used by the five boiler-based combi systems installed in the 

previous field monitoring project. Data from that project were used to determine whether DHW 

priority can improve occupant comfort and reduce peak loads on the system.

Occupant comfort was assessed by analyzing the delivered water and air temperatures during 

simultaneous DHW and space-heating events for systems with and without DHW priority.

Reductions and increases in delivered air and water temperatures were measured as simultaneous 

use starts or stops. Comparing the firing rates of systems with and without DHW priority showed 

the impact of this control on the separation of DHW and space-heating loads on sizing criteria.
17

5 Results and Discussion

5.1 Laboratory Tests

The increased efficiency and energy savings were computed using the daily average analysis 

results for the optimized set point (optimized SP), set point reset (SP reset) and full modulation

control strategies compared to a combi system operating with a set point of 140°F (SP 140). 

These strategies improved the efficiencies of combi systems with the SWHs and hybrid water 

heaters, but were less effective for the TWH- and boiler-based systems.

The hydronic AHU return water temperature had the greatest impact on space-heating efficiency. 

The control methods achieved lower return temperatures and higher annual operating efficiencies 

by reducing the set point temperature, increasing the airflow rate, or decreasing the water flow

rate.

Figure 6 shows the relationship between return water temperature and space-heating efficiency 

for the Vertex SWH tested at various water temperatures, airflow rates, and water flow rates.

There is a consistent trend of increasing efficiency with decreasing return water temperature. The 

return water temperature was a function of several other parameters (i.e., water flow, airflow, 

and set point temperature); these other parameters independently and significantly affected 

efficiency. For example, if a system were operated at two operating conditions, both of which 

had a return water temperature of 110°F, the same efficiency would be expected even if the flow 

rates were different. The laboratory test results, which are shown for the Vertex SWH in Figure 6

and Table 8, were used to select the combi system operating condition that defined the daily 

performance of the combi system.

Figure 6. Steady-state space-heating efficiency for the Vertex SWH

70 75 80 85 90 95 100 105 110 115 120

Return Water Temp

83%

84%

85%

86%

87%

88%

89%

90%

91%

92%

93%

94%

95%

96%

Spa 

915

915

915

915

915

915

915

800

700

600

915

915

700 500

915

500

Water Flow Rate (GPM)

0.500

1.000

2.000

3.000

4.000

5.000

121.24 140.00

Supply Water Te..
18

Table 8. Steady-State Space Heating Laboratory Results for the Vertex SWH

Test ID Flow Rate 

(GPM)

Airflow 

(CFM)

Set Point 

Temperature

(°F)

Return Water 

Temperature

(°F)

Output 

(Btu/h)

Efficiency

(%)

Test 1 2.5 900 140 95.3 52,736 90.3

Test 2 2.5 900 135 94.0 50,975 91.7

Test 3 2.5 900 130 92.2 47,228 93.2

Test 4 2.5 900 125 90.3 45,425 92.7

Test 5 2.5 900 120 88.8 41,607 93.4

Test 6 5 900 130 105.8 61,308 89.4

Test 7 4 900 130 101.1 56,100 90.5

Test 8 3 900 130 96.1 50,668 91.2

Test 9 2 900 130 89.3 43,338 92.6

Test 10 1 900 130 78.6 26,193 94.1

Test 11 2.5 800 130 93.1 46,166 92.8

Test 12 2.5 700 130 94.3 44,869 92.1

Test 13 2.5 600 130 95.9 43,188 91.2

Extra 1 1 500 120 80.6 21,819 95.7

Extra 2 0.5 500 120 77.5 13,524 95.2

Extra 3 1 500 120 71.4 23,385 95.7

Note: The set point temperature is not the same as the delivered water temperature. With SWHs the delivery 

temperature is typically below the set point because of the dead band on stored water temperature.

.

The operating conditions were selected for each control method to provide the maximum 

efficiency for the necessary output capacity each day. This improved annual operating 

efficiencies (Figure 7) and reduced annual energy use (Table 9) for the set point temperature (SP 

reset) and flow rate modulation (full modulation) controls. The energy savings and controls are 

compared to the fixed capacity, constant set point combi system (SP 140). The base case (SP 

140) used a water flow rate of 2.5 GPM, an airflow of 900 CFM, and a set point temperature of 

140°F. The base case was consistent with the most common operating conditions for the field 

installations. This configuration was the point at which the return water temperature from the 

AHU was lower than 105°F and the air temperature was at least 115°F—the parameters for the 

field study optimization. In addition to the base case and the control cases, a single-stage combi 

system with the optimized set point water temperature was analyzed (optimized SP). All cases 

assumed the use of the same high-performance AHU. Applying this analysis to a smallercapacity, less-expensive AHU would have increased the savings for the reset and fully 

modulating control strategies.
19

Note: These efficiencies are calculated based on laboratory testing and load data measured in the field.

Figure 7. Combined (space and DHW) efficiencies for both control strategies
20

Table 9. Energy Use and Savings from Control Strategies

Control Method

SWHs

Hybrid 

Water 

Heater

TWH

Phoenix Vertex Polaris Eternal Rinnai

Low-Load Home

Base Case—SP 140 Energy use 

(therms/year) 643 637 645 611 618

Optimized SP % saved 4.9% 3.0% 2.4% 3.4% 1.0%

SP Reset % saved 4.9% 3.0% 2.4% 3.4% 1.0%

Full Modulation % saved 5.6% 5.0% 5.7% 3.4% 1.0%

Medium-Load Home

Base Case—SP 140 Energy use 

(therms/year) 940 941 949 909 932

Optimized SP % saved 1.9% 2.7% 1.8% 3.0% 0.9%

SP Reset % saved 4.3% 2.7% 2.1% 3.0% 0.9%

Full Modulation % saved 4.8% 4.2% 5.2% 4.1% 0.9%

High-Load Home

Base Case—SP 140 Energy use 

(therms/year) 1403 1396 1394 1345 1396

Optimized SP % saved 0.0% 0.0% 0.0% 0.0% 0.0%

SP Reset % saved 4.8% 3.0% 1.8% 2.6% 1.1%

Full Modulation % saved 5.2% 3.7% 4.9% 3.1% 1.1%

The optimized SP control strategy produced greater savings for the low-load homes than for the 

high-load homes. The optimized SP control achieved 100% of the savings as that for the SP reset 

and greater than 75% of the savings as the full modulating controls in the low-load homes. In the 

medium-load homes the optimized SP control strategy produced an average of 85% of the 

savings for the SP reset control and 60% of the full modulation control. In the low- and mediumload homes the optimized SP control was often able to operate the system at or near the 

minimum set point temperature (typically 125°F or 120°F).

Further optimization from this condition resulted in only minor savings. For high-load homes the 

optimized SP control had no added savings because the system set point had to be 140°F to 

satisfy design load conditions. In these cases further control (SP reset and full modulation) were 

able to further reduce the system capacity and increase efficiency on nondesign condition days. 

For example, for a low-load home with a Vertex-based combi using an optimized SP control, the 

system always operated at a 120°F set point (Table 8 Test 5) and an efficiency of 93.4%. For the 

full modulation control the combi system would also be able to operate at lower capacity (test 

extra 1) with an efficiency of 95.7%, resulting in a small boost in efficiency at low-capacity 

operation. For a high-load home the optimized SP system would always operate at a 140°F set 

point (Table 8 Test 1) with an efficiency of 90.3%. The full modulation control allowed the 

combi system to reduce its capacity to the extra 1 operating mode and to increase the efficiency 

to 95.7%. This increased range of operation and lower efficiency in the optimized SP case 

allowed for greater savings.
21

The full modulation strategy always resulted in higher efficiencies and larger energy reductions 

than the set point control. The increased flexibility of airflow and water flow rate reductions 

allowed the system to operate at lower-capacity and higher-efficiency conditions than the 

temperature control alone. Set point temperature control allowed for reductions in space-heating 

output of 20% of the design load, which corresponded to an 8°F reduction in return water 

temperature. For the Vertex water heater, this return water temperature reduction corresponded 

to a 5.9% increase in efficiency. The full modulation control allowed for reductions in the spaceheating output capacity of up to 60% of design and an 18°F reduction in return water 

temperature, which increased efficiency up to 3.4% for the Vertex water heater.

5.1.1 Tankless Water Heaters

As previously mentioned, the control strategies were not as effective for the TWH- and boilerbased combi systems. The control strategies rely on reducing the operating output of the 

hydronic AHU as the space-heating load is reduced, which reduces the return water temperature 

and improves efficiency. The efficiency increase with reduced return water temperature was not 

as significant for the TWH- and boiler-based systems at low operating capacities. Figure 8 shows 

the impact of return water temperature on efficiency for the TWH. Efficiency for this unit peaked 

with return water temperatures of around 95°F, and the relationship between return water and 

efficiency was inconsistent at temperatures lower than 100°F. The operating points for the 

optimized combi system with constant set points (highlighted red in Figure 8) were either at or 

very near peak system efficiency, leaving little room for improvement for the SP reset and full 

modulation control methods. As shown by the TWH input to output relationship in Figure 9, at 

outputs lower than 43,000 Btu/h the TWH system had a significant change in the input output 

relationship, which corresponded to a reduced efficiency at lower output rates. A similar effect 

has been measured for TWHs installed for DHW performance, where efficiency decreased with 

firing rate (Schoenbauer et al. 2010).
22

Figure 8. Space-heating efficiency versus return water temperature for the Rinnai TWH

Figure 9. TWH space-heating input versus output relationship

75 80 85 90 95 100 105 110 115 120

Return Water Temp

76%

77%

78%

79%

80%

81%

82%

83%

84%

85%

86%

87%

88%

89%

90%

91%

Spa 

915

915

915

600

500

915

700

800

700

500

915

500

700

915

500

Water Flow Rate (GPM)

0.500

1.000

2.000

3.000

4.000

5.000

118.26 140.00

Supply Water Te..
23

5.1.2 Boilers

Combination boilers used with forced-air systems had a slightly different plumbing configuration 

than combi systems with water heaters (Figure 10). The boiler-based systems had a 

primary/secondary loop configuration. The manufacturers included the primary loop so the boiler 

could control the water flow rate and temperatures inside the boiler. If the water flow rate 

through the AHU (the secondary loop) was lower than the flow rate in the primary loop (through 

the boiler), some supply water from the boiler bypassed the AHU, which increased the return 

water temperature. The primary/secondary loop plumbing configuration overrode the benefit of 

reducing the return water temperature. This plumbing configuration impact has been observed 

for condensing boilers in other research studies (Arena 2010; Butcher 2006). Figure 11 shows 

the results from the steady-state testing. The results suggest that an optimized control strategy

might be identifiable. This strategy would not simply target the lowest possible return water 

temperature from the AHU. An ideal boiler control strategy would work with the internal 

primary loop control to achieve the lowest return water temperature in the primary loop, leading 

to a higher efficiency. This is apparently achieved from AHU water flow rates that match the 

internal boiler flow rates, which may be higher than flow rates used in a water heater-based 

system, and lower boiler set point temperatures.

Figure 10. Water heater and boiler-based combi system plumbing configurations
24

Figure 11. Steady-state space-heating test results: Navien boiler

5.1.3 Short Cycling

Combi systems were tested with various duty cycles in the laboratory to assess the impacts of 

short cycling. These tests were conducted by simulating a call for heat for a specified on cycle 

period, followed by an off period. The on/off cycles were repeated until a consistent efficiency 

was determined (with at least three on and off periods). The cycle length had a slight effect on 

the SWH-based system efficiency. For storage appliances with fixed burner input rates (Polaris 

and Vertex), the burner cycling was determined by the size of the set point temperature dead

band, or the degree of stored water temperature reduction that was allowed prior to reheat. The 

tested units were operated at 3°–8°F dead band. The dead band temperature range must balance 

idle losses and delivered water temperature. ND SWHs typically have dead bands higher than 

20°F. Figure 12 shows that the burner cycles were short, even for long space-heating events. The 

appliances maintained efficiencies within 2% of steady-state performance with these short burner 

cycles. The SWH with a modulating burner (Phoenix) reduced the burner capacity as the events 

reached completion to increase cycle length. Even during very short space-heating events, these 

burners ran for at least 5 minutes (Figure 13). 

80 85 90 95 100 105 110

Return Water Temp

83%

84%

85%

86%

87%

88%

89%

90%

91%

92%

Spa 

915

915

915 915

915

915 800

600

915

700 500

915

915

500

915

500

915

Water Flow Rate (GPM)

1.000

2.000

3.000

4.000

5.000

120.04 141.82

Supply Water Te..
25

Figure 12. Cycling operation of an SWH-based fixed input combi system

using a Polaris water heater

Figure 13. Cycling performance for combi systems with modulating burners

(Phoenix water heater)
26

The hybrid water heaters and boilers had larger reductions in efficiency with shorter spaceheating events. These products had larger startup and cooldown losses than other combi systems.

If cycling for part-load conditions had been included in the modeling, these effects would likely 

have caused even larger reductions in efficiency with longer off periods. Systems with small or 

no storage capacity have cold start losses when the heat exchanger and burner have cooled to 

ambient conditions and must be reheated before water can be heated. These effects have been 

well documented for water heaters (Hoeschele and Springer 2008; Bohac et al. 2010). Figure 14

and Table 10 show the efficiencies for short space-heating events (2 minutes on and 10 minutes 

off, 5 minutes on and 10 minutes off) and steady-state operation.

Figure 14. Impacts of short space-heating events on combi efficiency

Table 10. Impacts of Short Space-Heating Events on Combi Efficiency

Storage Tankless Hybrid Boiler

Phoenix Polaris Vertex Rinnai Eternal Navian Rinnai

2 min (on)/

10 min (off) 90.9% 90.2% 90.6% 87.5% 88.4% 76.7% 86.1%

5 min (on)/

10 min (off) 90.8% 90.3% 92.3% 89.1% 92.6% 84.5% 89.0%

Steady-State 91.4% 92.2% 92.2% 89.2% 94.4% 90.3% 93.7%

Reduction S-S to 2/10 0.6% 2.1% 1.7% 1.9% 6.4% 15.1% 8.1%

The combi system control strategies further minimized the short cycling impacts. The control 

strategies matched the space-heating capacity of the system to the heating load of the home. The 

space-heating events would be longer with control strategies than without. Therefore, the impacts 

of short cycling would be greater for the constant set point base case and the predicted savings 

would be higher for the SP reset and full modulation control methods.
27

5.1.4 Additional Benefits

Several benefits were identified for the set point temperature and flow rate control strategies. In

addition to the efficiency improvements and energy savings, both strategies can reduce the combi 

systems’ dependence on site-specific optimization. The analysis conducted for this report

assumed that the constant set point control strategy was accurately optimized for the design load 

of the system. To achieve this optimization in practice, the design load of the home must be 

accurately known and a careful optimization protocol followed. The SP reset and full modulation 

control methods can significantly simplify the optimization process. Installers would not need to 

worry about balancing the necessary design output capacity to achieve high system efficiency, 

because either control strategy can respond to large demands by ramping up to a higher output 

capacity stage.

The analysis used a high-performance AHU because it was capable of the largest water 

temperature drop (e.g., highest heating output), which resulted in the lowest return water 

temperatures. Table 11 shows that optimization would produce greater savings for a combi 

system with a standard AHU. AHUs with a smaller water temperature drop have higher return 

water temperatures and lower efficiencies. With these AHUs the benefit of reducing the output 

capacity in the shoulder seasons would be much greater than with the optimized, highperformance AHU. For a medium-load house with an Eternal combi system and a highperformance AHU, the set point reset temperature control strategy reduced the annual energy 

consumption by 3% over the constant set point control system (SP 140). Under the same 

circumstances with the standard AHU, the set point control would save 12% of the annual energy 

use.

Table 11. Eternal Water Heater Based Combi System Performance Bins for

Operation at 5 GPM and 900 CFM

Space-Heating 

Output

(Btu/h)

Set Point 

Temperature

(°F)

Return Water 

Temperature

(°F)

System 

Efficiency

(%)

High-Performance AHU

68,810 140 113 86.6%

63,437 135 110 90.2%

59.625 130 106 91.8%

55,032 125 103 92.3%

50,439 120 100 92.8%

Standard AHU

43,464 140 123 75.3%

40,712 135 119 79.7%

37,961 130 115 84.2%

35,102 125 111 88.6%

32,242 120 107 91.6%
28

These results also show the value of using the high-performance over the standard AHU for all 

control methods. For example, for a medium-load house with a combi system using an Eternal 

hybrid water heater and constant set point control (SP 140) the high-performance AHU saves 9% 

(90.4 therms/year) compared to the standard AHU. The added equipment cost is about $650.

5.2 Field Tests

5.2.1 Set Point Reduction

Five of the 20 field test sites from the previous study were available for continued monitoring.

Sites were selected if homeowners were willing to allow monitoring to continue and the design 

space-heating load could be provided with a reduced set point temperature. The five sites 

included systems with three different heating plants (Table 12).The set point temperatures were 

adjusted from 140°F to 130°F for four of the sites; the fifth site (1031) was reduced to 135°F.

Further reduction of the set point temperature at site 1031 would have reduced the capacity of the 

combi system below the design load plus a 20% safety factor.

Table 12. Field Monitoring Sites and Combi System Equipment

Site Heating 

Plant

Type of Heating 

Plant AHU

Set Point Temperature

(°F)

Pre Post

1027 Polaris SWH Ennerzone 140 130

1031 Polaris SWH Ennerzone 140 135

1056 Phoenix SWH Ennerzone 140 130

1049 Eternal SWH Ennerzone 140 130

1070 Eternal SWH Ennerzone 140 130

Average annual efficiency and annual energy consumption were computed for the combi systems 

with reduced set point temperatures. The analysis was completed using the linear input/output 

method used for the previous project (Schoenbauer et al. 2014). 

Table 13 shows the results from the field analysis. The set point reductions resulted in a 1%–4% 

increase in annual energy savings. Because of the small sample size and the differences in the 

combi system equipment and operation at each site, a detailed analysis of savings with other 

parameters was not possible. However, these results show general agreement with the laboratory 

estimates of increased efficiency with decreased return water temperature. These combi systems 

had been previously optimized as part of the initial installation. The set point temperature was 

reduced, but the water flow and airflow rates were not changed. The set point reductions at these 

five sites resulted in an average reduction in return water temperature of 4°F. The laboratory tests

indicate that a 10°F set point reduction will result in improvements in steady-state efficiency 

from 1.2 to 1.6 percentage points for the three SWHs included in this field test (Table 14).
29

Table 13. Efficiency and Energy Use Improvements from Set Point Reduction

Site Heating 

Plant

Set Point 

Reduction

(°F)

Annual Efficiency

(%)

Annual Energy Use

(therms)

Pre Post Change Pre Post Savings

1027 Polaris 10 85.3 89.0 3.6 809 776 33 4.1%

1031 Polaris 5 84.5 86.9 2.4 773 751 22 2.8%

1056 Phoenix 10 84.0 86.1 2.1 551 538 14 2.5%

1049 Eternal 10 89.9 91.0 1.1 668 661 8 1.2%

1070 Eternal 10 88.8 90.5 1.7 507 498 10 1.9%

Table 14. Steady-State Space Heating Laboratory Results for SWHs (2.5 GPM and 900 CFM)

Test ID

Set Point

Temperature

(°F)

Polaris Phoenix Eternal

Test 1 140 90.6% 90.1% 93.2%

Test 2 135 92.2% 91.2% 93.8%

Test 3 130 92.2% 91.4% 94.4%

5.2.2 Domestic Hot Water Priority

Field data from the previous study were used to evaluate the potential of DHW priority to 

improve space-heat delivery and reduce the required peak capacity of the combi system. The first 

area of evaluation for DHW priority was the impact on the conditioned air temperature. This 

control strategy prevents space heating during DHW draws. In a home with high continuous hot 

water use, this may prevent the space heating from running for long periods. The conditioned air 

temperature data in each home with DHW priority were analyzed. The biggest area of concern 

was homes with multiple morning showers back to back, simultaneous with recovery from a 

nighttime temperature setback. Site 1037 had DHW priority, an average daily hot water use of 75 

gal, and a 6°F heating nighttime setback. The average conditioned space temperature was 

compared for peak heating season days with low DHW use (less than 30 gal) and high DHW use 

(more than 90 gal). Figure 15 shows that when DHW use was low, the space heating system 

recovered from the nighttime setback in about 2 hours (from 5 a.m. to 7 a.m.). When DHW use 

was high and the combi system prioritized DHW over space heating, recovery from setback took 

4–7 hours and did not reach peak temperatures until noon. The system was able to recover about 

75% of the setback prior to 8:00 a.m. However, the recovery rate appears to be significantly 

higher for the low DHW days than for the high DHW days. This recovery problem was found

only in homes with nighttime setback and large (more than 90 GPD) DHW loads. The other four 

sites with DHW priority did not experience this effect, because they had smaller DHW loads 

and/or no nighttime setback.
30

Figure 15. Conditioned space temperature during peak heating season for days with

high and low DHW use (Site 1037)

The second potential impact of DHW priority is the variation in supply water temperature under 

simultaneous DHW and space-heating draws. Certain DHW draws, such as showers, require hot 

water at a specific, constant temperature. Without DHW priority, a call for space heat during this 

type of DHW draw has the potential to change the temperature of the hot water. Field data from 

past monitoring data were analyzed to determine the impact of space-heating events on DHW 

delivered water temperature.

A TWH-based combi system was most susceptible to simultaneous use impacts. With no storage 

capacity, the TWH relied on the modulation of the burner output to respond to the change in 

loads when a second draw was initiated or completed. Figure 16 shows a large DHW draw (3 

GPM) with a space-heating draw that starts during the water draw at time equals 0 seconds. 

TWHs restrict the water flow rate as necessary to ensure the outlet temperature requirement is 

met. For the combi systems installed in the field monitoring project, the water heaters were set to 

140°F (the blue line in Figure 16) and a mixing valve was used to temper the outlet water for 

DHW use (the black line in Figure 16). For this large DHW draw the flow restriction of the 

TWH prevented a great impact from the space-heating system (initially the space-heating water 

flow rate was restricted below 0.5 GPM).

The start of the space-heating draw triggered very slight instability in the tempered water 

temperature (less than 0.5°F). Figure 17 shows a smaller DHW draw (0.8 GPM) on the same 

system. Because the DHW flow rate was lower, the space heating water flow rate was not 

restricted and it had a greater impact on the water temperature. In this case a 5°F temperature 

decrease in the tempered DHW temperature was measured at the start of the space-heating draw

and an 8°F increase was measured at the end. These are significant fluctuations in water 

temperature, and in these instances a very low-flow shower head located close to the combi 

system may cause discomfort. Long pipe runs between the water heater and the end use will help

reduce some of the short-term fluctuations.
31

Figure 16. Simultaneous DHW and space-heating draws with a TWH-based combi system
32

Figure 17. A small DHW draw interrupted by a space-heating draw with a

TWH-based combi system

The SWH-based systems had no DHW priority or flow restriction controls in place. When DHW 

and space heating draws happened simultaneously, the system provided hot water to both end 

uses at the same rates as during independent uses. Figure 18 shows a simultaneous use event.

The DHW outlet water temperature (blue) and space heating water temperature (red) rise and fall 

(about 0.5°F per minute) as the storage tank burner cycles on and off to keep the storage water 

temperature at set point. The tempering value reduces the mixed temperature fluctuation even 

further. This event had no increased water temperature variance caused by the combi system in 

the SWH-based system.
33

Figure 18. Simultaneous DHW and space heating draws on a SWH-based combi system

Five boilers were installed and monitored in the previous field monitoring project. Each used 

DHW priority. During the full heating season no simultaneous space heating and DHW draws

occurred at any of these five sites. Occasionally a space-heating draw and a DHW draw would 

begin at about the same time, but the space-heating draw would give priority to the DHW draw

and shut down within 3 seconds. Figure 19 shows a 30-second DHW draw that occurred during 

the space-heating draw. When the draw was initiated, the boiler stopped the space-heating water 

flow and switched to its DHW supply mode. It provided hot water for DHW at 120°F until the 

DHW draw was completed, at which time it returned to space-heating operation.
34

Figure 19. DHW priority operation on a combi boiler system
35

6 Conclusions

Better insulation and tighter envelopes reduce space-heating loads for new and existing homes.

These reduced loads enable a combined space- and water-heating system to meet the combined 

loads. Combi systems have been used on a limited basis for many years but have only recently 

been used as a high-efficiency option. First-generation high-efficiency systems reduce annual 

space- and water-heating gas consumption by 19% but require site-specific design and 

optimization. These systems use a single space-heating stage with a fixed output capacity. This 

project has shown that controls can significantly benefit these systems. Two control methods 

were analyzed:

• Set point temperature reset control, which reduced the supply water temperature for 

periods of lower space-heating demand. This measure can be implemented with an 

outdoor air temperature reset control, which is commonly done for high-performance 

boilers.

• Flow rate modulation, which controlled the set point temperature and varied the water 

flow and airflow rates through the AHU.

Both strategies were used to reduce the output capacity of the space-heating system as the load 

was reduced on the home.

Laboratory tests and daily load/performance models showed that the set point temperature reset 

control produced a savings of 2.1%–4.3% (20–40 therms/year) for SWH and hybrid water heater 

combi systems operated in medium-load homes. The full modulation control showed additional 

savings over set point control (in high-load homes, which almost doubled the savings: 4%–5% 

over the no-control case). The reset control can be implemented for $200–$400, which would 

provide paybacks of 6–25 years for low-load houses and 3–15 years for high-load houses. Offthe-shelf equipment is not currently available for full modulation, but high-performance AHU

components could be used for full modulation with a controller that is similar in design to an 

outdoor reset. These components can also help control fan and pump speed. Implementation 

costs would be similar to the outdoor reset and would provide paybacks of 5-1⁄2–20 years for lowload houses and 2-1⁄2–10 years for high-load houses. The components required for these 

controllers are inexpensive and the implementation costs would be minimal when control is 

implemented with a new combi system installation. 

The laboratory tests and analysis were conducted with a high-performance hydronic AHU with a 

higher heating output that produced greater water temperature drops across the AHU. The reset 

and fully modulating controls would produce greater savings for a combi system with a standard 

AHU. AHUs with a smaller water temperature drop have higher return water temperatures and 

lower efficiencies. With these AHUs the benefit of reducing the output capacity in the shoulder 

seasons would be much greater than with the optimized high-performance AHUs. For a mediumload house with an Eternal combi system and a high-performance AHU, the set point 

temperature control strategy reduced the annual energy consumption by 3% compared to the 

constant set point control system (SP 140). Under the same circumstances with the standard 

AHU, the set point control would save 12% of the annual energy use. The analysis also shows 

that a high-performance AHU is necessary for producing high system efficiencies.
36

Field monitoring of five homes was conducted to measure the energy savings impact of set point 

reduction. The field testing and analysis showed that a reduction in set point temperature from 

140°—130°F resulted in an average reduction in return water temperature of 4°F. These homes 

saved 1%–4% on their annual energy bills with the reduced set point.

Although the control strategies provided energy cost savings, the biggest benefit may be a 

simpler commissioning process that more reliably results in high-efficiency operation. For

instance, previous NorthernSTAR research has shown that improper commissioning or inaccurate 

characterization of the design heating load can significantly reduce combi system efficiency.

These control strategies would allow the system to automatically reduce capacity to the most 

efficient operating point and still meet the load. For example, the study showed that design loads 

were typically overestimated and had additional safety factors applied. This approach resulted in 

combi systems being optimized for design loads much higher than the actual design load; 

therefore, the return water temperature increased and the efficiency decreased. If the same 

systems were installed with a set point reset or fully modulating control, the system would 

operate at a lower capacity stage; reduced water temperature would decrease and efficiency

would increase. The control system would thus reduce the design calculations and time that 

would be required to properly install and commission a combi system.
37

References

Arena, L.B. 2010. In-Field Performance of Condensing Boilers in Cold Climate Region. Final 

Report. Building America. Norwalk, CT: Prepared by Steven Winter Associates, Inc. of the

Consortium for Advanced Residential Buildings Team, for Building America

Bohac, D. 1992. “Once Heated, Twice Used.” Home Energy Magazine Online, July. 

http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/92/920708.html.

Bohac, D., M. Hewett, T. Dunsworth, M. Hancock, and T. Staller. 1991. Retrofit Savings for 

Dual Integrated Appliances in Small Commercial and Multifamily Buildings. Report to 

Minnegasco Inc. Center for Energy and the Urban Environment. 

www.mncee.org/pdf/tech_pubs/90-5.pdf. 

Butcher, T. 2006. “Condensing Boilers and Baseboard Hydronic Distribution Systems.” 

ASHRAE Transactions 112 (1).

Butcher, T. 2011. The DHW Performance of Heating Boilers presented at the ACEEE Water heating 

Forum, May 10, Berkeley, CA. http://www.aceee.org/files/pdf/conferences/hwf/2011/7A%20-

%20Tom%20Butcher.pdf.

Hoeschele, M., and D. Springer. 2008. “Field and Laboratory Testing of Gas Tankless Water 

Heater Performance.” ASHRAE Transactions 114 (Part 2).

Schoenbauer, B., M. Hewett, and D. Bohac. 2010. Actual Savings and Performance of Natural 

Gas Tankless Water Heaters. Minneapolis, MN: Center for Energy and Environment, October 

18. www.mncee.org/research/tankless_water_heaters/index.php.

Schoenbauer, B., D. Bohac, P. Huelman, M. Hewett, and J. McAlpine. 2012. “Retrofitting 

Combined Space and Water Heating Systems: Laboratory Tests.” Washington, D.C.: U.S. 

Department of Energy. http://www.mncee.org/getattachment/623786cd-c32f-4ba7-80fcb229f1fa8a37/.

Schoenbauer, B., D. Bohac, P. Huelman, and J. McAlpine. 2014. “Retrofitting Combined Space 

and Water Heating Systems: Field Assessment.” Washington, D.C.: U.S. Department of Energy.

Thomas, M. 2011. “Combination Systems Good or Bad?” Presented at the ACEEE Water 

Heating Forum 2011, May 10, Berkeley, CA. 

www.aceee.org/files/pdf/conferences/hwf/2011/7A%20-%20Martin%20Thomas.pdf.
38

Appendix: Solution Center Content

Guide Title: Combined Space and Water Heating: Next Steps to 

Improved Performance

Keywords: HVAC, Space heating, water heating, condensing, low-load homes

Climate Zone: Heating climates, zones 4–8

Construction Type: New construction and/or existing homes. 

Scope

Image Title: Combi system installed in a retrofit home providing both space and water heating

Image Source: Schoenbauer. 2012. Center for Energy and Environment. Unpublished.

Display Image Filename: Combi_Hybrid_001.jpeg

Efficient HVAC Equipment

ENERGY STAR Version 3 (Rev. 06)

Description

New housing construction practices and present-day retrofit measures are leading to more 

thermally efficient building envelopes and the decline of home space heating loads. At the same 

time, new water heating product lines with higher firing rates are becoming the dominant thermal 

engines in an increasing number of homes. Instead of continuing to specify two separate thermal 

engines—such as a separate gas-fired furnace and water heater—researchers, homebuilders, and 

HVAC/DHW equipment manufacturers are exploring more cost-effective packaging of space 

and water heating in a new generation of combination systems that use high-efficiency 

condensing heating plants and high-performance hydronic air coils to provide both space and 

water heating.

Condensing combi systems are still relatively new and complex, often requiring onsite 

engineering and optimization to achieve desired performance. Improved controls have the 
39

potential to reduce some of this installation and operational complexity and improve upon the 

measured performance. The control measures evaluated in the laboratory include temperature set 

point control (outdoor reset and/or turndown after heating season), space heating modulation 

(both water flow and airflow) and DHW priority. The benefits to installation, sizing, and 

optimization were characterized for all three control schemes. Set point control and space heating 

modulation were analyzed for efficiency and energy savings improvements through laboratory 

and field testing. 

Ensuring Success

High-efficiency combination systems are an emerging technology and many installers were 

unfamiliar with their installation and optimization. Verification of proper installation was 

necessary to achieve the desired results of a safe, comfortable, and efficient space and water 

heating system. After training contractors, installing systems, and verifying installations, the 

following process was developed to verify future installations. 

1. Combi system installed using approved high-performance equipment

2. Installer must have received an approved combi installation training

3. Field-verified performance of return water temperature or system efficiency during 

operation

Climate

Combi systems may be appropriate for all climate zones. However, control and optimization 

specifics will vary. The systems discussed here are optimized for heating-dominated climates.

Training

Presentations

1. Presentation Title: Performance and Optimization of Residential Condensing Combi Systems

Presentation Source: http://mncee.org/Innovation-Exchange/ResourceCenter/Webinars/Performance-and-Optimization-of-Residential-Conden/

Filename: 2013_04_23 IX Webinar- Combis.pptx

Compliance

More Info

Case Studies

1. NorthernSTAR. 2014. Combined Space and Water Heating: Next Steps to Improved Performance, 

Minneapolis, MN. Prepared by the Center for Energy and Environment for the U.S. Department 

of Energy Building America Program. Upcoming

Climate Zone: Zones 4-8. 

Case Study Type: Measure-specific 

Construction Type: New and existing.
40

2. NorthernSTAR. 2014. Retrofitting Combined Space and Water Heating Systems: Laboratory 

Tests, Minneapolis, MN. Prepared by the Center for Energy and Environment for the U.S. 

Department of Energy Building America Program. Upcoming

Climate Zone: Zones 4–8. 

Case Study Type: Measure-specific 

Construction Type: New and existing.
DOE/GO-102016-4600 ▪ July 2016

buildingamerica.gov

For more information, visit: energy.gov/eere