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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
info@neea.org
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.
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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
Table 7 – Baseline 1: Building Type 2 Predicted Annual Gas and Electricity Consumptions and Savings
.................................................................................................................................................................... 20
Table 8 – Baseline 2: Building Type 1 Predicted Annual Gas and Electricity Consumptions and Savings
.................................................................................................................................................................... 21
Table 9 – Baseline 2: Building Type 2 Predicted Annual Gas and Electricity Consumptions and Savings
.................................................................................................................................................................... 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
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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
Characterizations ......................................................................................................................................... 13
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
Characterizations ......................................................................................................................................... 15
Figure 14 – Forced-air Condensing Tankless Combi VTH Space Heating Electric Performance
Characterizations ......................................................................................................................................... 15
Figure 15 – Baseline 0.62 UEF 40-gallon Atmospherically Vented Water Heater VTH Gas Performance
Characterizations ......................................................................................................................................... 16
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
Characterizations ......................................................................................................................................... 17
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
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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.
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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.
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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 - - - -
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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.
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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)
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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.
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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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
Figure 8 – Baseline 80% AFUE Single-stage
Forced-air Furnace VTH Electric Performance
Characterizations
Figure 9 – Baseline 95% AFUE Single-stage
Forced-air Furnace VTH Gas Performance
Characterizations
Figure 10 – Baseline 95% AFUE Single-stage
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
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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
LABORATORY TEST REPORT: iFLOW/NAVIEN COMBI SYSTEM ASSESSMENT
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.
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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/
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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
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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
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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.
Table 6 – Baseline 1: Building Type 1 Predicted Annual Gas and Electricity Consumptions and Savings
Table 7 – Baseline 1: Building Type 2 Predicted Annual Gas and Electricity Consumptions and Savings
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)
Gas Electricity Total Gas Electricity Total Gas Electricity Total Gas Electricity Total
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)
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Table 8 – Baseline 2: Building Type 1 Predicted Annual Gas and Electricity Consumptions and Savings
Table 9 – Baseline 2: Building Type 2 Predicted Annual Gas and Electricity Consumptions and Savings
Gas Electricity Total Gas Electricity Total Gas Electricity Total Gas Electricity Total
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)
Gas Electricity Total Gas Electricity Total Gas Electricity Total Gas Electricity Total
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)
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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
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%
T03 Water Heater Hot
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%
T04 Water Heater Tempered
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%
T05 Water Heater Manifold
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%
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
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%
T08 Hydronic Loop Leaving
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%
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
T-Type Insulated Thermocouples
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
T-Type Insulated Thermocouples
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%
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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
Conditioned Air Flow
High
Duct-Mounted Air Flow
Measurement Station
Dwyer FLST-C10
100 to 10,000
FPM ± 2%
DP02
Furnace/AHU
Conditioned Air Flow
Differential Pres Low
Low Differential Pressure
Transmitter
Dwyer 607-2
0” to 0.5” wc ±0.5%
DP03
Furnace/AHU
Conditioned Air Flow
Differential Pres Hi
Low Differential Pressure
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)
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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”).
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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)
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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.
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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
Capacity 80 MBH 60 MBH
Thermostat set point 70°F Heating 70°F Heating
Location Indoor Indoor
Space heating demand NREL. (2017). BEopt 2.8 modeling
Space Heating Baseline EE Measure
Type Condensing Furnace Tankless Combi
AFUE 95% AFUE NA
Capacity 56 MBH 60 MBH
Thermostat set point 70°F Heating 70°F Heating
Location Indoor Indoor
Space heating demand NREL. (2017). Beopt 2.8 modeling
Recommended