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Heat Pump Water Heaters: Design Details

What do you see when you look at a building’s exhaust air stream, a restaurant kitchen so hot that the cook faints, or a commercial laundry where the workers are made miserable by heat and humidity? At Wescor we see hot water, cool air, and energy savings.

Air-to-water heat pump water heaters (HPWHs) can capture the latent and sensible energy from a building’s exhaust air or the ambient air in warm environments such as commercial kitchens and laundries and transfer the energy to water, providing:
  • potable hot water
  • cool air for air conditioning
  • energy savings
This paper explains how heat pump water heaters work, applications, design considerations, and expected energy savings.

How Heat Pump Water Heaters Work

Heat pump water heaters extract latent and sensible heat from air and use the extracted energy to heat water. Figure 1 is a schematic diagram of a HPWH system.

Figure 1 Heat Pump Water Heater System

The HPWH cools in-coming warm air by approximately 20ºF, with air-cooling capacity approximately 70% of water-heating capacity. The water is heated approximately 10ºF with each pass through the HPWH, with compressor energy adding to water heating.

There are two types of HPWHs: single-stage systems and two-stage systems.

Single-stage HPWHs

Single-stage HPWHs use an air-to-water heat pump and are effective down to 40ºF. A single-stage HPWH can both heat and cool water (if it has a reversing valve), and it can cool air without heating water (if it has a remote condenser). Figure 2 is a schematic diagram for a typical single-stage HPWH.

Figure 2 Single-stage HPWH

Single-stage HPWHs are sized from 50K to 500K BTUHs and typically come as a complete packaged system.

Capacity and Efficiency

The primary factors that determine a heat pump water heater’s efficiency are:
  • the supply air wet bulb (WB) temperature
  • the desired final water temperature
Average entering water temperature is a secondary factor.

The heating coefficient of performance (COP) is the ratio of the HPWH’s heat energy to the electrical energy input when both are in consistent units.

Figure 3 shows the COP of a typical single-stage HPWH at various air temperatures.

Figure 3 COP for a Single-stage HPWH at Various Air Temperatures

Figure 4 shows the capacity of a typical single-stage HPWH at various air temperatures.
Figure 4 Capacity of a Single-stage HPWH at Various Air Temperatures

Capacity is mainly a function of compressor BTU rating and air temperature. Figure 5 shows the capacity of a typical single-stage HPWH at various entering water temperatures (EWT). As shown, EWT has little effect on capacity.

Figure 5 Capacity of a Single-stage HPWH at Various Entering Water Temperatures

Table 1 compares the performance of a single-stage HPWH capturing heat from various sources: a commercial kitchen, a building’s exhaust air stream, and the average outside air temperature for one year and for the month of January in Portland, Oregon.

Table 1 Make-up Domestic Hot Water Performance Estimator



HPWH
specs
Heat sources
Commercial
kitchen
Building
exhaust
Portland
avg
annual
temp
Portland
avg
January
temp
Nominal capacity at 72o WB (BTUs per hour) 500,000 500,000 500,000 500,000 500,000
Design WB 72o 80o 63o 55o 40o
Avg input water temp 100o 100o 100o 100o 100o
Supply water temp 55o 55o 55o 55o 55o
Set point temp 140o 140o 140o 140o 140o
% rated capacity 100% 111% 89% 78% 58%
COP at design WB 3.47 3.81 3.10 2.76 2.13
Delivered BTU per hour 503,800 557,000 443,950 390,750 291,000
Recovery rate gal per hour 714 790 629 554 412
Gallons per 8-hour run 5,713 6,316 5,034 4,431 3,300
Gallons per 18-hour run 12,854 14,211 11,327 9,970 7,425

Cost per mmBUT* (assumes $.10 KWh and $1.40 therm)
WH series HPWH $8.43 $7.69 $9.46 $10.62 $13.76
95% efficient gas WH $14.74 $14.74 $14.74 $14.74 $14.74
85% efficient gas WH $16.47 $16.47 $16.47 $16.47 $16.47
100% electric WH $29.30 $29.30 $29.30 $29.30 $29.30

Two-stage HPWHs

A two-stage HPWH is an air-to-water heat pump (first stage) feeding a water-to-water heat pump (second stage). Two-stage HPWHs:
  • Can operate in lower air temperatures than single-stage HPWHs.
  • Maintain BTU capacity.
  • Have higher total boost.
  • Use staged operation for efficiency.
Figure 6 shows a schematic diagram for a typical two-stage HPWH.

Figure 6 Two-stage HPWH

The first stage operates alone until the air temperature drops to about 40ºF, when the second stage kicks in. A two-stage HPWH can operate effectively to15ºF. Even at that low temperature, heating water to 140ºF is easy. Figure 7 shows the COP of a typical two-stage HPWH at various air temperatures.

Figure 7 COP for a Two-stage HPWH at Various Air Temperatures

Applications

Applications with typical entering water temperature (EWT) of 100ºF (so-called “hot water” applications) include:
  • Potable hot water
  • 120-140ºF space-heating radiators
  • Process water for areas such as laundries, kitchens, and industrial applications
Compared to hot water applications, lower-water-temperature applications have advantages, including:
  • Lower water temperatures resulting in much higher COP.
    • Two-pipe boiler/chiller loops (50-90ºF)
    • Pool heating (80-90ºF)
    • Radiant floor heating (70-100ºF)
  • Even better pay-back
  • Same BTUH de-rating curves for air temperatur
Low EWT applications can be much more efficient than high EWT applications. Figure 8 shows the COP of a typical two-stage HPWH at various entering water temperatures.

Figure 8 COP for a Two-stage HPWH at Various Entering Water Temperatures

The following three sections list conditions to consider for three typical environments-—building exhaust air, commercial kitchen, and commercial laundry—-when deciding whether a HPWH is suitable for a particular application.

Building Exhaust Air

energy source
A building exhaust air stream is typically a constant 63ºF WB. For example, if there is 96,000 cfm from roof exhausts, then approximately 3.4 mm BTUH of recoverable heat is available 24/7.
buffer and backup
If the building is using a boiler/cooling tower for heating and cooling a water loop, then the loop volume can provide the primary buffer storage with the gas boilers providing additional heat.
cooling
The HPWH in this example could provide approximately 220 tons of cooling, which could be enough to eliminate cooling tower cost.
heating performance
In this example, the system has a 4.4 COP at loop temperature of 90ºF and a 6.3 COP at loop temperature of 50ºF.
size
Depends on building heating and cooling requirements.

Commercial Kitchen

energy source
A commercial kitchen’s supply air is typically 80ºF+ near the ceiling.
buffer and backup
An existing or new tank provides water storage; gas or electric water heaters provide additional heat.
cooling
Air ducted to spot-cool areas directly reduces AC load.
heating performance
COP > 4 (half the cost of gas per BTU).

size
Hot water use is based on meals per day and varies by type of kitchen:
Fast food: 46 kBTU per 100 meals.
Full service and cafeteria: 160 kBTU per 100 meals.

Laundries (commercial, athletic club, motel, hotel, etc.)

energy source
Ambient air from dryer or drain trough area.
buffer and backup
Electric tank for pre-heated water. Gas water heaters provide additional heat and high temperature.
cooling
Can provide spot cooling or full-time cooling with a remote condenser.
heating performance
COP > 3.5.

size
Typical hot water use is one to two gallons per pound of laundry.

Designing HPWH Systems

When designing a HPWH system, you need to consider three things: design strategies, environmental conditions, and hot water use.

Design Strategies

Use the following strategies when designing a HPWH system.
  • Use the HPWH for base demand with a conventional HW heater for back up.
  • Trade off buffer storage capacity and dynamic source capacity to meet peak demand.
  • Size the system for a minimum of eight hours per day run time.
  • Use the cool air the HPWH produces to increase ROI.
  • A separate remote condenser allows cooling when water heating is not needed.

Environmental Conditions

Consider the following environmental conditions when designing a HPWH system.
  • Energy source
    • Is the air source too hot (WB over 95º)?
    • Is the heat source constant or variable?
    • Is unconditioned or outside air sufficient?
  • Air flow
    • Typical air flow is about 2500 cfm per rated 100k BTUH.
    • Can cooled, dehumidified air be used for cooling a room or process?
  • Buffer storage
    • Requirement is based on the difference between peak and base loads. For example, an application that has a two-hour peak load in the morning and a two-hour peak load in the evening needs larger buffer storage than an application where use is consistent throughout the day.
    • Usually 50-200 gallons storage capacity per 100k BTUH is needed as a buffer.
  • Capacity
    • Best to choose a HPWH with a BTUH where the run time is greater than 50% of the operating time.

Hot Water Use

Table 2 is a handy water-use guide from the 2007 ASHRAE Handbook—HVAC Applications (Chapter 49, Table 7).

Table 2 Water Use Guide for Various Types of Buildings*

Type of Building Maximum Hour Maximum Day Average Day**
Men's dormitory

Women's dormitory

Motel:
20 units
60 units
100 units
3.8 gal/student

5.0 gal/student



6.0 gal/unit
5.0 gal/unit
4.0 gal/unit
22.0 gal/student

26.5 gal/student



35.0 gal/unit
25.0 gal/unit
15.0 gal/unit
13.1 gal/student

12.3 gal/student



20.0 gal/unit
14.0 gal/unit
10.0 gal/unit
Nursing home 4.5 gal/bed 30.0 gal/bed 18.4 gal/bed
Office building 0.4 gal/person 2.0 gal/person 1.0 gal/person
Full-meal restaurant, cafeteria

Drive-in, grille, luncheonette, sandwich, snack shop
1.5 gal/max meals/hr


0.7 gal/max meals/hr
11.0 gal/max meals/hr


6.0 gal/max meals/hr
2.4 gal/max meals/hr


0.7 gal/max meals/hr
Apartment building:
20 units
50 units
75 units
100 units
>200


12.0 gal/apt
10.0 gal/apt
8.5 gal/apt
7.0 gal/apt
5.0 gal/apt


80.0 gal/apt
73.0 gal/apt
66.0 gal/apt
60.0 gal/apt
50.0 gal/apt


42.0 gal/apt
40.0 gal/apt
38.0 gal/apt
37.0 gal/apt
35.0 gal/apt
Elementary school

Junior, senior high school
0.6 gal/student


1.0 gal/student
1.5 gal/student


3.6 gal/student
0.6 gal/student


1.8 gal/student
*Interpolate for intermediate values.
**Per day of operation

Financial Comparisons

As shown in Table 3, HPWHs have a lower first cost, higher annual energy savings, and shorter payback time than solar options. The incentive information in Table 3 is specific to Portland, Oregon. Available grants and incentives vary by location. ContactWescor’s Portland office for information about grants and incentives available in specific locations in Oregon and Idaho and Wescor’s Seattle office for information on locations in Washington.

Table 3 Financial Comparisons of Various Solar and HPWH Options



Heat pump water heaters transform waste heat or ambient air into hot water and air conditioning
Heating
option
Energy use Cost and incentives Savings and payback
Annual
energy
produced
Annual
electric
expense
First cost BETC*
pass
thru
ETO
E.B.**
Federal
credit
Net
equip
price
Heating only Heating and cooling
Annual
savings
Payback
years
Annual
savings
Payback
years
Added to
condensing
boilers
Therms - - - - - - - - - -
Solar thermal
8000 sqr ft
10,100 $1,825 $1,000,000 $350,000 $60,600 $300,000 $289,400 $11,305 25.6 No
cooling
No cooling
Solar thermal
2000 sqr ft
2,525 $456 $250,000 $87,000 $15,150 $75,000 $72,350 $2,826 25.6 No
cooling
No cooling
HPWH
8 hrs/day
12,963 $12,248 $100,000 $25,000 $12,963 NA $62,037 $4,604 13.6 $16,852 3.7
HPWH
12 hrs/day
19,445 $18,372 $100,000 $25,000 $19,445 NA $55,555 $6,906 8.0 $25,279 2.2
HPWH
18 hrs/day
29,168 $27,559 $100,000 $25,000 $29,168 NA $45,832 $10,359 4.4 $37,918 1.2
HPWH
24 hrs/day
38,890 $36,745 $100,000 $25,000 $35,000 NA $40,000 $13,812 2.9 $50,557 0.8
Added to
electric
boilers
kWh - - - - - - - - - -
HPWH
8 hrs/day
379,696 $12,248 $100,000 $25,000 $35,000 NA $40,000 $25,721 1.6 $37,970 1.1
HPWH
12 hrs/day
569,544 $18,372 $100,000 $25,000 $35,000 NA $40,000 $38,582 1.0 $56,954 0.7
HPWH
18 hrs/day
854,317 $27,559 $100,000 $25,000 $35,000 NA $40,000 $57,873 0.7 $85,432 0.5
HPWH
24 hrs/day
1,139,089 $36,745 $100,000 $25,000 $35,000 NA $40,000 $77,164 0.5 $113,909 0.4
*Business Energy Tax Credit (Oregon)
**Energy Trust of Oregon Existing Buildings Program

The Bottom Line on HPWHs

Properly applied and installed, HPWHs save energy in almost every situation.

The most cost-effective way to heat water. Because heat pump water heaters use electricity to move heat from one place to another rather than generating heat directly, they can be two to seven times more energy-efficient than conventional water heaters. That means that HPWHs can produce the same amount of hot water using less then half the amount of energy as conventional water heaters.

The best ROI with the shortest payback. As shown in Table 3, a typical HPWH application has a short payback time—as low as five or six months. Compare to solar thermal, which can have a payback time of 25 years or more.

Significantly lower first cost than solar. Even with subsidies, solar is approximately two to five times more expensive than HPWHs, with less annual energy savings.

Captures and recycles waste energy. In applications such as building exhaust air and commercial laundries and kitchens, HPWHs let you capture heat energy you have already paid for and recycle that energy back into hot water demand (and cool air for air conditioning).

Effectively reduces natural gas use and carbon footprint. For example, a continuous 500,000 BTU-demand HPWH reduces carbon emissions by 77.5 tons per year.

Whether you want to save the environment, save money, or both, HPWHs are as green as it gets.
Contact Wescor for additional information on heat pump water heaters
Oregon, SW Washington, and Idaho
Dave Baasch
503 231 4009
DaveB@wescorhvac.com
Southern Oregon
Dan Salter
619 823 1573
DanS@wescorhvac.com
Washington and Alaska
Spence Braden
206 933 9651
SpenceB@wescorhvac.com