By Tad Montgomery
In addition to heating and cooling buildings, air source heat pumps (ASHP’s) can be used for the heating water. Conventional electric water heaters can be converted, but it is more efficient to use one made for the purpose. The ASHP typically draws heat from the basement where it is located and delivers it to the water in the water tank, using roughly one third of the energy of a standard electric water heater.
It is also possible to replace oil-fired boilers with ASHP’s for combined space-and-water heating. These old boilers had to run during the summer to provide hot water, and this used a lot of oil. It has been estimated that an average homeowner saves 150-200 gallons of heating oil a year by replacing their oil boiler with an ASHP water heater. At $3.50 per gallon for heating oil and a price tag of $1,200 for a cheaper ASHP water heater, the payback is less than two years.
Another profound advantage of the ASHP hot water heaters is that they dehumidify the basement where they are located. Just as with a conventional dehumidifier, there is water condensate to deal with, but if a basement needs a dehumidifier, the ASHP will do the job as a side benefit, with additional cost savings.
A down side is that an ASHP water heater can cost significantly more than a conventional hot water system. ASHP’s for domestic hot water range from $1,200 to $2,500, but many utilities offer incentives up to $1,000 because of their amazing efficiency. They are also noisy, though the noise level, as well as the efficiency, varies substantially from unit to unit. They also can take longer to heat a tank of water than conventional water heaters do. Because of this homeowners are often encouraged to buy the larger units, which cost more. ASHP’s are also more complicated, with moving parts that can fail.
ASHP water heaters are not appropriate for every situation. Putting one in the air- conditioned basement of a house heated by electricity would not be a good idea because the space heating and hot water systems would work against each other. In the right situation, though, they have the potential to reduce energy usage dramatically. These systems have been used in Asia and Europe for decades, and now that energy prices are climbing in America we will be seeing a lot more of them here, too. In fact, the federal government has all but mandated their use starting in 2015 by increasing the performance standards of water heaters.
There are ways ASHPs could be made even more efficient. For example, warm outdoor summer air could be ducted to a water heater ASHP to increase its efficiency. We can envision an ASHP water heater being coupled with a refrigerator or air conditioner, or even used to cool a root cellar, since they have opposite and complementary needs.
Rebates: Efficiency Vermont & PSNH do not at present offer rebates for ASHP’s, though a representative for PSHN queried by Green Energy Times emailed: “For 2013-2014, all 4 (NH) electric utilities proposed heating and cooling system rebates, including a rebate for mini split / air source heat pumps. We think this technology is extremely efficient and works great with homes heated by Oil or LP FHW (liquid propane forced hot water – eds).” The NH Electric Co-Op offers rebates up to $2,000 for mini-splits and up to $1,000 for ASHP hot water units. Western Mass. Electric offers $1,000 for ASHP water heaters and $150-500 for “Central AC/Heat Pump.”
Models: Some ASHP space heater models: Mitsubishi’s ‘Mr. Slim,’ Fujitsu’s 12RLS.
Some ASHP water heater models: AirGenerate’s Air Tap hybrid, General Electric’s GeoSpring, Stiebel Eltron’s Accelera, Rheem’s Hybrid Electric and A.O. Smith’s Voltex.
Tad Montgomery is an ecological engineer based in the Connecticut River Bioregion: http://TadMontgomery.com