How heat pumps work and who makes them

Q. Tell me something about heat pumps? How do they operate? Are they energy-efficient? What are the advantages and disadvantages of this type of heating-cooling system?

* Also, I would like some information on how to install a hot-water recirculating system which makes hot water available instantly at every faucet. I understand this can be done with a pump, but I am interested in a gravity system. Robert Agnew Novato, Calif.

A. Three national manufacturers of heat pumps are General Electric, Lennox, and Carrier, but there are others.

Heat pumps, which appeared on the market about 40 years ago, combine both heating and cooling in one reverse-cycle unit. They keep the home cool in the summertime and warm in winter by using only electricity and heat extracted from the outdoor air as fuel.

Working like a no-frost refrigerator, a heat pump contains outdoor and indoor coils, a compressor, and an expansion device. In winter, a heat pump absorbs heat from the outside air even when the temperature is as low as zero. It then transfers that heat inside.

Even with the thermometer at zero degrees F., the air still contains more than 82 percent of the heat that was available at a sweltering 100 degrees. In summer the system simply reverses itself, expelling inside heat outdoors.

Compared with a ducted electrical heating system, one manufacturer says that a heat pump could reduce the season's electrical heating bills from 30 to 60 percent, depending on where and how you live. Since it's less expensive to move heat than to generate it, the heat pump delivers heat for less cost than electric resistance systems.

Under some conditions, heat pumps can deliver nearly three units of heat for every unit of electricity used. That's nearly 300 percent efficiency.

Uncertainty of gas and oil supplies and rising costs have popularized heat pumps more and more. Such systems have no flames or fuel to worry about, have no dirt or combustion odor, and operate quietly.

Disadvantages of heat pumps are minimal when made and installed by proven and reliable firms. If the power goes off, so does the heat pump. Of course, that problem exists with any heating or cooling system which is energized electrically.

It has to be said that heat pumps work better in moderate climates than in extremely cold areas. Too, the cost of operation is lower where the weather is warmer.

Here's an example: One study shows a 29 percent saving by heat pumps over conventional electric heat in Minneapolis, where winter temperatures are very low, compared with a 56 percent saving in much milder San Jose, Calif.

In colder climates you would very likely need a supplemental heating system when the temperature drops much below freezing.

* Now about that gravity hot-water system minus a pump. Hot water can be made to circulate between heater and faucet when the pipe is inclined from the heater to the faucet and declined from the faucet to the heater. This works in theory, but it may take an installation expert to make it work in practice.

In other words, you may be hard put to do th e job yourself and have it work well.

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