Magic Heat Pumps

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This article talks about magic — the sort of magic that allows you to heat and cool your home with solar energy stored in water, earth or thin air. That’s the essence of heat pump systems. The concept is simple: heat pumps use the Earth as a huge heat source or sink — borrowing heat in the winter and dumping heat in the summer. This exciting option offers long-term cost savings and less dependence on fossil fuels.

Heat pumps have been around since the early 1900s — refrigerators and air conditioners are types of heat pumps — but were not commonplace for residential buildings until the 1970s. Although they have been available for more than 30 years, heat pumps are still not understood by many homeowners.

Heat naturally flows from a warmer area to a cooler area. But with the aid of a compressor and a small amount of electricity, heat pumps can force heat (from water, air or the ground) to flow in the opposite direction. It’s a little like pumping water uphill. When the heat pump process is reversed in the summer, it will move warm air out of your house, eliminating the need for a separate air-conditioning system. And because heat pumps move heat from one place to another rather than create heat through combustion, they are up to four times more efficient than fossil-fueled furnaces.

There are two main types of heat pumps: air-source and ground-source. On the outside, they are unexciting — simple, rectangular, metal boxes with protruding pipes. It’s what’s inside that creates the magic. Here’s how they work:

The heat pump cycle begins as cold liquid refrigerant passes through a heat exchanger and absorbs heat from a natural source such as air, water or the ground. The refrigerant evaporates as the heat is absorbed, becoming a gas. This refrigerant gas then passes through a compressor, where it is pressurized, raising its temperature to more than 160 degrees. The gas then circulates through another heat exchanger, where heat is removed from the gas and transferred to air or water, which is then circulated into your house. (The temperature of the heated air or water is about 100 degrees.) As it loses heat, the refrigerant gas changes back to a liquid. The liquid is further cooled as it passes through an expansion device (which releases the pressure created by the compressor, causing the temperature of the refrigerant to drop), and the heat pump cycle is complete, ready to begin again.

Because a heat pump supplies lower-temperature air than a fossil-fueled furnace, a heat pump tends to run for longer periods of time. For people who are not accustomed to this, it may seem that the heat pump is always running. Yet, a properly designed and installed heat pump will deliver steady heating with less energy consumption than the fossil-fueled competition. For example, a typical air-source heat pump requires just 100 kilowatt-hours (kWh) of electricity to produce the equivalent of 300 kWh of useful home heat. And a ground-source heat pump can be even more efficient. This leads to substantial savings, especially in areas where fossil fuel prices are relatively high and electricity prices are relatively low. Savings are even greater with variable-speed heat pumps. These units adjust their output to match the actual heating or cooling needs of your home, reducing excessive cycling and wear on the system, especially during mild weather.