Q. I've been thinking about buying a heat pump to replace my aging forced-air oil furnace. How do they work, and are they really as efficient as advertised?

A. If you're shopping for a new home or looking for ways to trim costs, heat pumps can provide both heating and cooling, and in some cases, hot water heating.

In areas where natural gas is not available, heat pumps are a popular alternative, but using only a heat pump for all your heating needs wouldn't be economical, or even possible. Most heat pumps use electricity as a power source, and most of them just don't cut the mustard in very cold weather. Suplementary heat such as gas, oil, electric and wood are used when the temperature falls below about 5C, or 42F. However, if you already have a heating system and you're considering air-conditioning, a heat pump can be an economical way to heat or cool the house for most of the year.

Summer cooling will add to your annual energy bill! Heat pumps warm the house in cooler weather and cool it in warm weather. They are powered by electricity. If you add a heat pump or convert from another system, and you didn't have air conditioning before, you're going to find your electricity bills higher than before.

"How Do They Work?" Most heat pumps are "air-to-air" and use a fluid medium to absorb heat from the air at one location and transfer it to the air at another. Refridgerators and air conditioners are both examples of one-way heat pumps. But the cycle is reversible, and heat pumps can be used almost year-round to move heat either way. However, they can only extract heat from outdoor air down to a point where they are no longer economical; and at some lower point, most don't work at all. Ground and water source heat pumps are more efficient, but several times more expensive, and not suitable for all locations. "Bi-valent" heat pumps use a gas or oil-fired heater to pre-heat the outdoor coil, so that the heat-pump can continue to operate more efficiently in colder weather.

"What's the bottom line?"

There are many factors to consider before purchasing any heating or cooling system. Carefully weigh the ancicipated savings against the cost of the new installation. What kind of system do you already have, how old and how efficient is it, and will it be compatible, economical and convenient to use with a heat pump? What kind of energy sources are available in your area, and how reliable are they? Will you have to open finished walls and ceilings to install duct work?

Do your homework first!