By George Harvey
In physics, Gay-Lussac’s law says gasses expand as they are heated and contract as they cool. It explains why hot air rises. It also explains why a heat pump can work.
An air conditioner is a type of heat pump. Its compressor takes gas, typically at room temperature, puts gas under pressure, which increases its temperature. Next, the gas is pushed into a heat exchanger, which is outside the house. While the air outside is hot, it is not nearly as hot as the compressed gas, so the gas cools off. Next, the gas is depressurized, which cools it off. Since it had already lost its heat, it winds up being much colder than it originally was. It absorbs heat from inside the building, and is then ready to go through the cycle again. Put succinctly, it moves heat from inside a building to outside, even though the inside is relatively cooler and the outside is hot.
Some other heat pumps, such as those in refrigerators, are very familiar. Many people are unfamiliar with the heat pumps that can heat houses, however. They can extract heat from outside air, even when it is cold out there, and release it inside the house. Modern heat pumps are so efficient that they can do this even when it is very cold out.
The new heat pumps people are getting for their homes are not all alike, however. While they all can provide heat, and many can also provide efficient air conditioning, they should be understood before they are purchased so people get what suits them best.
One way to run a heat pump to heat and cool a building is to move the heat to or from the ground. This is the ground-source heat pump. The temperature of the earth is fairly stable, when you go more than a few feet below the surface. In most of New England, the temperature down there is about fifty degrees Fahrenheit. Heat dissipates through the ground. The earth is heated in summer and cooled in winter from above, but it is usually just heated from below, and the farther down you go, the warmer things usually get. So a simple way to run a ground-source heat pump is to put a heat exchanger consisting of a long section of pipe down a well. One variation on this is to do a relatively shallow excavation to put the pipes in a horizontal area a few feet below the surface. Another puts the pipes into a lake or river.
Another way to run a heat pump is to use the outside air as a heat source. Even cold air, sometimes below zero Fahrenheit, can be used as a heat source by a modern heat pump designed to do the job.
Which type of heat pump to use for heating a building depends on circumstances. The ground-source heat pumps are much more costly to install because they require drilling a well or excavating the land. Their advantage is that they are very much more efficient in terms of the amount of heat delivered per unit of electricity.
By contrast an air-sourced heat pump is very much less expensive to install but more expensive to operate. In some cases, the higher installation costs would be hard to recover from the differences in efficiency. So deciding which type of heat pump is better requires some thought.
We should note that there are other types of heat pumps, with other reasons to use them. One, sometimes used in industrial plants, takes heat from areas where processes create a lot of waste heat, and pumps it to other areas where heat is needed. An example of this would take heat from a place where ovens are being operated and move it to an office. Another type can move heat from a cool damp basement into a water heater, with the side benefit that the heat exchanger in the basement air can act as a dehumidifier.
Of course, backup heat should always be planned for. An ordinary wood stove might go for a long while without being used, but prove very valuable if the electricity goes out.
Heat pumps might seem like the latest gadgets in heating, but that does not make them expensive to run. The heat they provide is about as inexpensive as anyone could get today.