NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Condensing Hybrid Water Heater 

Monitoring Field Evaluation

Jeff Maguire, Lieko Earle, and Chuck Booten 

National Renewable Energy Laboratory

C.E. Hancock 

Mountain Energy Partnership

Produced under direction of the Sacramento Municipal Utilities District by 

the National Renewable Energy Laboratory (NREL) under Interagency 

Agreement CRD-05-168 and Task No WR49.3000. 

Technical Report

NREL/TP-5500-52234

October 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Condensing Hybrid Water Heater 

Monitoring Field Evaluation

Jeff Maguire, Lieko Earle, and Chuck Booten 

National Renewable Energy Laboratory

C.E. Hancock 

Mountain Energy Partnership

Prepared under Task No. WR49.3000

Technical Report

NREL/TP-5500-52234

October 2011
NOTICE

This manuscript has been authored by employees of the Alliance for Sustainable Energy, LLC (“Alliance”) under 

Contract No. DE-AC36-08GO28308 with the U.S. Department of Energy (“DOE”).

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, 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.

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
iii

Acknowledgments

The authors would like to thank the Sacramento Municipal Utility District (SMUD) for its 

contribution to this work, and Xia Fang, Dave Roberts, Stacey Rothgeb, and Cheryn Engebrecht 

of NREL and Mike Keesee of SMUD for their reviews and other contributions.
iv

Executive Summary

Water heating is the second largest energy use in U.S. homes. A typical gas water heater has an

efficiency of about 60%, although more efficient options are available. A hybrid condensing 

water heater with a high rated efficiency (thermal efficiency = 0.973, energy factor = 0.95) was 

installed in an occupied home in Sacramento, California, and monitored for one year to 

determine the actual in situ efficiency. This unit performed at an annual efficiency of 90.6% for 

this home and achieved its rated thermal efficiency during extended draws. The efficiency of this 

unit was largely independent of the mains water temperature, but varied significantly with the 

average daily draw volume. Due to the high hot water demand and temperate climate at this site, 

the measured in situ efficiency observed here is expected to be higher than what might be 

achieved by installations in other climates and in households with lower hot water demands.
v

Contents

Acknowledgments ...........................................................................................................iii

Executive Summary ........................................................................................................iv

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

Figures ............................................................................................................................ v

Tables ............................................................................................................................. v

Nomenclature..................................................................................................................vi

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

2 Procedure and Measurements ................................................................................. 3

3 Average Efficiency Calculations................................................................................ 4

3.1 Discussion of Factors Impacting Overall Efficiency............................................ 5

3.2 Short-Term Efficiency Measurements ................................................................ 8

4 Economic Analysis.................................................................................................. 10

5 Conclusions ............................................................................................................ 12

6 References ............................................................................................................. 13

Figures

Figure 2. (a) The installed water heater and (b) a schematic of key components....... 2

Figure 3. Measurement points for field monitoring...................................................... 3

Figure 4. Electrical energy used per hour during standby as a function of average 

ambient temperature ....................................................................................................... 5

Figure 5. Effect of inlet water temperature on water heater efficiency ........................ 6

Figure 6. Efficiency versus daily draw volume. As use increases, the water heater’s 

efficiency increases......................................................................................................... 7

Figure 7. Draw profile for one day compared to the energy factor test draw profile.... 8

Figure 8. Annual energy consumption using measured data and the BEopt models 11

Tables

Table 1. Installed Measurement Equipment in the Mascot Home ............................. 3

Table 2. Monthly Average Energy Use and Efficiency............................................... 4

Table 3. Uncertainty Analysis of Each Measurement Instrument During One 

Extended Draw................................................................................................................ 9

Table 4. Annual Operating Costs for the Monitored Water Heater and a Modeled 

Typical Gas Water Heater............................................................................................. 10
vi 

Nomenclature

cp specific heat of water 

Eelec electrical energy consumed 

Egas gas energy consumed 

E(x) error associated with variable x 

h natural gas heating value 

݉ሶ mass flow rate of water 

Qin thermal energy in 

Qout thermal energy out 

TF therm factor 

Tin inlet water temperature 

Tout outlet water temperature 

ܸሶ flow rate of water through water heater 

Vgas volume of gas drawn by the water heater 

ܸሶ

gas flow rate of gas to water heater 

η in-use efficiency 

ρ density of water 
1 

1 Introduction

The Sacramento Municipal Utility District (SMUD) Energy Efficient Remodel Demonstration 

program installs efficiency upgrades in homes to showcase their energy saving benefits. In 

partnership with the National Renewable Energy Laboratory (NREL), a package of efficiency 

upgrades was selected and installed at a home on Mascot Avenue in Sacramento, California (see 

Figure 1). This was an abandoned, 1950s vintage home that required extensive repairs, including 

a new roof, new heating, ventilation, and air-conditioning equipment, and wall repairs. The need 

for these repairs made it an ideal candidate for the efficiency upgrades, which included building 

envelope improvements, compact fluorescent lights, and a new water heater. Of these upgrades, 

the unique water heater installed in this home is of particular interest.

Field monitoring was performed to determine the in-use efficiency of the hybrid gas condensing 

water heater (Navien CR240-A) installed at the home. These results were then compared to the 

unit’s rated efficiency. This unit is ENERGY STAR® qualified and has one of the highest rated 

efficiencies of the gas water heaters currently on the market. It consists of a tankless condensing 

water heater and a 0.5 gallon buffer tank (See Figures 2 and 3). A condensing water heater 

condenses the exhaust from the water heater to extract more heat that can be used to heat the 

water. The buffer tank eliminates the “cold water sandwich” issue common to tankless water 

heaters, but increases standby losses. The “cold water sandwich” commonly occurs between two 

closely spaced domestic hot water (DHW) draws (for example, two morning showers). After the 

first draw, hot water may still be in the pipes, but the tankless water heater will have turned itself 

off because no water is being drawn. The second event will start with hot water from the pipes, 

but will be followed by a slug of cold water right before the tankless water heater burner ignites

and heats the water once the water heater fully fires.

Figure 1. The Mascot home

The water heater was monitored for a full year (from March 2010 to March 2011), during which 

time the home was occupied by a single family. Based on the data collected, this hybrid 

condensing water heater demonstrated that it can reach its rated thermal efficiency of 97.3% 

during extended draws of 15 minutes or longer, but has an in situ efficiency of about 90.6%. This NREL/PIX 19261
2 

value is lower than the rated efficiency; the energy factor for this unit is 95%. This is still quite 

good for a gas water heater; typical gas storage water heater efficiencies are 50%–60%. Past field 

studies have shown this unit generally performs below its rated efficiency when installed in 

homes, in some cases far below its rated efficiency [1]. The likely cause of the discrepancy 

between rated and actual efficiency is the difference between the draw profiles used in the 

standardized U.S. Department of Energy (DOE) efficiency tests [2] and the actual in-home use 

draw profile.

Figure 2. (a) The installed water heater and (b) a schematic of key components

Credit: Jeff Maguire/NREL

NREL/PIX 1926

0

a b
3

2 Procedure and Measurements

To determine the in situ efficiency of the water heater, the inlet and outlet temperatures for 

water, the flow rates for water and gas, and the electricity use were measured. Although the 

water heater is gas powered, electricity is required by the unit to operate the controls, ignition, 

venting fan, and freeze protection. A schematic of the measurement points is provided in Figure 

3 and measurement equipment and accuracies are given in Table 1. Most measurements were of 

sufficient resolution to draw detailed conclusions, but the limited accuracy of the gas flow meter 

made it difficult to determine short term efficiencies that could be compared to the thermal 

efficiency. A more detailed discussion of this issue is provided in Section 3.2. Data were

collected and recorded via a Campbell Scientific CR1000 data logger at 1 minute intervals,

reported at 1 minute, 15 minute, 1 hour, and daily intervals, then transmitted back to the 

laboratory via a wireless modem. This unit was monitored for a full year to examine seasonal 

effects. This was an occupied home, so the draw profile reflects the actual DHW use of these

particular occupants and may not be typical.

Figure 3. Measurement points for field monitoring

Table 1. Installed Measurement Equipment in the Mascot Home

Measurement Equipment Manufacturer Accuracy

Temperature Type T thermocouple Omega ± 1.8°F (1°C) or 0.75%

Water flow Hall effect flow meter Omega >10% cont. flow-max flow: ± 1.5%, 

<10% continuous flow: ± 2%

Gas flow Diaphragm gas flow 

meter Sensus ± 1 ft3

Electric power kWh transducer Continental Control 

Systems ± 0.5%
4 

3 Average Efficiency Calculations 

This water heater is rated at 97.3% thermal efficiency and 95% energy factor. Thermal efficiency 

provides a measure of the efficiency during continuous operation while energy factor provides an 

average overall efficiency based on a standardized daily draw profile. Although the rated thermal 

efficiency was observed during some events, on average the daily efficiency observed was lower 

than the rated energy factor. The efficiency can be calculated for any time interval using the 

equation: 

ߟ ൌ ொ೚ೠ೟

ொ೔೙

ൌ ௠ሶ ௖೛ሺ்೚ೠ೟ି்೔೙ሻ

ா೐೗೐೎ାா೒ೌೞ

 (1) 

In Equation 1, the energy out is calculated from the flow rate of water through the water heater 

(݉ሶ ሻ, the temperature difference between the inlet and outlet water ሺܶ௢௨௧ െ ܶ௜௡ሻ, and the water’s 

specific heat (ܿ௣ሻ. The energy out (ܳ௢௨௧ሻ is calculated internally by the data logger and is 

reported at every time interval. The electric energy term includes the energy used by the controls, 

ignition, venting fan and freeze protection from an electric heater. The energy from gas is 

calculated according to Equation 2 using the volume of gas used and the therm factor (an average 

gas heating value defined as the number of therms per 100 ft3

 of gas) as reported by the local 

utility, Pacific Gas and Electric (PG&E). 

ܧ௔௚௦ ൌ ௏೒ೌೞ

ଵ଴଴ ሺܶܨሻ (2) 

For this home, PG&E gave a therm factor of 1.01616 therms/100 ft3. Monthly measured energy 

consumption, DHW use, and efficiencies are given in Table 2. 

Table 2. Monthly Average Energy Use and Efficiency 

Month 

DHW 

Draw 

Volume 

(gal) 

Daily 

Draw 

Volume 

(gal/day) 

DHW 

Electricity 

Use 

(kWh) 

DHW 

Gas Use 

(therm) 

Electricity 

Use 

(% of total 

energy 

use) 

DHW 

Energy 

Out 

(therm) 

Efficiency

March 3424 110.5 7.35 16.3 1.54% 15.4 92.6% 

April 2712 90.4 6.80 12.5 1.85% 11.6 90.6% 

Map 2285 73.7 6.81 9.8 2.38% 8.8 88.2% 

June 2297 76.6 6.50 8.8 2.51% 8.2 90.6% 

July 2883 93.0 6.80 9.4 2.47% 8.8 91.5% 

August 2191 70.7 6.55 8.4 2.67% 7.6 88.7% 

September 2325 77.5 6.49 8.9 2.50% 8.2 89.8% 

October 2908 93.8 6.96 11.8 2.01% 10.9 90.4% 

November 2761 92.0 7.24 12.9 1.91% 11.8 89.5% 

December 3411 110.0 7.59 17.2 1.50% 15.9 90.6% 

January 3902 125.9 8.04 20.7 1.33% 19.2 91.8% 

February 2868 102.4 6.94 15.1 1.57% 13.9 90.5% 

Total (average) 33967 (93.1) 84.05 151.9 (1.89%) 140.2 (90.6%) 
5

3.1 Discussion of Factors Impacting Overall Efficiency

The freeze protection system for this water heater turns on a backup electric heater whenever it 

detects an ambient air temperature close to freezing. Typically, the freeze protection threshold 

temperature is set above 32°F, as any freezing in the water heater would lead to catastrophic 

failure. Freeze protection was seen to turn on at temperatures as high as 45°F for this unit. This 

threshold was crossed in very few instances (see Figure 4). Overall, the energy used for freeze 

protection accounted for 5.5% of the total energy used during standby (when no water was being 

drawn) over the year. The energy consumed during any hour for freeze protection varied 

depending on how long the freeze protection was on in that hour. The baseline energy 

consumption of 8 Wh comes from the controls.

Figure 4. Electrical energy used per hour during standby as a function of average 

ambient temperature 

Another factor that could reduce efficiency during colder months is the buffer tank. Past work 

has suggested that this tank is poorly insulated and could significantly affect the efficiency of the 

water heater, especially when it is located in unconditioned space [1]. During the year that the 

water heater was monitored, only one event was observed where the water heater had to fire to 

replace standby losses. Since such an event would be difficult to detect with the resolution of the 

gas flow meter, it is likely to have occurred more than once over the full year. Overall, the DHW

use was a much larger factor in the overall efficiency than ambient temperature. This is likely 

due to the temperate climate of Sacramento; past work where a significant impact was seen 

involved field monitoring of this water heater in a garage in Minnesota [1].

0

10

20

30

40

50

60

30 40 50 60 70 80 90 100 110

Electricity Use (Wh)

Ambient Temperature (°F)

Electricity Use Versus Ambient Temperature
6

One factor could increase the efficiency of the system during the winter. Condensing water 

heaters are more efficient when the inlet water temperature is lower because the cold temperature 

increases the amount of water vapor that can condense in the unit (or, if full condensation has 

occurred, further reduces the temperature of the remaining flue gas and condensate). Based on 

previous case studies that showed very low efficiencies in cold climates, it is expected that this 

unit should be less efficient overall in cold conditions. The current dataset suggests that the 

efficiency of this water heater is not a strong function of inlet water temperature (see Figure 5).

Figure 5. Effect of inlet water temperature on water heater efficiency

Efficiency is only weakly influenced by inlet water temperature, but it is strongly influenced by 

the volume of water drawn from the water heater (see Figure 6). However, there is still a wide 

spread in the data because of fluctuations in ambient temperature and daily draw profiles. This 

home is a particularly high DHW user (an “average” home is 64.3 gallons/day [2], and this home 

averages 93.1 gallons/day), so the in situ efficiency of this unit may be higher than what would 

be seen in other homes with more typical use.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

45 50 55 60 65 70 75 80 85 90

Efficiency

Inlet Water Temperature (°F)

Water Heater Efficiency Versus Inlet Water Temperature
7

Figure 6. Efficiency versus daily draw volume. As use increases, the water heater’s 

efficiency increases.

The discrepancy between the water heater’s rated efficiency and its measured in situ efficiency is 

most likely due to significant differences between the draw profiles used in the rating tests and 

actual draw profiles used by the homeowners, although other factors such as fluctuations in

mains temperature and ambient temperatures also have some effect. Thermal efficiency is 

determined with a continuous use test, which means that the efficiency decrease from heating the 

heat exchanger are unaccounted for. For the energy factor test, six draws of equal volume over 

the course of six hours are used as an estimate of a home’s water use. The energy factor test also 

includes an 18 hour period when no water is drawn so standby losses can be determined. In 

actual use, there are many short draws over the course of a day (see Figure 7). These short draws 

require the heat exchanger to heat up and cool down with each draw, reducing the system 

efficiency. The energy factor test draws a set amount of water (64.3 gallons) that is intended to 

represent typical use, although use can vary widely from one home to another based on the 

number of occupants and their behavior.

0.50

0.60

0.70

0.80

0.90

1.00

0 25 50 75 100 125 150 175 200

Efficiency

Daily Draw Volume (gallons)

Water Heater Efficiency Versus Draw Volume
8 

Figure 7. Draw profile for one day compared to the energy factor test draw profile 

3.2 Short-Term Efficiency Measurements 

To determine the water heater’s efficiency during actual use, the one minute interval data was 

analyzed during periods when the water heater was in use. However, the limited resolution of the 

gas flow meter made detailed analysis of the short term performance of the water heater difficult. 

The gas flow meter is accurate only to the nearest cubic foot, and the water heater draws gas at 

less than 1 cfm when it is in use. This meant that the reading for any one minute period would 

either be 1 or 0 ft3

. This created such a large error that doing detailed analysis of the short term 

performance for most draws was not possible. Fortunately, some longer draws could be analyzed 

to look at the performance of the water heater under constant use. The one minute data was used 

to identify periods where there was a continuous 15 minute draw, and those periods were 

analyzed to determine the water heater’s efficiency during use. During these periods, the 

electrical energy use was very small compared to the gas energy use, typically affecting the 

efficiency by less than 0.1%, and was therefore ignored. The equations used to calculate the 

uncertainty in energy in, energy out, and efficiency are given in Equations 3–5. The error 

propagation calculations were done with Engineering Equation Solver. A sample efficiency 

calculation is given in Table 3. In Equations 3-5, h is the natural gas heating value, Vgas is the 

gas flow rate, V is the water flow rate, Tin is the inlet water temperature, and Tout is the outlet 

water temperature.

 ܧሺܳ௜௡ሻ ൌ ቀ ௛

ଷସଵଶቁ ܧ൫ܸ௚௔௦൯ (3) 

0

1

2

3

4

5

6

7

8

0 3 6 9 12 15 18 21 24

Flow Volume (gallons)

Time (hr)

Energy Factor Test Compared to Actual Draws

Draw

Approximate EF Profile
9 

 ܧሺܳ௢௨௧ሻ ൌ ቀ ఘ௖೛

ଷ଺଴଴଴଴଴ቁ ܧ൫ܸ௚௔௦൯ሾܸሺܶ௢௨௧ െ ܶ௜௡ሻሿටቀ

ாሺ௏ሻ

௏ ቁ

ଶ

൅ ாሺ்೚ೠ೟ሻమାாሺ்೔೙ሻమ

ሺ்೚ೠ೟ି்೔೙ሻమ (4)

 ܧሺߟሻ ൌ ቀொ೚ೠ೟

ொ೔೙

ቁ ටቀ

ாሺொ೚ೠ೟ሻ

ொ೚ೠ೟

ቁ

ଶ

൅ ቀாሺொ೔೙ሻ

ொ೔೙

ቁ

ଶ

 (5)

Table 3. Uncertainty Analysis of Each Measurement Instrument During One Extended Draw 

Variable ± Uncertainty Partial Derivative % Uncertainty 

η = 0.9464 ± 0.0968 – – 

Tin = 14.74 ± 1 (°C) ∂η/∂Tin = –0.02826 8.52% 

Tout = 48.42 ± 1 (°C) ∂η/∂Tout = 0.02794 8.33% 

V = 79.61.42 ± 1.592 (l) ∂η/∂V = 0.01189 3.82% 

Vgas = 11 ± 1 (ft3) ∂η/∂Vgas = –0.08621 79.32% 

From the start of testing in March through June of 2010, 57 continuous draw events lasted for 15 

minutes or longer. A more accurate efficiency measurement could be calculated during 

continuous use, although the accuracy of each event was still not particularly high. The 

uncertainty was as large as ±20% for the efficiency of each event, largely because of the error in 

gas flow measurement. By averaging these events together, an in situ efficiency of 99.0±1.7% was 

found. This is consistent with the rated thermal efficiency of 97.3%, because the thermal 

efficiency is within the error bounds. 

To more accurately determine the efficiency of the water heater while it is in use, a more 

accurate gas flow meter with an accuracy of at least ±0.1 ft3 would be required to keep the 

uncertainty in gas flow measurement on the same order of magnitude as the other measurements. 

A potential replacement for the currently installed gas flow meter is the AC-250 diaphragm gas 

flow meter available from IMAC Systems, Inc. [3]. This gas flow meter can be fitted with a 

pulse output attachment to achieve an accuracy of ± 0.025 ft3, which would be sufficient for 

future studies on tankless gas water heaters. However, it is significantly more expensive than the 

gas flow meter that was used in this study. 
10

4 Economic Analysis

To determine how cost-effective this unit is, a comparison was made to a simulated standard gas 

storage water heater at the same house. The draw profile, ambient temperature, and inlet water 

temperature recorded during field monitoring were all used as inputs to the model to ensure the 

comparison would be as direct as possible. The gas water heater modeled is a 50 gallon unit with 

an energy factor of 0.59, a typical efficiency for this style of water heater. The model is based on 

a specific unit made by a major water heater manufacturer. Model parameters were derived from 

standard rating test results [4]. The modeling was done using the TRNSYS simulation program. 

Modeling results and operating costs for both units are shown in Table 3.

Table 4. Annual Operating Costs for the Monitored Water Heater and a Modeled Typical Gas 

Water Heater

Month

Hybrid Condensing 

Water Heater Total 

Energy Use

(therm)

Gas Storage 

Water Heater 

Gas Use

(therm)

Hybrid Condensing 

Water Heater 

Operating Cost

($)

Gas Storage 

Water Heater 

Operating Cost

($)

January 16.6 31.23 $20.16 $37.07

February 12.7 23.68 $16.07 $29.06

March 10.0 25.64 $12.79 $31.56

April 9.0 20.08 $12.98 $28.07

May 9.6 16.47 $15.23 $25.38

June 8.6 14.32 $14.34 $23.16

July 9.1 16.60 $16.49 $29.36

August 12.0 13.83 $19.44 $21.84

September 13.1 14.71 $18.33 $19.97

October 17.5 19.47 $22.25 $24.27

November 21.0 20.09 $21.76 $20.23

December 15.3 27.43 $15.79 $27.29

Annual total 154.7 243.6 $205.64 $317.27

Annual operating costs were calculated based on monthly gas and electricity prices from SMUD 

and PG&E for 2008 [5][6]. The cost of installing a gas storage water heater was estimated based 

on the NREL National Residential Efficiency Measures Database as $780 [7]. The condensing 

hybrid water heater installed in this house had an installed cost of $4924. However, this home 

underwent extensive repairs and it is unknown how much of this cost is associated with repairs to 

this home, which makes it difficult to determine the installed cost in more typical retrofit 

situations. To determine the cost effectiveness of this unit, the simple payback of the hybrid 

water heater was calculated. For these calculations, both units were assumed to have zero 

maintenance cost, although the hybrid water heater may need regular maintenance [1]. If no 

incentives (tax credits, rebates, etc.) are considered, the simple payback is about 37 years, far 

longer than the expected lifetime. A typical water heater has a life of about 13 years. Most 

tankless water heater manufacturers advertise a 20 year lifetime, but these units have not been on 

the market long enough to validate that claim. There is especially large uncertainty in the lifetime 
11 

of the hybrid unit because it will depend on the lifetime of the buffer tank as well. An incentive 

of about $1910 (39% of the installed cost) would give this unit a 20 year simple payback; an 

incentive of about $2690 (55% of the installed cost) would give it a 13 year simple payback. 

Current federal incentives for this unit are $300 [8], so significant state and local incentives 

would be necessary to make this unit cost effective in this case. Using this water heater would 

save 91.8 therms of gas (about 9000 ft3

 for this site) relative to the 0.59 energy factor storage 

tank while using 85 kWh of electricity annually. 

The actual water heating energy use for this home was larger than the predicted energy use. 

BEopt modeling had suggested that the annual energy use for this water heater would be 119.7 

therms per year; the actual use was 147.7 therms per year, 23% larger than the predicted value 

(see Figure 8). This additional energy use can largely be attributed to differences in the estimated 

and actual amounts of DHWused per day. The original assumed amount was only 54.5 

gallons/day, significantly less than the actual average 93.1 gallons/day drawn by this household. 

The actual energy consumption was not accurately captured, but the magnitude of the savings 

was captured fairly well. BEopt predicted a 39% energy savings, while the savings when 

comparing the actual monitored energy use to the energy use predicted by the TRNSYS storage 

tank model is 36%. 

Based on this economic analysis, this unit is not a cost-effective replacement for a standard gas 

water heater. However, one issue not captured by this modeling is that the standard gas storage 

water heater will not always be able to keep up with demand, especially in high use cases such as 

the one studied here. Thus, the occupants would likely either use less hot water or raise the set 

point temperature of the water heater. Either behavior would alter the amount of energy used by 

the gas storage water heater, although it probably would not be enough to significantly change 

the cost effectiveness of the hybrid water heater considered here. 

Figure 8. Annual energy consumption using measured data and the BEopt models 
12

5 Conclusions

• Actual in situ efficiency for this water heater is about 5% lower than the rated efficiency 

(energy factor). The in situ efficiency was significantly influenced by the daily DHW

draw profiles. Differences between the actual draw profile and that used in the energy 

factor test procedure is the primary reason for this difference between the in situ

efficiency and the rated efficiency. Under conditions similar to those used for the thermal 

efficiency test, this unit does achieve its rated efficiency.

• The occupants of this home used larger than typical DHW volumes. The actual in situ

efficiency will vary depending on DHW use patterns, and is likely to decrease for many 

homes with lower DHW demands because of standby losses and more energy being 

wasted in heating and cooling the heat exchanger when compared to the delivered energy 

in the heat exchanger.

• Ambient air temperature and mains water temperature had little impact on the measured 

efficiency of this water heater. However, mains and ambient temperatures have the 

potential to have a large impact under extreme conditions, as demonstrated by the 

efficiency of this unit when compared to that monitored in Minnesota [1]: This unit’s in 

situ efficiency was about 90%, compared with 57%–67% for units tested in Minnesota.

• For future studies of tankless and hybrid gas water heaters, a more accurate gas flow 

meter should be installed so efficiency during short draws can be determined. Such gas 

flow meters are more expensive than the unit that was used in this study.

• Finally, this water heater is not currently a cost-effective solution compared to a typical 

gas storage water heater, because of its high installed cost.
13

6 References

1. Bohac, D.; Schoenbauer, B.; Hewett, M.; Lobenstein, M.S.; Butcher, T. 2010. Actual Savings 

and Performance of Natural Gas Tankless Water Heaters. Minneapolis, MN: Center for 

Energy and Environment.

2. U.S. Department of Energy. 1998. 10 CFR Part 430 Energy Conservation Program for 

Consumer Products: Test Procedure for Water Heaters, Federal Register Vol. 63, No. 90.

3. IMAC Systems, Inc. (n.d.). IMAC Systems. Retrieved March 31, 2011, from American Meter 

AC-250 Diaphragm Gas Meter: www.imacsystems.com/ac250.htm.

4. Burch, J.; Erikson, P. 2004. “Using Ratings Data to Derive Simulation-Model Inputs for 

Storage-Tank Water Heaters.” Proceedings of the 29th Passive Solar Conference, pp. 393-

398. Portland, OR.

5. U.S. Energy Information Agency. 2010. Electric Sales, Revenue, and Average Price 2008.

Washington, DC: EIA.

6. U.S. Energy Inforamtion Agency. 2010. Natural Gas Monthly. DOE/EIA-0130 (2010/06). 

Washington, DC: EIA.

7. National Renewable Energy Laboratory. 2010. National Residential Efficiency Measures 

Database. Retrieved May 09, 2011, from Tank Water Heaters: 

www.nrel.gov/ap/retrofits/measures.cfm?gId=6&ctId=40&scId=493

8. Database of State Incentives for Renewables and Efficiency (DSIRE). 2011. 

www.dsireusa.org/.