oikos®

Performance
Energy Use and Savings
Cost and Savings
Ventilation Effectiveness
Customer Satisfaction
Performance and Hot Water Demand
How an EAHP Works
Research Partners

In highly energy-efficient houses, water heating can use more energy than space heating. After you pile on the insulation and caulk every crack in sight, you should consider more efficient water heating. One promising technology is the exhaust air heat pump (EAHP).

The need for a high-efficiency electric water heater prompted researchers to find out how well EAHPs performed in the real world. The study, sponsored by the Bonneville Power Administration (BPA), tracked 31 Therma-Stor brand EAHPs for up to 13 months. The researchers installed monitors to ecord energy use and run time. They also measured system air flow and air leakage in the test houses.

Performance

Monitoring results showed that EAHPs are reliable and efficient. Efficiency, measured as Coefficient of Performance (COP), averaged 2.0 for all the units. There was no significant difference between the units installed in the colder climate (Idaho) and the warmer coastal climate (western Washington). However, there was wider COP variation between homes in the Idaho sample. The COP also varied little across the seasons. That stands to reason because the primary heat source is conditioned interior air.

During the test period, maintenance visits were made to nine sites. Most repairs involved thermostat failures or improper refrigerant charges.

Energy Use and Savings

Average energy savings for all units was 2200 kWh per year (183 kWh per month) compared to electric resistance water heating. The table shows monthly energy use for different components.

Cost and Savings

Translating energy savings into cost savings depends on the local cost of electricity and the amount of hot water used. In the Pacific Northwest, where the average electric rate is about $0.05/kWh, annual savings would be $110. At the national average rate of $0.08/kWh, the average savings increase to $177/year. At $0.12/kWh annual savings would climb to $264.

The report cited the typical cost of Therma-stor's HPV-80 at $1449. Installed cost averaged $2272. However, the EAHP replaces several other pieces of equipment, such as the standard water heater and bath fans. Taking these "avoided costs" into account, the net installed cost was reported at $1802.

To be economically acceptable to most consumers the initial cost would have to be recovered within five to eight years. At the national average electric rate of $0.08/kWh the savings would pay off the installation cost in slightly more than 10 years. The investment would be more attractive at higher electric rates. Electric utilities in some areas have helped improve the economics by offering cash rebates and other incentives to consumers who purchase EAHPs. Until recently, BPA offered a $1300 rebate for the Therma-stor HPV-80, although that program has now expired. The rebate reduced the payback to 4.5 years, even at the low electric rates in the Northwest.

Dataloggers, kilowatt-hour meters and a water meter (not shown) monitor system performance.

While electric rates are one important factor, the amount of hot water used also has a big impact. The average house in this study used about 65 gallons of hot water per day, which is close to the national average. Even at low electric rates, households that use large amounts of hot water would see more favorable economics.

Few building products are judged strictly on financial payback. The EAHP also offers other benefits, such as excellent ventilation and less lime buildup in the water tank.

Ventilation Effectiveness

The EAHP compressor must run six to eight hours per day to keep 54 gallons of water at about 120°F. That virtually guarantees adequate background ventilation for all but the largest homes.

Blower door and tracer gas measurements established the air leakage of the tested houses. Although a few individual houses varied widely, the average total leakage rate was 0.38 ACH. Of that, the EAHP accounted for 0.17 ACH. Without the ventilation system many of the houses would have fallen to 0.29 ACH, below the ASHRAE minimum ventilation rate of 0.35 ACH.

The EAHP exhaust fan can be controlled by a 24-hour timer. Some homeowners didn't set this ventilation timer at all. Instead they received ventilation only when the system made hot water. This demonstrates one advantage of the technology. Successful ventilation is automatic and resistant to operator error. Ventilation also coincides with periods of hot water use when showers, dish washing or clothes washing injects water vapor into the air and creates a need for ventilation.

Air flow measurements taken in 19 sites at the beginning and end of the test period revealed one potential problem. Four sites showed a drastic drop in air flow through the EAHP. In only two cases did the resulting air flow fall below the ASHRAE minimum background ventilation standard (0.35 air changes per hour or 15 cfm per occupant). However, the EAHP needs a minimum air flow to function properly. Low air flow could reduce COP and cause ice to build up on the coil. The unit stops heating water when the coil freezes, but the equipment isn't harmed.

The report didn't identify reasons for the reduced air flow. However, the most likely cause would be dirty filters. While it's easy to change a filter, homeowners are often less than vigilant about such basic maintenance.

Customer Satisfaction

Homeowner reaction was generally favorable. None of the occupants complained of noise or persistent condensation. As with all water heaters, there were times when the demand for hot water exceeded the supply. Those with large bathtubs or spas ran out of water most often. Most people said they were happy with the system. However, several said that they would not have purchased the EAHP without the utility rebate. Two of the 31 omeowners said they wouldn't choose the EAHP, regardless of the rebate, because of hot water shortages.

This exhaust air heat pump was installed in a utility closet.

Performance and Hot Water Demand

Colder water means higher efficiency, because heat moves from the heat pump's condenser to the water more rapidly when the water is cold. So, households that draw more water out of the tankwhich draws more cold water in create better operating conditions. The EAHP study confirmed this principle. Efficiency was lower in the households with lowest hot water use.

Hot water demand also affects energy and cost savings. The EAHP cut the average hot water bill in half. Large households that spend more money to heat water would save more than households that use less hot water.

Deciding Factors

Should you install an EAHP in the next house you build? It depends. The Therma-stor units in this study were reliable and efficient. Equally important, EAHPs virtually guarantee adequate ventilation. In areas with low electric rates, the long payback could discourage homeowners, unless utility rebates are available. However, in regions with higher rates, the economics are more favorable. Households with above average hot water demand would benefit more than below average users. The match of efficient water heating and enforced ventilation makes an EAHP a valuable addition to tightly sealed, highly insulated houses.

How an EAHP Works

An exhaust air heat pump heats water using a small compressorabout the size of a window air conditioner. A duct connected to the EAHP draws stale house air through the unit. The EAHP strips the stale air of much of its heat, then blows that air outside at a temperature between 45° and 50°F. Branched collection ducts are often used to pick up stale air from bathrooms, so bath fans aren't needed. When connected to a properly installed duct system, the air flow rate is about 150 cubic feet per minute (cfm). Long or convoluted duct runs could reduce the flow rate.

Houses need regular ventilation at least 15 cfm per person. The exhaust air heat pump is a way to get adequate ventilation while reducing the amount of heat that escapes. Since some people don't operate bath and kitchen fans as much as they should, the enforced ventilation of the EAHP guarantees at least a minimum level of ventilation.

A technician puts the finishing touches on the monitoring equipment.

EAHPs deliver about 7,000 Btu/hr of heat to the water, about half the rate of a standard electric water heater with a 4.5 KW element. To compensate for slow recovery, EAHPs should have larger storage tanksat least 80 gallons. This reduces the chance that occupants will run out of hot water. With a compressor that draws only about 800 watts, the unit contributes less to the electric utility's peak demand.

Standard heat pump water heaters draw heat from room air which reduces indoor temperature. That can be an advantage during warm weather, but in most heating-dominated climates its called the "heating enalty." EAHPs operate differently. They expel the chilled air outside, reducing the heating penalty.

HPWHs use CFC-22 refrigerant, which is considerably less harmful to atmospheric ozone than some other refrigerants. Nevertheless, CFC-22 will be phased out over the next 15 years or so. By then a suitable replacement should be available.

Very little maintenance is required for the Therma-stor EAHP. Filters should be changed every 6 to 12 months. The blower motor requires a few drops of oil every six months. The sacrificial anode that protects the tank from hard water should be inspected periodically and changed when needed.

Research Partners

Several organizations collaborated on this research project. It was funded by the Bonneville Power Administration (BPA) and coordinated by the Idaho Department of Water Resources (IDWR) Energy Division. Kootenai Electric Cooperative helped recruit participants in Idaho, while Puget Power help locate them in western Washington. Pacific Science & Technology of Bend, Oregon, conducted much of the field work.