Q: I need a new central air conditioner. I've heard of ground-source heat pumps that can produce $4 of free heat for each $1 on my utility bills. Are they also super-efficient for air conditioning?
A: The correct name is "geothermal heat pump" and they are also super-efficient central air conditioners in the summer. Replacing a 10-year-old central air conditioner with a geothermal heat pump can cut your air conditioning bills by up to 50 percent. The steady, high cooling capacity ensures comfort even on the hottest days.
Even if you have a gas furnace, it may be economical to add on a geothermal heat pump. With some of the new geothermal units, you pay only 20 cents on your electric bill for each $1 of heat you get in the winter.
Another advantage is free hot water in the summer. With a geothermal unit, the waste heat removed from your house runs through a heat exchanger that transfers the heat to your water heater. It can also heat water in the winter less expensively than an electric water heater.
A geothermal heat pump is very efficient because it uses the earth as a source for heat in the winter and a sink to exhaust heat in the summer. Since a condenser fan is not needed, the entire system (in a soundproof cabinet) is located inside your utility room or basement. Since it is indoors, there is little maintenance and risk of damage.
In summer, ground temperatures several feet deep can be 30 degrees cooler than the outdoor air. This makes it easier for the heat pump to exhaust heat from your house. In the winter, the ground can be 40 to 50 degrees warmer than the air, so it's easier to draw out the free heat.
A closed-loop geothermal heat pump system circulates a water-antifreeze solution in a small plastic pipe buried in the ground. This long pipe can be laid horizontally in a very narrow, trench that is several feet deep. Another closed-loop system uses a long vertical pipe in a hole drilled in the ground. An open-loop system can use water from ponds or wells.
There are several design variations. One manufacturer has a two-speed compressor. It runs in the super-efficient slow speed 80 percent of the time. It switches to high-speed only during very hot or cold weather. Another design runs the refrigerant through the pipes in the ground. This eliminates one of the heat exchangers and increases efficiency.
You can write to me for Utility Bills Update No. 595 showing a buyer's guide to geothermal heat pump manufacturers. It lists maximum cooling and heating outputs, efficiency levels, types of compressors, prices, and special features. Include $1.50 and a self-addressed business-size envelope.
Correspondence should be addressed to James Dulley, c/o Baltimore Sun, 6906 Royalgreen Drive, Cincinnati, Ohio 45244.