© 2010, 2011 Western Environmental Services Corporation. All rights reserved.
Permission granted to reproduce for personal and educational use only.

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.


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.

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 4 shows the capacity of a typical 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.

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.

	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.

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.

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.


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

Commercial Kitchen

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

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).

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.

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
Jim McKillip
503 452 2498
JimM@wescorhvac.com

Washington and Alaska
Spence Braden
206 933 9651
SpenceB@wescorhvac.com

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© 2010, 2011 Western Environmental Services Corporation. All rights reserved.