Heat Pump Primer

I used to think that electric heating could never be justified—for any reason. As energy guru Amory Lovins said in the 1970s, using electricity for heating was “like cutting butter with a chain saw.” It just didn’t make sense to use such a high grade of energy for such a low-grade energy need.

But since then, electric heat pumps have become widely available. This has changed my mind and I find myself recommending electric heat all the time. In fact, a new house that my wife and I are planning in southern Vermont will have electric heat—from a heat pump. Let’s find out what heat pumps are, and why they make sense.

Electric Heating Basics

What I thought about as electric heat in the 1970s, when Lovins’ preaching was ringing in my ears, was electric-resistance baseboard heating. With electric-resistance heat, electric current is converted directly into heat. Special “resistance wire” is used that resists the flow of electrons, producing heat in the process. Electric-resistance heat is most commonly delivered through baseboard “radiators,” but it can also be delivered through radiant ceiling panels, fan-driven convection heaters, and radiant-floor systems.

Electric-resistance heating is 100% efficient—each kilowatt-hour (kWh) of electric energy is converted into a kWh of heat. However, that only considers the conversion efficiency in the unit—and does not factor in the utility transmission losses or the efficiency of the source (see “Improving Source & Site Efficiency” sidebar).

Instead of using electricity directly to produce heat, the electrical energy can be used to move heat from one place to another—like from outdoors to indoors. That’s the basic principle of a heat pump: it extracts heat from the outdoor air (air-source heat pump) or from the ground (ground-source heat pump) and delivers that heat inside the building.

The idea of moving heat from a cooler to a warmer place is counterintuitive, but it’s exactly what your refrigerator does. Peter Temple, an architecture professor Keene State College, refers to this concept as “pumping heat uphill.” While electric resistance is 100% efficient at converting electricity into heat, a heat pump can deliver (move) two to four units of heat for every unit of electricity it consumes.

Every substance that is at a temperature above absolute zero (about –460°F, where all molecular motion stops) contains heat energy. Cooler objects have less heat energy (energy of motion of the molecules) than warmer objects. But even air molecules at 0°F have enough heat so that we can extract some, and leave the air just a little cooler than it was before.

A heat pump’s coefficient of performance (COP) is the ratio of energy delivered compared to the operating energy. A heat pump with a COP of 1 delivers 1 unit of heat for each unit of electricity consumed—no better than electric-resistance heating. A COP of 2 provides twice as much heat from the electricity consumed. That can be thought of as 200% efficiency—though it really isn’t a measure of efficiency, since the electricity is used for moving, rather then generating, the heat. Some heat pumps, in some situations, can have a COP as high as 5 or 6.