Q. I have heard about an electric water heater that can help air-condition and dehumidify my house. What is it and is it very energy efficient? Can I attach one to my present water heater tank? — H.C.

A. You are referring to heat pump water heaters (HPWH) and they are very energy efficient. They are designed to draw heat from the surrounding room air and transfer it to your water heater tank. They can produce hot water at 140, hot enough for all household needs.

Some HPWH’s can operate at a coefficient of performance (COP) above three. This means that you get three Btu of heat into your water for each Btu worth of electricity you pay for. Depending on your local utility rates, the operating cost for a HPWH can compare favorably with a gas water heater. You also get the bonus summer air-conditioning.

A HPWH works very much like a small window air conditioner. Instead of exhausting the heat outdoors, it is transferred through a heat exchanger to your hot water tank. This leaves the room air around the HPWH cool and dry. A low-voltage wire from the tank thermostat signals the HPWH to come on when more hot water is needed.

There are two types of HPWH’s — integral and add-on. The integral type has the heat pump mounted on top of the tank. The add-on type is designed to be used with your existing water heater tank.

The small add-on HPWH (less than two feet wide and high) offers the advantage of being located remotely from your water heater. Therefore, you can locate it in a room or area that you want to cool and dehumidify. This can be a family room or a musty basement, for example.

You can write to me for Utility Bills Update 106 listing the manufacturers of add-on and integral HPWH’s, hot water output specifications, and a worksheet to determine the payback of installing a HPWH. Please include $1.25 and a self-addressed business-size envelope. Send to James Dulley, 6906 Royalgreen Drive, Cincinnati, Ohio, 45244.

James Dulley studied energy management at the doctoral level at Harvard.