Laboratory Test Report: iFlow/Navien Combi System Assessment · forced-air tankless combi applications because they can achieve condensing operation at high firing rate turndowns

August 5, 2019

Laboratory Test Report: iFlow/Navien Combi System Assessment

Prepared by:

Gas Technology Institute

1700 South Mount Prospect Road

Des Plaines, IL

847-768-0500

Northwest Energy Efficiency Alliance PHONE

503-688-5400 FAX

503-688-5447 EMAIL

[email protected]

mailto:[email protected]

LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT

ii

DISCLAIMER

This information was prepared by Gas Technology Institute (“GTI”) for the Northwest Energy Efficiency Alliance.

Neither GTI, the members of GTI, the Sponsor(s), nor any person acting on behalf of any of them:

a) Makes any warranty or representation, express or implied with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately-owned rights. Inasmuch as this project is experimental in nature, the technical information, results, or conclusions cannot be predicted. Conclusions and analysis of results by GTI represent GTI's opinion based on inferences from measurements and empirical relationships, which inferences and assumptions are not infallible, and with respect to which competent specialists may differ.

b) Assumes any liability with respect to the use of, or for any and all damages resulting from the use of, any information, apparatus, method, or process disclosed in this report; any other use of, or reliance on, this report by any third party is at the third party's sole risk.

c) The results within this report relate only to the items tested.


iii

TABLE OF CONTENTS

1.0 INTRODUCTION/BACKGROUND .............................................................................................. 6

2.0 TECHNOLOGY DESCRIPTION ................................................................................................... 7

3.0 METHODS FOR LABORATORY EVALUATIONS .................................................................... 9

VTH System Diagram and Instrumentation Plan ................................................................................... 10

Baseline and Combi Equipment Evaluated ............................................................................................ 11

4.0 LABORATORY TEST RESULTS ............................................................................................... 12

Forced-air Space Heating Performance Characterizations ..................................................................... 12

Water Heating Performance Characterizations ...................................................................................... 15

5.0 BUILDING ENERGY MODELING RESULTS .......................................................................... 17

Energy Savings Predictions Between Baselines and Combi Energy Efficiency Measure ..................... 20

APPENDIX – INSTRUMENTATION, DATA ACQUISITION, AND CALCULATIONS ........ 22

Instrumentation ....................................................................................................................................... 22

Data Acquisition and Calculations ......................................................................................................... 23

APPENDIX B – MODELING MEASURES ................................................................................. 27


iv

LIST OF TABLES

Table 1 – Air Handler Unit Market Landscape (Atributes and Shortfalls) ................................................... 8

Table 2 – iFLOW AHU Component List ...................................................................................................... 9

Table 3 – Baseline and Combi Equipment Evaluated ................................................................................. 11

Table 4 – Peak Annual Modeled Heating Loads by Zone .......................................................................... 15

Table 5. Summary of building model characteristics for Type 1 and Type 2 ............................................. 18

Table 6 – Baseline 1: Building Type 1 Predicted Annual Gas and Electricity Consumptions and Savings

.................................................................................................................................................................... 20


.................................................................................................................................................................... 20


.................................................................................................................................................................... 21


.................................................................................................................................................................... 21

Table 10 – Baseline and EE Measure Water Heater Assumptions ............................................................. 27

Table 11 – Baseline 1 and EE Measure Space Heating Assumptions ........................................................ 27

Table 12 – Baseline 2 and EE Measure Space Heating Assumptions ........................................................ 27

LIST OF FIGURES

Figure 1 – Part Load Space Heating Histograms .......................................................................................... 6

Figure 2 – Space Heating System Performance ............................................................................................ 6

Figure 3 – Forced-air Tankless Combi Configuration .................................................................................. 7

Figure 4 – Flue Gas Dewpoint Temperatures ............................................................................................... 7

Figure 5 – iFLOW Combi Configuration (Including Heat Pump) .............................................................. 10

Figure 6 – Virtual Test Home Test Rig Diagram ........................................................................................ 11

Figure 7 – Baseline 80% AFUE Single-stage Forced-air Furnace VTH Gas Performance Characterizations

.................................................................................................................................................................... 13


v

Figure 8 – Baseline 80% AFUE Single-stage Forced-air Furnace VTH Electric Performance

Characterizations ......................................................................................................................................... 13

Figure 9 – Baseline 95% AFUE Single-stage Forced-air Furnace VTH Gas Performance Characterizations

.................................................................................................................................................................... 13

Figure 10 – Baseline 95% AFUE Single-stage Forced-air Furnace VTH Electric Performance


Figure 11 – California Weather Zone 16 Combi System Outdoor Temperature Reset Strategy ................ 14

Figure 12 – California Weather Zone 9 Combi System Outdoor Temperature Reset Strategy .................. 14

Figure 13 – Forced-air Condensing Tankless Combi VTH Space Heating Gas Performance


Figure 14 – Forced-air Condensing Tankless Combi VTH Space Heating Electric Performance


Figure 15 – Baseline 0.62 UEF 40-gallon Atmospherically Vented Water Heater VTH Gas Performance


Figure 16 – Forced-air Condensing Tankless Combi VTH/LHC Gas Performance Characterizations ...... 17

Figure 17 – Forced-air Condensing Tankless Combi VTH Water Heater Electric Performance


Figure 18 – Building Simulation Geometries and Orientations (Two Building Types Two Orientations) 18

Figure 19 – California Weather Zones ........................................................................................................ 18

Figure 20. Comparison of Modeled Space Heating Load Predictions to RASS Averages and DOE IECC

2006 Prototype Residential Building Models ............................................................................................. 19

Figure 21. Comparison of Modeled Water Heating Load Predictions to RASS Averages and DOE IECC

2006 Prototype Residential Building Models ............................................................................................. 19

Figure 22 – Leaving Air Temperature Gradient ......................................................................................... 24

Figure 23 – Leaving Air Mass Flow Gradient ............................................................................................ 24


6

1.0 INTRODUCTION/BACKGROUND

As building envelope performance for new homes continues to improve, new gas forced-air combined space

and water heating (combi) technologies have emerged. They can serve the home with one condensing tankless

water heater (TWH) and an air handler unit (AHU) providing a single energy efficiency measure that can be

more efficient for space heating and domestic hot water (DHW) than traditional baseline equipment such as

separate furnaces and water heaters.

For this project, GTI has focused gas space heating research on gaining an understanding of exactly why system

performance suffers in part-load conditions, and more importantly, what can be done to improve low-load

performance. GTI believes forced-air combis using condensing tankless water heaters offer a unique

opportunity to improve gas space heating performance at very low loads by controlling water and air flows, and

temperatures to modulate capacity. Figure 1 is presented in tandem with Figure 2, and shows the frequencies at

which space heating systems would operate at various space heating loads in two representative homes

including a 1,700 sq-ft single-story home (Type 1) and 2,500 sq-ft two-story home (Type 2) in California

climate zone 16, Portland, Oregon, and Seattle, Washington. Figure 2 shows part-load performances of a

single-stage condensing and a single-stage non-condensing furnace, and an advanced tankless forced-air combi.

The figure is highlighted yellow where space heating loads occur most frequently in the climates as depicted by

the histograms in Figure 1. Together, these figures illustrate why it is important for space heating systems to

operate efficiently at low loads and why combis offer tremendous opportunity for energy savings.

Figure 1 – Part Load Space Heating Histograms

Figure 2 – Space Heating System Performance

Part of GTI’s gas space heating research uses a novel method of testing to determine part-load performance of

residential appliances. This method is GTI’s Virtual Test Home (VTH), which consists of multiple test rigs and

associated algorithms that simulate real world conditions in a controlled, repeatable laboratory environment.

The VTH can be used for gas and electric space and water heating equipment. Unlike the Annual Fuel

Utilization Efficiency (AFUE) and Heating Seasonal Performance Factor (HSPF) that are typical single-rating

point methods for space heating equipment, the VTH provides performance characterizations across a wide

range of part-loads ranging from 1-100% and at various temperatures. The performance characterizations are

then incorporated in building energy modeling software, such as EnergyPlus, to estimate annual appliance

efficiencies, and quantify potential annual energy, emissions, and cost savings for various equipment in various

buildings and climates.


7

2.0 TECHNOLOGY DESCRIPTION

Figure 3 shows the general configuration of a gas

forced-air tankless combi consisting of a hydronic

AHU, TWH and external pump. The primary

heating section of the AHU is a hot water coil heat

exchanger. With reasonably short and insulated

piping runs, the entering water temperature (EWT)

into the coil is roughly the same as the TWH outlet

temperature; and the leaving water temperature

(LWT) returning to the TWH varies based on water

flows and airflows. The entering air temperature

(EAT) into the coil is about the same as the return

air from the conditioned space. Air flows across the

coil and picks up heat to supply warm air to the

space at the leaving air temperature (LAT). DHW

is tapped directly into the hot water loop between

the AHU and TWH.

Figure 3 – Forced-air Tankless Combi Configuration

Condensing tankless water heaters will condense

only if enough thermal energy is removed to cool

the flue gas below its dewpoint temperature. That is

because moisture in combustion gas begins to

condense out of the gas at the dewpoint temperature,

which varies depending on combustion excess air

(Figure 4). Some TWHs will operate at higher

dewpoint temperatures than others. Those high

dewpoint (>120°F) TWHs are well-suited for

forced-air tankless combi applications because they

can achieve condensing operation at high firing rate

turndowns.

Figure 4 – Flue Gas Dewpoint Temperatures

Advanced forced-air tankless combi technologies can achieve exceptionally high space heating performance at

very low part-loads if the combi systems have the following attributes:

1. The system implements a condensing TWH that can achieve dewpoint temperatures greater than 120°F.

This can be accomplished if the TWH employs a fully modulating burner.

2. The system can provide hot water at two temperature setpoints. This can be accomplished if the TWH

has the ability for a DHW temperature set point and a different space heating temperature setpoint, and

it switches between the setpoints automatically depending on the hot water demand.

3. The temperature difference (ΔT) across the AHU coil (EWT - LWT) is maximized thereby minimizing

LWTs to induce TWH condensing operation and maintain comfortable supply air LATs. This can be

accomplished if the AHU has a coil along with air and water flow controls that can reduce LWTs below

100°F while maintaining required space heating capacities at sufficient LATs.

4. The system can be configured to preheat the AHU and TWH heat exchangers before supply air is

delivered to the space. This can be accomplished if the AHU has the ability to delay blower operation

until the water pump has pushed hot water through the system and heated the coils.

5. The system can be configured to flush heat out of the AHU and TWH after space heating calls. This

can be accomplished if the AHU/TWH system has controls to keep the blower and pump on and the

TWH burner off until the space heating loop is reduced below about 90°F.

6. Short-cycling even at high turndowns is minimized. This can be accomplished if the system has

outdoor temperature reset – or if it can reduce its capacity as outdoor temperatures rise.


8

Using the six key qualifying attributes above, GTI conducted a forced-air tankless combi technology market

landscape assessment to identify commercial-ready combi technologies that can deliver exceptionally high

space heating performance in mild climates. The first two key qualifying combi system attributes involve the

TWH. First is the use of a condensing tankless water heater that can achieve dewpoint temperatures greater

than 120°F. These TWHs have negative pressure gas valves and fully modulating burners with 10:1 turndown

or better. The second key TWH attribute is the dual temperature setpoint capability for DHW and space

heating. TWHs with these attributes are readily available in the market and include the Navien NPE series and

Rinnai Sensei series TWH product lines. Moreover, those Navien and Rinnai TWH product lines can be

programmed to keep the burner off while the pump is on for heat flushing after space heating calls.

Qualifying combi system attributes 3 through 6 from the list above are related to the AHU and include the

hydronic coil design and associated air and water flow controls; blower and pump preheating and heat flush

controls; and outdoor temperature reset capabilities. Hydronic AHUs are prevalent in the market, but AHUs

with all of those attributes are not – primarily because few manufacturers recognize the market need for high

performance condensing tankless combi systems. iFLOW HVAC Inc. does recognize the need and

manufactures the only AHU GTI has identified with all the key qualifying attributes above. Table 1 shows the

attributes and shortfalls for each of the AHU product lines GTI researched. Some AHUs did not have hydronic

coils designed to generate enough ΔT to induce TWH condensing operation. As such, other attributes for those

AHUs were not investigated.

Table 1 – Air Handler Unit Market Landscape (Atributes and Shortfalls)

AHU

Manufacturer

Product

Series

Coil

ΔT

Coil

LATs

Preheat

Controls

Heat Flush

Controls

OAT

Reset

iFLOW IFLH Sufficient 130°F Yes Yes Yes

NTI GF200 Sufficient 130°F Yes No Yes

Bosch AHU Sufficient 100-110°F Yes No No

Redzone DVS Sufficient 100-110°F Yes No No

Rheem RW1PT Sufficient < 100°F Yes No No

Rheem RHWB Sufficient < 100°F Yes No No

First Co. HBQB 48 Sufficient < 100°F No No No

Comfort-aire AHG Insufficient - - - -

Ecologix ZCx/EC/ECR Insufficient - - - -

First Company CDX Insufficient - - - -

First Company Most HBQB Insufficient - - - -

First Company VMB Insufficient - - - -

Hi-Velocity All Insufficient - - - -

Magic Aire All Insufficient - - - -

Mortex All Insufficient - - - -

RedZone RXAH/HVR/ HVS Insufficient - - - -

Rosemex All Insufficient - - - -

ThermoPride All Insufficient - - - -

Williams All Insufficient - - - -


9

The iFLOW IFLH series air handler product line is offered in three sizes with nominal space heating capacities

at 140°F EWT of about 38 MBH, 48 MBH, and 60 MBH. All of the AHUs in the IFLH series are configured

with the attributes previously mentioned and can be paired with various sized fully modulating TWHs. As

such, the combi space heating capacities can range from about 5 MBH to over 60 MBH maintaining high

performance across that range.

Base AHU prices provided by iFLOW and including the external circulating pump range from about $2,200 to

$2,600. The iFLOW AHUs are comprised of off-the-shelf components adding up to about $2,500 in retail costs

for the 60 MBH AHU. A retail cost breakdown for the AHU components is shown in Table 2. Component

prices purchased at volume would likely be significantly lower than those shown in the table.

Table 2 – iFLOW AHU Component List

Component DHW Load

Hydronic coil $1,100

Blower fan $300

Blower motor $250

Control board (PCB) $310

Temperature instruments $110

Transformer/power $50

Cabinet $110

External pump/fittings $270

Total $2,500

3.0 METHODS FOR LABORATORY EVALUATIONS

Single-rating points such as AFUE provide a single performance metric for forced-air space heating systems at

their optimal operating condition. However, space heating systems rarely operate at optimal conditions.

Rather, they operate most of the time in part-load conditions where cycling and other factors negatively impact

performance. Part-load is defined as the ratio of load to capacity, occurring when space heating appliances

serve loads less than their design capacities. The lower the part-load, the shorter the heating cycles, and the

lower the efficiencies. To properly assess how efficient forced-air space heating systems serve building heating

loads, detailed performance characterizations are developed to define how the systems perform across a wide

range of part-load conditions where they operate most of the time.

For water heaters, the Uniform Energy Factor (UEF) evolved from the EF rating and attempts to better account

for real world water heater performance by implementing realistic draw patterns with typical usage of 84, 55,

38, and 10 gallons per day. However, field research has indicated the draw patterns utilized in the UEF test may

not fully represent real-world performance. To properly assess how efficient water heaters serve DHW loads,

detailed performance characterizations are developed to define how they perform across a wide range of DHW

draw rates.

Alternative methods for space heating, condensing water heating, and forced-air tankless combi testing

collectively referred to herein as Virtual Test Home (VTH) methods are used to develop repeatable part-load

performance characterizations of baseline and combi technologies. The VTH consists of multiple laboratory

test rigs and associated algorithms that simulate real world conditions in a controlled laboratory environment.

The performance characterizations are then implemented in building energy modeling software, such as

EnergyPlus to estimate annual appliance efficiencies, and quantify potential annual energy, emissions, and cost

savings for various equipment in various buildings and climates.


10

VTH System Diagram and Instrumentation Plan

Figure 5 shows the iFLOW AHU/TWH configuration along with an electric heat pump and refrigerant coil.

Combi evaluations for this project did not include the heat pump. However, the electric heat pump along with

the refrigerant “A-coil” are used for space cooling and can be used in a hybrid configuration with the TWH to

provide space heating in mild conditions. GTI is underway with further research on combis in this hybrid

configuration.

Figure 6 depicts the equipment and instruments included in the VTH laboratory test setup, delineates the system

components and subsystems, and identifies input and output energy streams for space heating and water heating

systems tested. The instruments used in the test setup identified in Figure 6 as well as data acquisition and

calculation methods associated with the instrument measurement points are provided in Appendix A.

Figure 5 – iFLOW Combi Configuration (Including Heat Pump)


11

Figure 6 – Virtual Test Home Test Rig Diagram

Baseline and Combi Equipment Evaluated

The combi system incorporating the iFLOW IFLH AHU and Navien TWH was evaluated in the VTH and

compared to an 80% AFUE furnace (Baseline 1) and a 95% AFUE furnace (Baseline 2) evaluated in the VTH

together with a 0.62 EF storage water heater. Make and models for the equipment evaluated for this project are

shown in Table 3.

Table 3 – Baseline and Combi Equipment Evaluated

Type Manufacturer Model Efficiency

Single-stage ECM Furnace Goodman GMVC8 80% AFUE

Single-stage ECM Furnace Rheem R95T 95% AFUE

Tank Water heater Rheem XG40T 0.62 EF

Modulating Combi iFLOW + Navien IFLH-180000 0.94 TPF

Tankless Water Heater Navien NPE-180S 0.97 UEF


12

4.0 LABORATORY TEST RESULTS

Forced-air Space Heating Performance Characterizations

The experimental methods employed in the VTH for space heating are, collectively, an alternative to ASHRAE

standard test method 103 for estimating seasonal furnace performance (AFUE). Forced-air systems are

configured in a laboratory test setup to draw preconditioned air from the laboratory into the forced-air system,

and dump the heated supply air out of the laboratory. Algorithms developed specifically for single-stage, two-

stage, and modulating forced-air system evaluations are used to control on/off or modulating thermostat calls.

Data sets of direct energy input and output measurements are collected for each forced-air system operating

under incremental part-loads, including 1%, 5%, 10%, 15%, 20%, 30%, and continuing at 10% intervals to

100%. For example, a 30,000 Btu/hr (30 MBH) forced-air system operating at 10% part-load, would have to

deliver 3,000 Btu/hr (3 MBH) for that test. The data sets are then used to calculate part-load thermal

efficiencies based on energy input from natural gas and energy output delivered in supply air (reconciled with

water-side measurements for combis). Those thermal efficiencies are then used to develop part-load

performance characterizations for each forced-air system. Annual heating therms are then calculated using

building energy modeling software applying the performance characterizations together with weather data and

hourly load calculations for specific climates and buildings.

Steady state measurements for calculating combustion efficiencies are also collected under the same

incremental part-loads. Combustion efficiency is a key performance calculation for determining AFUE per the

AFUE 103 standard. It is calculated assuming output energy equals input energy minus the stack losses.

Combustion efficiencies are determined by measuring the temperature and oxygen content of the exhaust gases,

as well as condensate production while the forced-air systems operate under part-load conditions at steady-state.

Combustion efficiencies are calculated to validate thermal efficiencies at certain conditions in the VTH.

The VTH laboratory test setup is designed such that energy in warm air delivered by the forced-air system is

determined using accurate airflow measurements along with supply- and return-air temperature measurements.

However, that energy is not delivered to a conditioned space, like it would be in a home. Rather, the test setup

is designed to expel the warm air from the laboratory space. As such, the forced-air system does not operate on

actual calls from its thermostat. Instead, for each incremental part-load test, algorithms in a computer-

generated, or virtual home are used to calculate room air temperatures and simulate thermostat cycle times

(on/off or modulation calls depending on the type of system). During the on cycles, the amount of energy

delivered in warm air by the forced-air system is calculated every five seconds using measurements from the

test setup. Knowing the energy delivered at 5-second intervals, and the required heating demand for the

particular part-load test, room air temperatures in the virtual home are calculated every five seconds as well. It

is important to note, conditioned space volume is also needed to determine how the room air temperatures

change. Part-loads are based on specific building reference models. Therefore, building volume and

construction are arbitrarily defined by that reference model. The model used to define the volume can be

arbitrary as only the resulting cycle times are important to map the part-load space heating performance for any

given forced-air system design.

Thermostats operate based on room air temperature set points and deadbands, which are adjustable. The

thermostat deadband is important because space heating efficiencies are negatively affected by smaller

deadbands. The smaller the deadband, the shorter the space heating cycle time, and the lower the efficiency.

The deadband allows the forced-air system to cycle on or modulate between allowable minimum and maximum

room air temperatures. The cycling algorithms for baseline and combi research used a 2°F deadband, allowing

the room air temperature in the virtual home to be between 65°F and 67°F. Therefore, the forced-air systems

come on at 65°F and shut off at 67°F. The thermal efficiency for each incremental part-load test is based on an

average of three full on/off cycles.


13

Baseline – 80% and 95% AFUE Single-stage Forced-air Furnace Performance Characterizations

The 80% AFUE and 95% AFUE single-stage furnaces were configured in the VTH laboratory test setup to

characterize their performance. Algorithms developed specifically for single-stage furnace evaluations were

used to control the on/off and thermostat calls. Datasets of direct energy input and output measurements were

collected for the furnaces operating under incremental part-loads. The datasets were then used to calculate part-

load performance characterizations. The gas performance characterizations for the baseline furnaces run

through the VTH are shown in Figure 7 (80% AFUE) and (95% AFUE). For ease of modeling, Electric

performance is defined as a percentage of the gas consumption at each part-load and those are shown in Figure

8 (80% AFUE) and (95% AFUE). Electricity includes all power to the furnace.

Figure 7 – Baseline 80% AFUE Single-stage

Forced-air Furnace VTH Gas Performance

Characterizations


Forced-air Furnace VTH Electric Performance

Characterizations


Forced-air Furnace VTH Gas Performance

Characterizations


Forced-air Furnace VTH Electric Performance

Characterizations

Energy Efficiency Measure – Forced-air Condensing Tankless Combi Space Heating Performance

Characterizations

The forced-air tankless combi was configured with a 160 MBH condensing TWH rated at 0.94 UEF that could

achieve dewpoint temperatures greater than 120°F. The forced-air tankless combi was also configured with an

AHU that had a hydronic coil along with air and water flow controls that could reduce AHU LWTs below

100°F while maintaining required space heating capacity. The AHU/TWH combi system had controls to keep

the ECM blower and pump on and the TWH burner off until the space heating loop could be reduced below


14

90°F; and the AHU/TWH combi system had outdoor temperature reset that allowed it to reduce its capacity

upon rising outdoor air temperatures (OAT). Two examples of the combi system outdoor temperature reset

strategy are shown in Figure 11 and Figure 12. Figure 11 shows the combi outdoor temperature reset for

California weather zone 16. The maximum space heating capacity for the one-story reference model in that

zone was about 33,000 Btu/hr (33 MBH) at about 12°F OAT. The building space heating load linearly

decreases to 0 MBH at about 63°F as (shown by the orange line). The combi system operated at about 23 MBH

output down to about 25°F OAT (shown by the grey line) at which point it began to step up in capacity to about

33 MBH at 12°F OAT. Figure 12 shows the combi outdoor temperature reset for California weather zone 9.

The maximum space heating capacity for the one-story reference model in that zone was about 12 MBH at

about 32°F OAT. For that model, the combi system operates at about 23 MBH and never increases in capacity

because the heating demand never exceeds 23 MBH. For reference, Table 4 shows the peak annual space

heating demands by zone as modeled.

The forced-air condensing tankless combi was configured in the VTH laboratory test setup to characterize its

performance. The same algorithms developed for the single-stage furnace evaluations were used to control the

on/off and thermostat calls for the combi system. Datasets of direct energy input and output measurements

were collected for each operating capacity under incremental part-loads. The datasets were then used to

calculate part-load performance characterizations for each capacity as shown in Figure 13. For ease of

modeling, Electric performance is defined as a percentage of the gas consumption. For the forced-air tankless

combi in space heating mode, electric accounts for power supplied to the AHU and TWH including hydronic

pumping power and is shown in Figure 14.

Figure 11 – California Weather Zone 16 Combi

System Outdoor Temperature Reset Strategy

Figure 12 – California Weather Zone 9 Combi

System Outdoor Temperature Reset Strategy


15

Figure 13 – Forced-air Condensing Tankless

Combi VTH Space Heating Gas Performance

Characterizations

Figure 14 – Forced-air Condensing Tankless

Combi VTH Space Heating Electric Performance

Characterizations

Table 4 – Peak Annual Modeled Heating Loads by Zone

Water Heating Performance Characterizations

Baseline – Tank-type Atmospherically Vented Water Heater Performance Characterizations

Building America is an industry-driven research program sponsored by the U.S. Department of Energy (DOE)

that applies systems engineering approaches to accelerate the development and adoption of advanced building

energy technologies in new and existing residential buildings. The House Simulation Protocols (HSP)

document* provides guidance on analysis methods that are proven to be effective and reliable in investigating

* NREL. 2014 Building America House Simulation Protocols Retrieved from

https://www.nrel.gov/docs/fy14osti/60988.pdf

Zones 2-story 1-story

CZ01 20,636 15,722

CZ02 18,990 14,809

CZ03 17,388 14,299

CZ04 18,629 14,899

CZ05 20,271 16,903

CZ06 12,648 10,277

CZ07 13,754 10,737

CZ08 13,709 10,954

CZ09 13,352 10,215

CZ10 13,722 9,903

CZ11 19,514 14,466

CZ12 20,566 15,132

CZ13 20,669 15,328

CZ14 24,692 18,127

CZ15 11,072 8,265

CZ16 30,370 23,208

Peak Annual Heating Demands


16

the energy use of advanced energy systems including water heaters. Baseline atmospherically vented water

heater performance characterizations are well developed in the HSP and were used for the baseline water heater

performance characterizations. The gas performance characterizations for the baseline water heater are shown

in Figure 15.

Figure 15 – Baseline 0.62 UEF 40-gallon Atmospherically Vented Water Heater VTH Gas Performance

Characterizations

Energy Efficiency Measure – Forced-air Condensing Tankless Combi Water Heating Performance

Characterizations

As with combi space heating, the experimental methods employed for evaluating tankless combi water heating

capabilities also used the VTH. For combi water heating, the VTH methods are collectively an alternative to

ASHRAE standardized test method 118.2 for rating water heaters. The VTH is used to conduct simulated-use

tests under controlled conditions with water draw events at various conditions. Efficiencies are calculated using

an input-output method rather than a uniform energy factor (UEF) as applied in ASHRAE 118.2.

A series of 20 water heating tests are conducted for each of the combis to develop efficiency profiles for various

DHW draws. Data sets of direct energy input and output measurements are collected for the combis operating

at 1, 2, 3, and 4 gpm draws for 1, 3, 5, 7, and 10 minutes. Between each scenario, the water heater is cooled

down to about 65°F. This VTH method provided data points for the TWH up to about 22,000 Btu of daily

DHA load as shown in Figure 16. The VTH TWH data corresponds very well with the thermodynamic model

originally developed by NREL called the Lumped Heat Capacity (LHC) model.† The advantage of this model

is that it can be used to accurately predict energy consumption of the TWH when subjected to realistic draw

patterns and mains temperatures, which can be done in the VTH, but would be time consuming. Figure 16

shows the VTH data overlaid on the LHC model data resulting in a combined VTH/LHC performance

characterization for the TWH. For the forced-air tankless combi in water heating mode, electric accounts for

power supplied to the TWH only when it is providing DHW, and is shown in Figure 17.

† J. Burch, J. Thornton, M. Hoeschele, D. Springer and A. Rudd, "Preliminary Modeling, Testing, and Analysis of a Gas

Tankless Water Heater NREL/CP-550-42917," NREL, 2008.


17

Figure 16 – Forced-air Condensing Tankless Combi VTH/LHC Gas Performance Characterizations

Figure 17 – Forced-air Condensing Tankless Combi VTH Water Heater Electric Performance

Characterizations

5.0 BUILDING ENERGY MODELING RESULTS

The building models used to estimate energy savings were developed using BEopt‡. Two building types were

developed and simulated for two different orientations, pictured in Figure 18, for each of the 16 California

climate zones shown in Figure 19. The Type 1 building represents a high use scenario and the Type 2

represents a low use scenario. Two orientations were used to account for differences in annual loads that would

occur due to variations in external building loads such as solar irradiation and infiltration. The characteristics of

both building types are summarized in Table 5.

Both building types were configured with a forced-air condensing combi utilizing the 160 MBH condensing

TWH and AHU with hydronic coil. To account for higher DHW demands in the larger 2-story Type 1 building,

‡NREL. (2017). BEopt 2.8. Retrieved from https://beopt.nrel.gov/

https://beopt.nrel.gov/


18

a second standalone TWH was used in addition to the one used for the combi system. Additional Baseline and

Energy Efficiency Measure assumptions are provided in Appendix B.

Figure 18 – Building Simulation Geometries and

Orientations (Two Building Types Two Orientations)

Figure 19 – California Weather Zones

Table 5. Summary of building model characteristics for Type 1 and Type 2

Characteristic Building Type 1 Building Type 2

Finished sq-ft 2,500 1,700

Stories 2 1

Bedrooms 5 3

Bathrooms 3 1.5

Garage 2-car attached N/A

Foundation Crawl space

Vintage ~2005

Construction/schedules/internal loads

According to 2008 CEC Residential Alternative Calculation Method (ACM) Approval Method, where applicable

Default assumptions from BEopt 2.8, i.e., 2014 Building America House Simulation

Protocols, were used where other sources of information were not available

Hourly space heating and domestic hot water loads were generated using these building models. The loads were

then post-processed using the performance characterizations developed in the VTH and otherwise previously

described to estimate baseline and combi systems energy consumption. For validation, the average baseline

space heating and domestic hot water energy consumption of the models were compared to values reported in

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwiuqOK8wPviAhVPnKwKHVodD7YQjRx6BAgBEAU&url=/url?sa%3Di%26rct%3Dj%26q%3D%26esrc%3Ds%26source%3Dimages%26cd%3D%26ved%3D%26url%3Dhttps://engineering.purdue.edu/DLAT/help/cacz.htm%26psig%3DAOvVaw23bKZlV1BatReVhxNp3sVw%26ust%3D1561238418606965&psig=AOvVaw23bKZlV1BatReVhxNp3sVw&ust=1561238418606965


19

the 2009 California Residential Appliance Saturation Study (RASS) and DOE IECC 2006§ prototype residential

building models for California. The comparisons are summarized in Figure 20 and Figure 21.

Figure 20. Comparison of Modeled Space

Heating Load Predictions to RASS Averages and

DOE IECC 2006 Prototype Residential Building

Models

Figure 21. Comparison of Modeled Water

Heating Load Predictions to RASS Averages and

DOE IECC 2006 Prototype Residential Building

Models

The modeled predictions compare favorably with the results of the RASS. Discrepancies can be attributed to

two primary factors. First, there are inconsistencies between the zones used in the RASS and the 16 California

climate zones. A best effort was made to match the zones where possible, however it cannot be determined that

the climate regions represented match exactly. Second, the RASS energy use data is averaged for a particular

region while the model predictions are averaged for the two types of buildings.

The results of the IECC 2006 prototype building models are on average higher than the predictions of the

project models. The IECC 2006 prototype models are 2,400 sq-ft models, use Typical Meteorological Year 3

data**, and use different modeling assumptions than those in the House Simulation Protocols. However, the

orders of magnitude and trends with heating degree hours are consistent. The comparison of the present model

predictions and literature values indicate that the models provide a reasonable estimate of energy consumption

in all California climate zones.

§Department of Energy (DOE) – Residential Prototype Building Models – (2019) Retrieved from:

https://www.energycodes.gov/development/residential/iecc_models

** NREL. (2015). Typical Meteorological Year 3. Retrieved from National Solar Radiation Data Base:

http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3

https://www.energycodes.gov/development/residential/iecc_models


20

Energy Savings Predictions Between Baselines and Combi Energy Efficiency Measure

The modeled predictions shown in Table 6 and Table 7 are for building Type 1 and Type 2 respectively with an

80% AFUE furnace and 0.62 EF storage water heater (Baseline 1) compared to the Navien+iFLOW combi

system.

The modeled predictions shown in Table 8 and Table 9 are for building Type 1 and Type 2 respectively with a

95% AFUE furnace and 0.62 EF storage water heater (Baseline 2) compared to the Navien+iFLOW combi

system.



Gas Electricity Total Gas Electricity Total Gas Electricity Total Gas Electricity Total

1 56.76 1.04 57.80 34.54 1.18 35.72 22.22 -0.13 22.09 39% -13% 38%

2 45.34 0.75 46.09 26.45 0.82 27.27 18.89 -0.07 18.82 42% -9% 41%

3 38.96 0.54 39.50 22.45 0.62 23.06 16.51 -0.08 16.44 42% -14% 42%

4 38.16 0.55 38.71 21.90 0.61 22.51 16.26 -0.06 16.20 43% -11% 42%

5 36.38 0.46 36.85 20.28 0.51 20.79 16.10 -0.04 16.06 44% -9% 44%

6 26.64 0.22 26.86 14.54 0.26 14.81 12.10 -0.04 12.06 45% -20% 45%

7 24.24 0.16 24.41 13.02 0.20 13.22 11.22 -0.04 11.19 46% -22% 46%

8 25.71 0.21 25.92 13.77 0.24 14.01 11.94 -0.03 11.91 46% -15% 46%

9 25.77 0.23 25.99 13.95 0.27 14.22 11.81 -0.04 11.77 46% -18% 45%

10 27.33 0.28 27.61 14.60 0.30 14.90 12.73 -0.02 12.71 47% -9% 46%

11 47.13 0.86 47.99 28.09 0.94 29.03 19.04 -0.08 18.96 40% -9% 40%

12 43.63 0.73 44.35 25.56 0.80 26.36 18.07 -0.07 18.00 41% -10% 41%

13 39.11 0.60 39.71 21.56 0.66 22.22 17.56 -0.07 17.49 45% -11% 44%

14 41.56 0.66 42.22 24.21 0.73 24.94 17.35 -0.07 17.28 42% -11% 41%

15 20.27 0.19 20.46 10.00 0.21 10.21 10.26 -0.02 10.24 51% -13% 50%

16 86.79 1.96 88.75 56.25 2.03 58.28 30.54 -0.07 30.46 35% -4% 34%

California

Weather Zone

Baseline 1 (MMBTU)

Two-story Building Model

Savings (MMBTU) Savings (Percentage)EE Measure (MMBTU)


1 53.39 1.08 54.48 31.52 1.23 32.75 21.87 -0.15 21.72 41% -14% 40%

2 41.01 0.75 41.76 22.81 0.82 23.64 18.20 -0.08 18.12 44% -10% 43%

3 35.50 0.56 36.06 19.46 0.65 20.11 16.04 -0.09 15.96 45% -15% 44%

4 34.58 0.56 35.14 18.83 0.63 19.46 15.75 -0.07 15.68 46% -12% 45%

5 34.02 0.52 34.54 18.10 0.58 18.68 15.92 -0.06 15.86 47% -11% 46%

6 23.70 0.25 23.95 11.99 0.30 12.29 11.71 -0.05 11.66 49% -20% 49%

7 20.69 0.17 20.86 10.04 0.21 10.25 10.65 -0.04 10.61 51% -21% 51%

8 22.43 0.23 22.66 10.99 0.26 11.25 11.44 -0.03 11.41 51% -15% 50%

9 22.48 0.24 22.72 11.18 0.28 11.46 11.31 -0.04 11.26 50% -18% 50%

10 23.84 0.28 24.13 11.67 0.31 11.98 12.17 -0.03 12.14 51% -9% 50%

11 40.86 0.79 41.65 23.02 0.87 23.89 17.84 -0.08 17.76 44% -10% 43%

12 38.29 0.69 38.98 21.18 0.76 21.95 17.11 -0.07 17.04 45% -11% 44%

13 33.92 0.57 34.48 17.74 0.63 18.38 16.17 -0.07 16.11 48% -12% 47%

14 36.74 0.63 37.37 20.17 0.72 20.88 16.57 -0.09 16.49 45% -14% 44%

15 17.55 0.19 17.74 7.73 0.22 7.95 9.81 -0.02 9.79 56% -13% 55%

16 77.13 1.82 78.95 47.77 1.92 49.68 29.36 -0.10 29.26 38% -5% 37%

One-story Building Model

California

Weather Zone

Baseline 1 (MMBTU) Savings (MMBTU) Savings (Percentage)EE Measure (MMBTU)


21




1 45.58 1.04 46.63 34.54 1.18 35.72 11.05 -0.13 10.91 24% -13% 23%

2 38.70 0.75 39.45 26.45 0.82 27.27 12.25 -0.07 12.18 32% -9% 31%

3 34.09 0.54 34.63 22.45 0.62 23.06 11.64 -0.08 11.57 34% -14% 33%

4 33.35 0.55 33.90 21.90 0.61 22.51 11.45 -0.06 11.39 34% -11% 34%

5 31.94 0.46 32.41 20.28 0.51 20.79 11.66 -0.04 11.62 37% -9% 36%

6 24.77 0.22 24.99 14.54 0.26 14.81 10.22 -0.04 10.18 41% -20% 41%

7 22.91 0.16 23.08 13.02 0.20 13.22 9.90 -0.04 9.86 43% -22% 43%

8 23.89 0.21 24.10 13.77 0.24 14.01 10.12 -0.03 10.09 42% -15% 42%

9 23.97 0.23 24.19 13.95 0.27 14.22 10.01 -0.04 9.97 42% -18% 41%

10 24.94 0.28 25.22 14.60 0.30 14.90 10.34 -0.02 10.31 41% -9% 41%

11 39.90 0.86 40.76 28.09 0.94 29.03 11.82 -0.08 11.74 30% -9% 29%

12 37.35 0.73 38.08 25.56 0.80 26.36 11.79 -0.07 11.72 32% -10% 31%

13 34.28 0.60 34.87 21.56 0.66 22.22 12.72 -0.07 12.65 37% -11% 36%

14 35.66 0.66 36.31 24.21 0.73 24.94 11.45 -0.07 11.37 32% -11% 31%

15 19.08 0.19 19.27 10.00 0.21 10.21 9.08 -0.02 9.05 48% -13% 47%

16 70.31 1.96 72.27 56.25 2.03 58.28 14.06 -0.07 13.98 20% -4% 19%

EE Measure (MMBTU)

Two-story Building Model

Savings (MMBTU) Savings (Percentage)California

Weather Zone

Baseline 2 (MMBTU)


1 42.03 1.08 43.11 31.52 1.23 32.75 10.51 -0.15 10.36 25% -14% 24%

2 34.44 0.75 35.18 22.81 0.82 23.64 11.62 -0.08 11.55 34% -10% 33%

3 30.51 0.56 31.07 19.46 0.65 20.11 11.05 -0.09 10.97 36% -15% 35%

4 29.70 0.56 30.26 18.83 0.63 19.46 10.87 -0.07 10.80 37% -12% 36%

5 29.21 0.52 29.73 18.10 0.58 18.68 11.10 -0.06 11.05 38% -11% 37%

6 21.65 0.25 21.89 11.99 0.30 12.29 9.66 -0.05 9.61 45% -20% 44%

7 19.32 0.17 19.49 10.04 0.21 10.25 9.28 -0.04 9.25 48% -21% 47%

8 20.53 0.23 20.75 10.99 0.26 11.25 9.53 -0.03 9.50 46% -15% 46%

9 20.62 0.24 20.85 11.18 0.28 11.46 9.44 -0.04 9.40 46% -18% 45%

10 21.43 0.28 21.71 11.67 0.31 11.98 9.75 -0.03 9.73 46% -9% 45%

11 34.23 0.79 35.02 23.02 0.87 23.89 11.21 -0.08 11.13 33% -10% 32%

12 32.36 0.69 33.05 21.18 0.76 21.95 11.18 -0.07 11.11 35% -11% 34%

13 29.35 0.57 29.91 17.74 0.63 18.38 11.60 -0.07 11.54 40% -12% 39%

14 31.07 0.63 31.70 20.17 0.72 20.88 10.91 -0.09 10.82 35% -14% 34%

15 16.35 0.19 16.54 7.73 0.22 7.95 8.62 -0.02 8.59 53% -13% 52%

16 61.56 1.82 63.38 47.77 1.92 49.68 13.79 -0.10 13.69 22% -5% 22%

Baseline 2 (MMBTU) EE Measure (MMBTU)

One-story Building Model

California

Weather Zone

Savings (MMBTU) Savings (Percentage)


22

APPENDIX – INSTRUMENTATION, DATA ACQUISITION, AND CALCULATIONS

Instrumentation

ID Parameter Instrument Range Accuracy

T01

Water Heater

Conditioned Water

Temp

T-Type Closed End Thermocouple

Omega

CP-SS-18-E-12

–100 to 250°F at >32 to 662°F

±1.8°F or 0.75%

T02 Water Heater Supply

Water Temp


Omega

CP-SS-18-E-12

–100 to 250°F at >32 to 662°F

±1.8°F or 0.75%

T03 Water Heater Hot

Water Temp


Omega

CP-SS-18-E-12

–100 to 250°F at >32 to 662°F

±1.8°F or 0.75%

T04 Water Heater Tempered

Water Temp


Omega

CP-SS-18-E-12

–100 to 250°F at >32 to 662°F

±1.8°F or 0.75%

T05 Water Heater Manifold

Temp


Omega

CP-SS-18-E-12

–100 to 250°F at >32 to 662°F

±1.8°F or 0.75%

T06 Water Heater Exhaust

Temp

Open-Ended Direct Exposure RTD

Omega

P-L-A-1/8-6-0-T-3

–100 to 250°F ± 0.65°F at 130°F

T20-T25 Water Heater Tank

Temp

T-Type Insulated Thermocouples

KK-T-20-36 –100 to 250°F

at >32 to 662°F

±1.8°F or 0.75%

F01 Water Heater Gas Flow Gas Flow Diaphragm Meter

Elster American Meter AC-250

0 to 656 SCFH

(5 psig) ± 0.5%

F02 Water Heater Water

Flow

Low Flow Water Meter

Seametrics SEB-075 0.2 to 18 GPM ± 1%

J01 Water Heater Power WattNode Pulse

WNB-3Y-208P

48 to 62Hz

at -20% to +15%

Voltage

± 0.5%

T07 Hydronic Loop

Entering Water Temp


Omega

CP-SS-18-E-12

–100 to 250°F at >32 to 662°F

±1.8°F or 0.75%

T08 Hydronic Loop Leaving

Water Temp


Omega

CP-SS-18-E-12

–100 to 250°F at >32 to 662°F

±1.8°F or 0.75%

F03 Hydronic Loop Water

Flow

Low Flow Water Meter

Seametrics SEB-075 0.2 to 18 GPM ± 1%

T09 Furnace/AHU Exhaust

Temp

Open-Ended Direct Exposure RTD

Omega P-L-A-1/8-6-0-T-3 –100 to 250°F ± 0.65°F at 130°F

T30-T38 Furnace/AHU Entering

Air Temp


KK-T-20-36 –100 to 250°F

at >32 to 662°F

±1.8°F or 0.75%

T40-T48 Furnace/AHU Leaving

Air Temp


KK-T-20-36 –100 to 250°F

at >32 to 662°F

±1.8°F or 0.75%

F04 Furnace/AHU Gas

Flow

Gas Flow Diaphragm Meter

Elster American Meter AC-250

0 to 656 SCFH

(5 psig) ± 0.5%

J02 Furnace/AHU Power WattNode Pulse 48 to 62Hz ± 0.5%


23

WNB-3Y-208P at -20% to +15%

Voltage

DP01 Furnace/AHU Total

Static Pressure

Low Differential Pressure

Transmitter

Dwyer 610-01A-DDV

0” to 1” wc ±0.25%

F05

Furnace/AHU

Conditioned Air Flow

Low

Duct-Mounted Air Flow

Measurement Station

Dwyer FLST-C8

100 to 10,000

FPM ± 2%

F06

Furnace/AHU


High

Duct-Mounted Air Flow

Measurement Station

Dwyer FLST-C10

100 to 10,000

FPM ± 2%

DP02

Furnace/AHU


Differential Pres Low


Transmitter

Dwyer 607-2

0” to 0.5” wc ±0.5%

DP03

Furnace/AHU


Differential Pres Hi


Transmitter

Dwyer 607-2

0” to 0.5” wc ±0.5%

T50-

T511

Outdoor Unit Ambient

Temp

T-type Insulated Thermocouples

KK-T-20-36 –100 to 250°F

at >32 to 662°F

±1.8°F or 0.75%

RH01 Outdoor Unit Ambient

RH

Humidity and Temperature

Transmitter

Vaisala HMT-120

0% to 100% RH

-40 to 176°F

at 0 to 90 %RH

±1.5 %RH

at 59 °F to 77 °F

±0.36 °F

at 32 °F to 59 °F and at

77 °F to 104 °F

±0.45 °F

at -40 °F to 32 °F and at

104 °F to 176 °F)

±0.72 °F

J03 Outdoor Unit Power WattNode Pulse

WNB-3Y-208P

48 to 62Hz

at -20% to +15%

Voltage

± 0.5%

Data Acquisition and Calculations

Measured data from all of the instruments listed in the table above were continuously collected and recorded at

5-sec intervals. All calculations were post-processed using the raw data from the data acquisition system as

follows:

Space Heating Energy Input

The following basic equation was used to calculate energy input to the systems in natural gas:

NGVHHVQ =NG

Where:

QNG = Energy input from natural gas (Btu/day or Btu/hr)

HHV = Higher heating value (HHV) of natural gas (Btu/ft3)


24

NGV = Volumetric flow rate of natural gas (ft3/day or ft3/hr)

The gas meter used to measure volumetric flow was temperature compensated. Gas pressure was recorded

before each test and the flow rate was corrected for the actual pressure.

Space heating Energy Output

Return air to the forced-air system (entering air temperature) is controlled using two two-row water coils in the

duct upstream of the forced-air system inlet. An air-to-water heat pump is used to supply hot or cold water to

the coil at a constant temperature, which is set depending on the laboratory room air and outdoor ambient

temperatures. Laboratory room air is preconditioned to about 65˚F so the conditioning coil is used only to trim

the return air temperature down.

Forced-air system leaving air temperatures are measured using a nine-point horizontal thermocouple array. In

addition, leaving airflows are measured at those same nine points. Forced-air testing research has found

temperatures and airflow measurements have distinctly similar gradients across the horizontal measurement

plane. Figure 22 shows an example of a temperature gradient with red being the hottest and blue the coldest.

Figure 23 shows an example of the corresponding mass flow gradient with green being the highest flow and

blue the lowest. These gradients show a clear correlation between temperature and mass flow. As such, rather

than taking an average across the thermocouple array, thermocouples are proportionately weighted by mass

flow. For example, the thermocouple that receives the highest weighting is the one at the point where mass

flow is the highest.

Figure 22 – Leaving Air

Temperature Gradient

Figure 23 – Leaving Air Mass

Flow Gradient

Heat supplied by the furnace/AHU was determined by measuring the air flow rate and supply and return air

temperatures at the furnace/AHU inlet/outlet as follows:

timeTcVCQ pltime = sup

Where:

Qsup = Summation of supplied heat by the furnace/AHU for each time step (Btu/hr or Btu/day)

𝑉𝑙 = Volumetric flow rate of air (ft3/min) calculated by:

𝑉𝑙 = 3.1415926 ∙ (𝐷

12)2∙1

4∙ (1096.7 ∙

𝑃

0.07368)1/2

Where:

P = Velocity Pressure at each recorded interval

D = Duct diameter for the flow measurement station (8”).


25

T = the difference between supply and return temperatures at each recorded interval (oF)

Cp = Specific heat of air at the average temperature between the supply and return air, (Btu/lb-oF)

ρ = Density of air based on air temperature at the flow meter for each recorded interval, (lb/ft3)

C = Unit conversion factor

time = time interval used in the data collection program (i.e. 5 second)

Space Heating Efficiency

Space heating efficiency (ηsh) was calculated as the ratio of the heat supplied by air to the energy carried by the

natural gas at the same time interval (i.e. hourly efficiency or daily efficiency), as shown in the following:

100sup

=

NG

shQ

Q

Where:

ηsh = Space heating efficiency (%)

Qsup = Total energy supplied by the space heating (Btu/day or Btu/hr)

QNG = Total natural gas energy input (Btu/hr or Btu/day)

Qe = Total electrical energy input (Btu/hr or Btu/day)

Note: Qe is added to the denominator when accounting for electrical energy. Results for this project are

reported with and without electrical energy used during the tests.

Water Heater Efficiency

The water heater was characterized with a daily efficiency (ηDHW) defined as the ratio of daily energy carried by

the water flowing out of the water heater to the daily water heater energy consumption (Qd). The energy carried

by the water was defined as the product of the domestic hot water draw flow and the temperature difference

between the city water and the water at the water supply (QDHW).

100=d

DHWDHW

Q

Q

( )citypDHWDHW TTCVQ −= sup

Where:

DHW = Daily efficiency of the water heater (%)

QDHW = Energy output from the water heater (Btu/day)

DHWV = Total volume of domestic hot water during the 24-hour test period (gal/day)

Cp = Specific heat of water at the average temperature between the supply and return water, (Btu/lb-oF)

ρ = Density of water based on water temperature at the flow meter for each recorded interval, (lb/ft3)

supT = Temperature of water flowing out of the water heater at each recorded interval (oF)


26

cityT = Temperature of water from conditioning tank to the water heater at each recorded interval (oF)

Qd = Daily water heating energy consumption (Btu/day)

Note: Reference ASHRAE 118.2 section 6.3.4 Daily Water Heating Energy Consumption for Qd calculation

methods. These methods include calculations for recovery efficiency per ASHRAE 118.2 section 6.3.2.

Results for this project are reported with and without electrical energy used during the tests.


27

APPENDIX B – MODELING MEASURES

Table 10 – Baseline and EE Measure Water Heater Assumptions

Table 11 – Baseline 1 and EE Measure Space Heating Assumptions

Table 12 – Baseline 2 and EE Measure Space Heating Assumptions

Water Heater Baseline EE Measure

Type 40 gallon tank Tankless Combi

Energy Factor 0.62 EF 0.94 EF

Recovery Efficiency 76% NA

Capacity 40 MBH 160 MBH

Supply water setpoint 135°F 140°F

Location Indoor Indoor

Hot water usage NREL. (2017). BEopt 2.8 modeling

Space Heating Baseline EE Measure

Type Furnace Tankless Combi

AFUE 80% AFUE NA


Thermostat set point 70°F Heating 70°F Heating


Space heating demand NREL. (2017). BEopt 2.8 modeling

Space Heating Baseline EE Measure

Type Condensing Furnace Tankless Combi

AFUE 95% AFUE NA


Thermostat set point 70°F Heating 70°F Heating


Space heating demand NREL. (2017). Beopt 2.8 modeling

Documents

Laboratory Test Report: iFlow/Navien Combi System Assessment · forced-air tankless combi applications because they can achieve condensing operation at high firing rate turndowns