Heat Pump Water Heaters: Controlled Field Research of Impact on
Space Conditioning and Demand Response Characteristics
Graham Parker,1 Sarah Widder,1 Ken Eklund,2 Joe Petersen,1 and
Greg Sullivan3
1
Pacific Northwest National Laboratory
Washington State University Energy Program
3
Efficiency Solutions, LLC
2
Abstract
A new generation of heat pump water heaters (HPWH) has been introduced into the U.S. market that
promises to provide significant energy savings for water heating. Many electric utilities are promoting
their widespread adoption as a key technology for meeting energy conservation goals and reducing
greenhouse gas emissions. There is, however, considerable uncertainty regarding the space
conditioning impact of an HPWH installed in a conditioned space. There is also uncertainty regarding
the potential for deployment of HPWHs in demand response (DR) programs to help manage and
balance peak utility loads in a similar manner as conventional electric resistance water heaters
(ERWH).
To help answer these uncertainties, controlled experiments have been undertaken over
30 months in a matched pair of unoccupied Lab Homes located on the campus of the Pacific
Northwest National Laboratory (PNNL) in Richland, Washington. The water heater closet in the Lab
Homes is in conditioned space; therefore, taking supply air and injecting cool exhaust air from an
HPWH will interact with space conditioning within the homes. Initial experiments were conducted with
the second-generation General Electric (GE) “Brillion™” heat pump water heater in several
configurations: 1) unducted, 2) exhaust-air ducted, and 3) fully ducted to ascertain the impact of the
HPWH on space conditioning. Experiments also were conducted under simulated occupancy
conditions on the GE units to determine the DR of the units under multiple scenarios.
Subsequent DR experiments were conducted in the Lab Homes with Sanden carbon-dioxide (CO2)
refrigerant HPWHs as part of a project conducted by Washington State University Energy Program
with funding from Bonneville Power Administration. These HPWHs are new to the U.S. market but
not yet commercially available. Two units were evaluated: 1) a split system with an outdoor
compressor and an 80-gal (303 L) indoor water tank and 2) a 40-gal (151 L) unitary unit installed in
the Lab Home water heater closet and fully ducted (supply and exhaust air).
1.0 Introduction
In the United States, water heating represents the second largest household energy expense behind
space heating, representing approximately 18% of residential energy consumption, or approximately
1.8 Quads annually [1]. Efficient water heater options are necessary to achieve significant energy
savings in the residential sector and therefore the U.S. Department of Energy (DOE) recently
completed new energy conservation standards for natural gas, oil, and ERWHs between 20 and
80 gal (76 and 303 L) [2]. For storage water heaters with volumes of 55 gal (208 L) and below
(representing the majority of new water heaters for dwellings) the new standards will increase the
efficiency by approximately 4% [2].
For water heaters larger than 55 gal (208 L), a much bigger jump in efficiency is mandated. The new
standards for these larger water heaters can be met using electric heat pump (HPWH) and gas
condensing technology. HPWHs save at least 50% and condensing gas units about 25% compared
to today’s conventional tank-type ERWHs [3].
Electric HPWHs ranging in size from 40 to 80 gal (151 to 303 L) offer the most efficient option for the
1
41% of homes equipped with ERWHs, with a theoretical energy savings of up to 63%.
1
1
Based on the DOE test procedure [9], and comparison of an ERWH (Energy Factor, EF = 0.90) versus a HPWH (EF = 2.4)
However, significant barriers must be overcome before this technology will reach widespread
2
adoption. One barrier noted by the Northwest Energy Efficiency Alliance (NEEA) is that HPWH
products are not ideal for northern climates, especially when installed in conditioned spaces. HPWHs
exhaust cool air as they heat the water, and there may be complex and detrimental interactions with
the homes’ space conditioning system for units installed in a conditioned space [4]. Such complex
interactions may decrease the magnitude of whole-house savings available from HPWHs installed in
the conditioned space in cold climates and could lead to comfort concerns [4, 5]. Modeling studies
indicate that the installation location of HPWHs can significantly impact their performance and the
resultant whole-house energy savings [6, 7]. As a result, NEEA’s Northern Climate HPWH
Specification, which describes the characteristics a HPWH must have to be incentivized in cold
climates in the Pacific Northwest, requires exhaust ducting for a Tier II product and full ducting for a
Tier III product (see www.neea.org/northernclimatespec).
In addition, if exhaust-air ducting on HPWHs is found to be beneficial in some or all climates, it will be
important to understand the source and temperature of supply air (whether it is the home’s
conditioned air or outside air brought into the home) and the implications for interior depressurization,
particularly for tight homes.
Another barrier is the impact of HPWHs on DR programs because DR characteristics currently are
unknown for HPWH technology. Many utilities currently employ large storage tank ERWHs to reduce
peak load by turning off the water heater during times of peak demand. Some utilities also are
demonstrating the potential of using ERWH to increase load for areas with high renewable energy
penetration and to provide additional balancing and ancillary services. As HPWHs begin to penetrate
the market in the residential sector, utilities will need to better understand the DR capability and
characteristics of HPWHs.
2.0 Demand Response
DR benefits to utilities include increased system reliability, defrayed cost of new infrastructure
investment, reduced fuel consumption, improved system efficiency, and decreased carbon emissions
through increased penetration of intermittent renewable resources. When considering grid stability
and reliability, several types of DR are considered:
• Peak curtailment, or peak load reduction, which drops non-critical loads for a period of
4-6 hours during the time when power use is highest and the strain on the grid is greatest.
This can prevent the need to bring on inefficient, fossil fuel-fired “peaking plants” that exist
solely to generate electricity during the peak 4-6 hour period and are otherwise turned down or
off.
• Balancing reserves, or regulation services, responds to hourly or sub-hourly changes in
generation capacity either because of inherent variability in the generation resource or large
disturbances in the grid (e.g., transmission fault). Balancing reserves can be implemented for
either a shortage of generation capacity (commonly referred to as INC) or a surplus of
generation capacity (commonly referred to as DEC).
• Ancillary services that adapts to sub-minute fluctuations in voltage or frequency to maintain
consistent electricity service and distribution.
Several studies have previously evaluated the potential of ERWH to provide peak curtailment,
balancing reserves, and ancillary services using models and found significant potential and benefit for
ERWH to perform these grid functions [8, 10, 11, 12]. However, no extensive field testing has verified
these model results. In addition, new HPWH technology has the potential to dramatically decrease
the electricity use and peak load of residential water heating. Consequently, use of more efficient
heat pump technology may also limit the magnitude of the water heater for DR.
2
2
www.neea.org
3.0 HPWH Experiments
Given there is a need to understand both the efficiency and impact of a HPWH installed in a
conditioned space in a home and the potential for deploying a HPWH for DR compared to the DR
3
characteristics of ERWHs, a set of experiments were conducted in the PNNL Lab Homes over a
3-year period to better understand the attributes of a HPWHs. Three diverse types of HPWHs were
tested: 1) a GE second-generation 50-gal (189 L) GeoSpring Hybrid Water Heater (model
GEH50DEEDSR), which is enabled with Brillion™ wireless communication and control technology to
4
test the thermal envelope and HVAC impact and DR characteristics; 2) a Sanden GES-15QTA 40 gal
5
(151 L) integrated heat pump water heater using CO2 as the heat pump refrigerant to test DR; and 3)
a Sanden GAU-315EQTA split system heat pump with an 80 gal (303 L) storage tank also using CO2
as the refrigerant to test DR. The Sanden unitary HPWH was fully ducted during all experiments with
this unit per manufacturer’s specifications.
3.1 PNNL Lab Homes Testing Platform
The matched pair of PNNL Lab Homes (designated Lab Home A and Lab Home B) provide a unique
platform in the Pacific Northwest region for conducting experiments on residential-sector technologies.
These unoccupied electrically heated and cooled 1500 square-foot homes are sited adjacent to one
another on the PNNL campus in Richland, Washington. The homes are fully instrumented with enduse metering (via a 42-circuit panel), indoor and outdoor environmental sensors, and remote data
collection. Figure 1 is a floor plan of the Lab Homes noting the location of the water heater and the
required ventilation (transfer) grills installed in interior walls of the home to allow air circulation for the
integral HPWHs that were tested.
Water heater
Figure 1. Floor plan of Lab
Homes with water heater and transfer grills locations noted.
3.2 Experimental Design
The experiments were designed evaluate the HVAC system (space conditioning) impact and DR
characteristics of the GE GeoSpring HPWH, and the DR characteristics of the Sanden integrated CO2
heat pump water heater and the Sanden CO2 split-system heat pump water heater. Occupancy and
hot water use schedules were simulated to isolate the performance, impact, and DR of the HPWHs
from all other variables. Table 1 summarizes the six experiments.
Table 1. Summary Description of HPWH Experiments in the PNNL Lab Homes.
Experiment
3
4
5
Equipment
Experiment Description
Time Period
A full description of the Lab Homes is found at http://labhomes.pnnl.gov.
A full description of the features and controls capability of the GE GeoSpring HPWH is found elsewhere [14].
CO2 has a very low Global Warming Potential of 1. A full description of the features of both Sanden units tested is found
elsewhere [15].
3
1
GE GeoSpring HPWH
2
GE GeoSpring HPWH
3
GE GeoSpring HPWH
4
GE GeoSpring HPWH
ERWH
5
Sanden GES-15QTA
HPWH Unitary System
Sanden GAU-315EQTA
HPWH Split System
6
HVAC impact of ducting compared to
unducted configuration
HVAC impact of ducting compared
to unducted configuration
Water heating energy use of ducted
and unducted configurations
DR characteristics of the GeoSpring
HPWH in unducted configuration
compared to DR of ERWH
DR characteristics
June-August
DR characteristics
October
Dec-Jan
June-August
Dec-Jan
April-May
October
As a part of thermal impact experiments 1 and 2, the comfort (i.e., room temperature) was
determined; however the results are not reported in this paper but are reported elsewhere [14]. In
addition, the COP of the Sanden split system under DR stress is currently being evaluated in the
laboratory and will be reported in late CY2015. The baseline efficiency in the laboratory was
established in research prior to the Lab Homes testing [15].
Figures 2a and 2b show the GE GeoSpring HPWH installed in the Lab Homes for experiments 1-4.
Figures 3a and 3b show the Sanden HPWHs installed in the Lab Homes for experiments 5 and 6.
Figure 2a. Installed GE HPWH
without ducting in Lab Home B.
Figure 2b. GE HPWH with exhaust and
supply ducting in Lab Home A.
The supply air for the fully ducted configuration of the GE GeoSpring units was supplied from the
6
home crawlspace near the center of the home through a 6-in. flexible duct. The exhaust air was
drawn by a small fan through 6-in. flexible duct from the unit and exhausted via a standard dryer-type
vent installed at the bottom of the closet door. The exhaust duct fan was needed to provide sufficient
airflow across the heat pump, as the GE GeoSpring HPWH was not designed for exhaust or full
ducting as purchased. The measured airflow through the ducting during these experiments, with the
3
supplemental exhaust fan running, was 166 CFM (282 m /hour) for the exhaust-only ducting and
3
117 CFM (199 m /hour) for the full ducting, both of which are in accordance with installation
recommendations [4]. For the unducted configuration, the master bedroom closet was closed and
any air circulation around the HPWH was provided by four transfer grills (two in the closet wall and
two in the hallway wall).
6
4
Note that the homes are both completely skirted by ~2 inch insulating vinyl-covered foam panels.
Figure 3a. Sanden unitary
unit installed in Lab Home A.
Figure 3b. Sanden split system
installed in Lab Home B.
3.3 Data Capture and Experimental Configuration
Following are summary descriptions of the experimental setup for the Lab Homes. These
descriptions include how energy (electricity) was measured, how occupancy was simulated, how the
HVAC system was operated, and how temperature, humidity, and water flow were measured. A more
detailed description can be found elsewhere [14,16].
3.3.1 Electrical Energy
Whole-house electrical power and circuit level (true) power was measured at the 42-circuit breaker
panel with current transformers. Data were captured at 1-minute intervals via a Campbell Scientific
data logger.
3.3.2 Simulated Occupancy
To simulate occupancy, hot water draw profiles were implemented identically in both homes. The hot
water draws used a modulating solenoid valve at the kitchen sink hot water supply and were
controlled via the Campbell data acquisition system.
Controllable breakers were programmed to activate connected loads on schedules to simulate human
occupancy. The bases for occupancy simulation were data and analysis developed in previous
residential simulation activities [15, 16]. Detailed information on the electrical loads used to simulate
occupancy and the relevant schedules was reported previously [18].
3.3.3 Hot Water Draw and Draw Profile
PNNL selected a hot water draw and draw profile that was representative of a typical daily draw
pattern for a population of homes, rather than a single home and that was feasible to implement
reliably and repeatable using existing equipment in the PNNL Lab Homes. The draw profile was
based on the U.S. DOE Building America House Simulation Protocols, which specify typical daily
draw volumes for different appliances based on the number of bedrooms and an hourly draw pattern
7
based on fraction of total daily load [17, 20] .
7
The hot water draw profile in the Lab Homes for this experiment is not shown in this paper but can
be found elsewhere [16]
5
A high hot water draw volume was chosen to create a worst-case scenario to evaluate the maximum
space conditioning interaction for the experiments. Therefore, the hot water flow rate was set to
2.0 gpm (7.6 L/minute), for a total draw volume of 130 to 140 gal/day (492 to 530 L/day) in both lab
homes.
3.3.4 HVAC System Operation
Throughout testing, the HVAC system in both homes was operated identically. During the cooling
season, the 2.5-ton SEER 13 heat pumps maintained an interior set point of 76°F with no setback
[17]. During the heating season, the heat pumps were set to “Emergency Heat,” to operate like
electric resistance furnaces and maintain an interior set point of 71°F with no setback [17].
3.3.5 Temperature and Humidity Measurements
Identical networks of temperature sensors were deployed in both homes. Each defined area of the
home (individual rooms, hallway, and open living areas) had at least one thermocouple; a total of 17
space temperature thermocouples were installed per home.
Two type T thermocouples were installed to measure supply and exhaust process air through the heat
pump compressor. Three crawlspace temperature sensors were installed to monitor the temperature
of the crawlspace, which was be the temperature of the supply air for the GE GeoSpring HPWH
tested in the fully ducted arrangement.
3.3.6 Water Flow Measurements
The water flow rate was measured using a low-flow, impeller-type flow meter with 375 to 1380 pulses
per gallon (0.07–5 gal [0.26–19 L] or 0.2–20 gal [0.76–7.6 L] range, depending on the model) with a
6–24 VDC output.
4.0 Testing Protocol
The testing protocol was tailored depending upon the experiment conducted. The GE GeoSpring
units were tested over an approximately 2-year period. They were tested for space conditioning
(HVAC) during relatively long time periods during the summer (cooling) and winter (heating) in both
ducted and unducted configurations. The GE GeoSpring DR experiments were conducted during
relatively shorter time periods in the spring and fall.
4.1 Baselining the Homes
Prior to initiating the experiments, the homes were extensively baselined. For experiments 1–4, the
homes were baselined with the GE GeoSpring water heaters operating in electric resistance (only)
and heat pump (only) modes. For experiments 5 and 6, the homes were similarly baselined although
the heat pumps were operated only in heat pump mode as the home thermal/HVAC impacts of the
units were not part of the experiment.
4.1.1 Air Leakage
Blower door measurements were taken on both homes as part of the baseline period for experiments
1 and 2 as these (thermal/HVAC impact) experiments depended upon near-identical building thermal
characteristics. Air leakage through the building shell was quantified in both homes using a
Minneapolis Blower Door Model 3 and DG-700 digital pressure gauge in accordance with ASTM
E779, “Standard Test Method for Determining Air Leakage Rate by Fan Pressurization,” and
manufacturer recommendations [21, 22].
4.1.2 Null Testing
Following blower door measurements, the homes went through an active null testing period, with fulloccupancy simulation to verify equivalent performance. Null testing with full occupancy (lighting,
human-related, and equipment sensible loads) and simulated hot water draws showed similar energy
use between the two homes, within 1.9 ±2.0% over the cooling season baseline testing period for
6
experiments 1 and 2. The differences in whole-house energy use between Lab Home A and Lab
Home B were observed to be within 0.7 ±0.5% over the heating season baseline testing period for
experiments 1 and 2.
5.0 Experiments and Results
The results of each of the six experiments described in Table 1 are summarized below. A discussion
of those results and conclusions will follow. Recommendations for further investigations follow the
discussions and conclusions.
5.1 Experiment #1: Cooling Season HVAC Impact of Ducting the GE GeoSpring HPWH
Fully Ducted Water Heater
Unducted Water Heater
Exhaust-Only Ducted Water Heater
% Difference in HVAC Energy Use
35
14%
30
12%
25
10%
20
8%
15
6%
10
4%
5
2%
0
0%
Exhaust-Only Ducted
Comparison
Fully Ducted Comparison
Percent Change in HVAC Energy Use
Due to Ducting Configuration
Daily HVAC Energy Use [kWh/day]
Over a period of several weeks during the cooling season, both exhaust-only and fully ducted
configurations were tested and compared to the unducted unit. Figure 4 illustrates the results of this
experiment. Both the exhaust-only (green bar) and fully ducted configuration (red bar) led to
increased HVAC energy usage as compared to the HVAC energy use in an unducted HPWH
configuration (blue bar). This was attributed to the supplemental space cooling from the unducted
HPWH exhaust being available for cooling the homes whereas the cool air was not available in either
ducted configuration. The unducted HPWH provided a space cooling benefit equivalent to
approximately 1.5 kilowatt-hours per day (kWh/day). Because this additional space cooling is not
available in the exhaust-only and fully ducted scenarios, these ducting configurations resulted in
increased space conditioning energy use of 9.3 ±1.0% for the exhaust-only and 9.3 ±2.2% for the fully
ducted scenario during the cooling season.
Figure 4. Daily home HVAC energy use and % difference in HVAC energy use for the exhaustonly ducted and the fully ducted GE GeoSpring HPWH experiments in the cooling season.
In Figure 4, the duct configuration (exhaust-only and fully ducted) is compared directly to the
corresponding unducted control case (blue bar). The average difference in HVAC energy use during
each experimental period is represented by the yellow diamonds, where positive values indicate
increased energy use resulting from ducting. The difference in the HVAC energy use for the
unducted HPWH between the exhaust-only ducted comparison and the fully ducted comparison
periods is due to weather differences during the two discreet time periods of the experiments.
5.2 Experiment #2: Heating Season HVAC Impact of Ducting the GE GeoSpring HPWH
Over a several week period in the heating season, both exhaust-only and fully ducted configurations
of the GE GeoSpring HPWH were tested and compared to the unducted unit. The results are
summarized in Figure 5. The HVAC energy use in Lab Home A in the exhaust-only ducted
configuration (green bar) increased 3.2 ± 2.5 kWh/day, or 4.0 ± 2.8% as compared to the unducted
HPWH in Lab Home B (blue bar). The HVAC energy use in Lab Home A in the fully ducted
7
configuration (red bar), decreased 5.7 ± 1.6 kWh/day or 7.8 ± 2.3% as compared to Lab Home B with
the HPWH in an unducted configuration (blue bar).
Fully Ducted Water Heater
Unducted Water Heater
Exhaust-Only Ducted Water Heater
% Difference in HVAC Energy Use
120
8%
6%
100
4%
2%
80
0%
60
-2%
-4%
40
-6%
-8%
20
-10%
0
-12%
Exhaust-Only Ducted
Comparison
Fully Ducted Comparison
Percent Change in HVAC Energy Use Due
to Ducting Configuration
Daily HVAC Energy Use [kWh/day]
In Figure 5, the ducting configuration is compared directly to the corresponding unducted control
case. The average difference in HVAC system energy use during each experimental period is
represented by the yellow diamonds, where positive values indicate increased HVAC system energy
use resulting from ducting.
Figure 5. Daily home HVAC energy use and % difference in HVAC system energy use for the
exhaust-only ducted and the fully ducted GE GeoSpring HPWH experiments in the heating
season.
5.3 Experiment #3: Water Heating Energy Use for Ducted and Unducted Configurations
Ducting can impact the energy consumed by the HPWH as the efficiency of the HPWH will be
affected by the temperature of the inlet air. For example, while the unducted water heater may
provide space conditioning benefits in the cooling season, such a configuration may increase water
heating energy use because the colder inlet air decreases the efficiency of the HPWH. Table 2
summarizes the annualized water heating energy use difference between an unducted HPWH and an
exhaust-only and fully ducted HPWH. The energy use difference is calculated based on data taken
during the summer and winter experiments and annualized to the Richland, Washington, climate. The
8
measured temperatures during the winter and summer experiments also are shown in Table 2.
Table 2. HPWH energy use for exhaust-only and fully ducted configurations compared to an
unducted HPWH.
HPWH
Configuration
Exhaust Only
Fully Ducted
Energy Use Difference
vs. Unducted HPWH
−144 ±74 kWh/yr
−6.8 ± 3.5%
48 ±49 kWh/year
2.3 ±2.3%
Living Space Average
Temperature (season)
75.9 ±2.1°F (cooling)
71.6 ±1.6°F (heating)
74.7 ± 0.4°F (cooling)
71.3 ± 1.5°F (heating)
Crawlspace Temperature
(season)
Not applicable to this
configuration
73.0 ±1.3°F (cooling)
44.2 ±2.2°F (heating)
5.4 Experiment #4: GE GeoSpring HPWH Demand Response Compared to ERWH
This experimental was undertaken to evaluate the DR capability of the GE GeoSpring HPWH as
compared to the ERWH under two types of DR events: peak curtailment and balancing reserves (INC
8
Note that the HPWH energy use difference does not include fan energy used for the exhaust-only and fully ducted
configurations.
8
and DEC, respectively). Table 3 summarizes the DR experiments that were designed to be a range
of typical times during which DR may be required across a broad spectrum of utilities.
To implement DR for the GE GeoSpring HPWH, PNNL deployed the built-in capability using the GE
®
Nucleus™, GE’s Home Energy Management System, via ZigBee communication protocol. The
mode and tank set points can be controlled through the Nucleus either by a homeowner or a utility
employing conserve signals or peak pricing.
Table 3. DR experiments with the GE GeoSpring HPWH compared to ERWH.
DR
Experiment
Description
Time of
Day
Duration
Purpose of Experiment
AM Peak
Curtailment
Turn off heating
elements/HP
0700
3 hours
PM Peak
Curtailment
Turn off heating
elements/HP
1400
3 hours
EVE Peak
Curtailment
Turn off heating
elements/HP
1800
3 hours
Balancing
INC
Turn off heating
elements/HP
1 hour
Balancing
DEC
Set tank temp to 135°F
0200
0800
1400
2000
0200
0800
1400
2000
Evaluate HPWH load shedding
potential (dispatchable kW and
thermal capacity) as compared
to ERWH to manage peak load
Evaluate HPWH load shedding
potential (dispatchable kW and
thermal capacity) as compared
to ERWH to manage peak load
Evaluate HPWH load shedding
potential (dispatchable kW and
thermal capacity) as compared
to ERWH to manage peak load
Evaluate HPWH potential to
provide balancing reserves for
dispatchable kW as compared to
ERWH
Evaluate thermal capacity of
HPWH, as compared to ERWH,
when temp set point is increased
to 135°F (57°C)
1 hour
Table 4 is a summary of the results of the experiments showing the average water heater power use
during the DR event, water heater energy use during the DR event, daily energy use, and the ratio of
the HPWH/ERWH for peak curtailment, INC experiment, and DEC experiment.
Table 4. Data from DR experiments for the GE GeoSpring HPWH and ERWH.
Experiment
Duration
Water
Heater
Mode
Peak
3 hours
HP
9
Average
Power
Draw
a
Impact (W)
−439
Average
Energy Use
During DR
Event (Wh)
−1,285
Average
Daily Energy
Use Impact
(Wh/day)
−498
Ratio
HPWH/ERWH
2.64
b
Curtailment
Balancing
c
INC
Balancing
DEC
1 hour
1 hour
ER
HP
−1,158
−442
−3,320
−442
258
−159
ER
HP
ER
−1,185
220
1,174
−1,185
220
1,174
86
−158
1,543
2.67
d
17.1
a. Positive numbers indicated increased energy use, and negative numbers indicate decreased energy use.
b. The 0700 peak curtailment event was not successfully implemented because of a communication failure
between the GE Nucleus and the GE server. Therefore, the data shown are the average for the combined PM
and EVE events. Disaggregated data for the PM and EVE events are available elsewhere [13].
c. Does not include the 0200 event when both water heaters had zero load per hot water draw schedule.
d. Ration range is from 2.12 for the 0200 event to 50.6 for the 0800 event when the HPWH ramping capability in
heat pump mode is significantly decreased.
5.5 Experiment #5: Sanden GES-15QTA HPWH Unitary System Demand Response
A DR schedule to simulate an oversupply (peak curtailment) condition and a balancing reserves
condition was generated to ascertain the performance of the Sanden unitary water heater and
demonstrate the peak shift or reduction associated with each experiment. The peak curtailment
experiment is of considerable interest to utilities deploying ERWHs in DR to manage peak demand.
Only the oversupply experiment shown in Table 5 is reported. The balancing reserves experiment is
reported elsewhere [16].
Table 5. Sanden unitary HPWH oversupply experiment.
Day
1
2
3
4
5
6
7
Start Time
1800
1700
1600
1500
1400
1300
1200
End Time
2400
2400
2400
2400
2400
2400
2400
Duration (hours)
6
7
8
9
10
11
12
As noted in Table 5, the experiment consists of increasing the time the unit is off in 1-hour increments,
beginning at 6 hours on Day 1 to a total of 12 hours by Day 7. This increased daily strain on the
system was imposed to determine at which hour the system cannot meet the delivered water
temperature during the daily draw schedule. The experiment also was designed to determine the load
impact from the shift in the HPWH operation. The unit was baselined for 7 days prior to initiating the
DR experiments.
Figure 6 shows the water temperature profile for the second DR event (Day 2) where the water heater
was off for 7 hours. The corresponding water heater power draw for Day 2 is shown in Figure 7 along
with the outdoor air temperature (red dotted line) and crawl space temperature (green solid line). The
Day 2 event is shown here as this is the time period during which the hot water temperature drops
below the tank set point to about 115°F (46°C) after 6 hours, thus making this demand event resulting
in a likely unacceptable water temperature and thus unavailable for residential application.
The sawtooth pattern shown in Figure 6 (and Figure 8) is a result of the temperature monitoring via an
insertion thermocouple in a thermowell at the outlet of the mixing valve. As water is drawn, the
temperature peaks to the set delivery temperature, a nominal 120°F (49°C), followed by the
temperature decay after the draw is concluded. This decay approached ambient temperature (i.e.,
temperature of the water heater closet) until the next draw occurs at which time the cycel begins again
10
Figure 6. Sanden unitary HPWH oversupply experiment delivered water temperature profile:
second DR event (7 hours powered-down).
Figure 7. Sanden unitary HPWH oversupply experiment power profile: second DR event
(7 hours powered-down).
5.6 Experiment #6: Sanden GAU-315EQTA HPWH Split System DR
The same peak curtailment DR schedule used for the unitary system (Table 5) was used to ascertain
the performance of the Sanden split system water heater. The balancing reserves DR experiment
schedule for the split system was identical to the unitary system experimental schedule. These
schedules are reported elsewhere [16].
The split system was baselined for 7 days prior to initiating the DR experiments. Figure 8 shows the
water temperature profile for Day 7 of the experiment when the water heater was off for 12 hours, the
longest duration of power-down. The corresponding water heater power draw for Day 7 is shown in
Figure 9 along with the outdoor air temperature (red dotted line).
11
Figure 8. Sanden Split System HPWH Oversupply Experiment Delivered Water Temperature
Profile for Last DR Event (12 hours powered-down).
The temperature profile in Figure 8 shows that the delivered temperature still maintains the 120°F
values across the DR period because of both the elevated set point and the larger tank capacity. This
observation was expected given the size of the storage tank.
Figure 9. Sanden split system HPWH oversupply experiment power profile for last DR event
(12 hours powered-down)
Table 6 presents the summary finding for the oversupply experiments. The Dispatchable Power is the
peak watts available to be shifted through oversupply implementation. The Recovery Energy Shift is
the value of energy (kWh) that is shifted to the post-oversupply period. The oversupply duration
indicates the number of hours the protocol was enacted while still delivering hot water at an
acceptable temperature to the household.
12
Table 6. Summary of the oversupply DR experiments in the Sanden units:
power, energy shift and off-period delivered acceptable hot water temperature.
Experimental Results
Dispatchable Power (kW)
a
Recovery Energy Shift (kWh)
b
Oversupply Duration (hours)
Maximum Off-Period while Acceptable Hot
Water Temperature Met (hours)
Unitary System
1.3
2.65
6
6
dispatchable
Split System
1.2
2.95
6
12
a
The Oversupply Recovery Energy Shift is the water heater energy use at the conclusion of the oversupply
period.
b
The Oversupply Duration of the split system presented was for the 6-hour interval and provided for comparison
to the unitary system.
6.0 Discussion and Conclusions
The following is a summary discussion of the experiments and conclusions drawn from the
experiment. Experiments 1 and 2 are combined in this discussion because the purpose of those
experiments was to determine overall/annual space conditioning (heating/cooling) impact from HPWH
ducting. Additional details on the cooling season impact and the heating season impact can be found
elsewhere [15].
6.1 Experiments 1 and 2: Whole-House Space Conditioning Annualized Impact of HPWH
Ducting
The experimental results were annualized for the climate in Richland, Washington, based on the
average number of heating days and cooling days in a year. Specifically, the daily difference in
HVAC energy usage for the exhaust-only ducted versus unducted configuration are combined to yield
a daily whole-house difference in energy use for that case, assuming all other loads are identical.
The annual difference in whole-house energy use for the fully ducted HPWH compared to the
unducted HPWH is calculated in a similar manner. The expected annual difference in HVAC and
whole-house energy use using this method is shown in Table 7.
Table 7. Comparison of HVAC and whole-house energy use for exhaust-only and fully ducted
HPWH configurations vs. an unducted HPWH.
HPWH
Configuration
HVAC Energy Use Difference
vs. Unducted
Whole-House Energy Use Difference vs.
Unducted
Exhaust Only
858 ±440 kWh/yr
6.2 ±3.2%
−1,079 ±408 kWh/yr
−7.8 ±3.0%
714 ±446 kWh/yr
2.9 ±1.8%
−1,031 ±411 kWh/yr
−4.2 ±1.7%
Fully Ducted
As shown in Table 7, the fully ducted configuration has a significant energy benefit on the HVAC and
whole-house energy use (7.8% and 4.2% less energy use, respectively) whereas the exhaust-only
configuration has a negative impact on the HVAC and whole-house energy use (6.2% and 2.9%
increase in energy use). Also note that the differences for both ducting configurations in HVAC and
whole-house energy use is much larger than the difference in water heater energy use (shown in
Table 2). The annualized differences in space conditioning energy usage is dominated by space
heating, which could be considered a worst-case comparison in this experiment because of the use of
an electric resistance furnace for space heating and heat pump for space cooling.
These data can be used to ascertain the cost-effectiveness of ducting (exhaust only or fully ducted) a
HPWH taking into account the climate (i.e., degree-days), the cost of ducting, and the cost of energy
for water heating and space heating and cooling.
The theoretical annualized differences for exhaust only and ducted configurations also were
determined from modeling. Modeling suggests that HPWHs installed in conditioned space will
increase HVAC energy use in the heating season because of the use of air that has been initially
13
heated by the HVAC system to heat water and the introduction of cool exhaust air into the space.
Therefore, models assume, any heat that has been extracted from the space must be made up, or
reheated, by the HVAC system to maintain interior thermostat set points. These models also indicate
that exhaust ducting will mitigate the impact of HPWHs on space conditioning systems by preventing
cool exhaust air from being introduced into the conditioned space. However, data collected during
this experiment suggest that exhaust-only ducting did not decrease space conditioning energy use, as
compared an unducted HPWH.
The experimental data may suggest that the reduced HVAC impact is due to buffering of the HPWH
space conditioning impacts by the interior walls. For the unducted HPWH, the water heater closet
experienced localized cooling while the thermostat, located in the hallway near the kitchen (see
Figure 1), was not affected by the HPWH thermal loads. Data taken during the experiments were not
sufficient to fully address this observation and further discussions are published elsewhere [14].
6.2 Experiment 3: GE GeoSpring Water Heater Energy Use in Ducted Configurations.
From Table 2, exhaust-only ducting decreased water heater energy use by 144 ±74 kWh/year on an
annualized basis. However, this decrease was offset by an increase in whole-house energy use of
714 ±446 kWh/year on an annualized basis (Table 7). Therefore, exhaust ducting in a climate similar
to that of Richland, Washington, would not be cost effective. Annualized estimation would be needed
for other climate regions to determine overall net energy impacts.
Fully ducting the HPWH resulted in a net increase in water heater energy use by 48 ±49 kWh/yr on an
annualized basis (Table 7). However, this increase is relatively small compared to the decrease in
whole-house energy use of 1,031 ±411 kWh/year on an annualized basis (Table 7). Therefore, from a
whole-house perspective, the net energy impacts of HPWHs installed in conditioned space are driven
by the HVAC system interaction and not by the impact of the ducting on the water heater
performance.
The magnitude of these energy impacts, their costs and the costs for ducting were determined and
reported elsewhere [15]. This analysis suggests that exhaust-only ducting may not be advisable but
that full ducting may be cost effective over the lifetime of the water heater in climates comparable to
that of Richland, Washington. However, additional analysis would be needed to take these data and
develop recommendations for the variety of energy rates, climates, and installations/configurations
where HPWHs may be installed.
6.3 Experiment 4: GE GeoSpring HPWH DR Compared to ERWH
Based on the data collected in these DR experiments, both ERWH and HPWH are capable of
performing peak curtailment and regulation services. However, their characteristics differ, as can be
seen in Table 4 which shows the average impact on power use during the DR event, energy use
during the DR event, and daily energy use for ERWH and HPWH for peak curtailment, INC events,
and DEC events.
The HPWH provides approximately 38% of the peak reduction or INC balancing response of the
ERWH, when accounting for differences in power use and use profiles of the water heaters. The
ERWH provides more dynamic response with a large amount of power increase or decrease per
water heater. However, the HPWH has longer and more frequent operating times, which means the
HPWH is able to respond when an INC event or peak curtailment is needed. In addition, the inherent
measured efficiency savings of HPWH (61.7 ±1.7%) compared to a standard ERWH will result in
some permanent peak savings as well (not quantified here), resulting in overall peak load reduction
for a utility with significant penetration of HPWHs.
However, the DEC response of the HPWH is limited during some parts of the day when especially
high hot water use occurs. During the nighttime when there is little to no hot water draw activity, the
HPWH has significant capacity to increase load because it takes much longer for increases in tank
temperature to saturate, as compared to an ERWH. And, in experiments reported elsewhere [19], a
modification to the HPWH control algorithm can allow the HPWH reach tank temperatures above
135ºF (57°C) using only the heat pump mode for overnight DEC response.
14
With regard to hot water delivery, decreased hot water delivery temperature was measured for all DR
events. The most dramatic delivered hot water temperature decrease was ~18ºF (~8°C) for the
HPWH during an afternoon peak curtailment event. However, an extremely high hot water draw
profile was implemented during these events to provide a “worst-case scenario.” Based on the results
of these experiments, decreased hot water delivery temperature is not likely to be an issue for the
majority of participants in a DR program providing peak curtailment or INC balancing services even
when only the heat pump is used to heat water. A more comprehensive discussion of this experiment
can be found elsewhere [19].
6.4 Experiments 5 and 6: Sanden HPWH DR Peak Curtailment
The Sanden split system with the 80 gal (303 L) tank was able to maintain ~120ºF delivered hot water
temperature under the relatively extreme DR experiment where the unit was powered off for 12 hours.
And, when the DR event was terminated, the ramp up peak power draw was ~1500 W at midnight,
thus shifting this peak to a typically off-peak period for most utilities. This is approximately the same
peak draw as a standard ERWH (not reported here; see reference [16]). Given the Day 7 experiment
profile (Figure 9), it is likely the water heater could provide near 120ºF (49°C) hot water to the home
during the initial early morning (0600) hot water draw (~4 gal/hour/~15 L/hr) to later (0700) hot water
draw (~11 gal/hr/~43 L/hr) without being powered. This is an advantage of the large tank capacity
and flexibility of this unit should a utility wish to avoid an early morning peak. Additional analysis and
discussion of the energy impacts of DR compared to ERWH can be found elsewhere [16].
The Sanden integrated system with the 40-gal (151-L) tank was unable to maintain ~120ºF (~49°C)
delivered hot water temperature for more than 5 hours after powered off (Figure 6). The delivered
water temperature after 5 hours was ~108ºF (42°C) would likely not be acceptable to home
occupants. These results were anticipated with a 40 gal tank and were similar to the observation with
the GE GeoSpring HPWH DR experiment. However, both the unitary and split systems can
effectively recover from peak curtailment up to a minimum of 5 hours in the case of the 40-gal (151-L)
unitary system and 12 hours for the 80-gal (303-L) split system. As with the GE GeoSpring HPWH, a
utility seeking 3 to 4 hours of peak curtailment and/or peak shifting could successfully deploy the
integrated system while simultaneously harvesting the advantage of using less energy to deliver hot
water to a home [16].
7.0 Recommendations
Results obtained from the experiments described in this paper answered many questions regarding
installation, performance, and DR potential for the current and future generation of HPWHs.
Observations made during these experiments also led to questions that could be answered through
additional research. These issues and additional research areas include, but are not limited to:
• The GE GeoSpring HPWH demonstration found significant differences between the modeled
energy impacts of an unducted HPWH compared to the measured impacts. A more thorough
measurement and analysis of air infiltration and movement and temperature gradients inside
the home with an unducted HPWH installed in conditioned space needs to be undertaken.
• As delivered, the GE GeoSpring HPWH was not configured for operating with ducting, so the
ducting had to be custom designed. A similar study of an unitary HPWH, and perhaps a
HPWH with a larger (60-80 gal) tank that was configured for ducting at the factory, may
provide additional data on the overall impact of ducting and the optimal ducting configuration
for installation in a conditioned space.
• The impact of exterior temperature, water supply temperature, and closet temperature for the
Sanden split system on the overall system performance is not well understood. A systematic
and controlled evaluation and analysis would be of value given a small number of field
research studies have indicated that these variables are not determinative of overall system
performance [23].
15
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