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Cold weather, heat pumps and thermostats

By: Edna Schack

We’ve all heard the mantra, “in the winter, turn your thermostat down when you are away from home or sleeping to save energy.” Is this always true? If you have a conventional furnace, this will clearly save heating costs because you are not creating and expending unneeded heat. However, if you have a heat pump, the answer is a little more complex.

There are several types of heat pumps; the most common in this climate is an air-to-air exchange heat pump. This type of heat pump takes the heat from the air and puts the heated air where you want it. In the summer, acting as an air conditioner, the heat pump takes heat from inside your home and sends it outdoors, making the air in your home cooler and more comfortable. Heat pumps (and conventional air conditioners) are given a SEER (Seasonal Energy Efficiency Ratio). A higher SEER rating indicates lower energy use needed to cool a given area. In the winter, a heat pump is reversed so it takes heat from the outdoor air (even winter air has some heat) and sends it indoors, warming your indoor air. This is accomplished with the heat pump’s compressor and a refrigerant that cycles through the coils of the heat pump. In addition to the SEER rating, heat pumps are also given an HSPF (Heating Seasonal Performance Factor) rating. A higher HSPF rating indicates lower energy use needed to heat a given area. An excellent description of how an air-to-air heat pump works is at http://www.flex.net/~lonestar/hpumps.htm  or
http://www.progress-energy.com/custservice/carres/energytips/heating.asp#b2.

A heat pump is an energy efficient heating choice for a moderate climate because it doesn’t need to generate all the needed heat. Heat pumps increase efficiency by also making use of existing outdoor heat, transferring the heat indoors, whereas most other heat sources generate all the needed heat through electricity, oil, coal, wood, or natural gas. An energy efficient heat pump, when used in combination with a well-sealed and insulated home envelope, can save homeowners about 30% to 40% of energy used to heat over a conventional heating system, according to the U.S. Department of Energy. ( www.energysavers.gov ) A further benefit of heat pumps is their ability to also cool, functioning essentially like a conventional air conditioner. The dual functions and energy efficiency for heating are why Frontier Housing homes are heated and cooled with a heat pump.

You might be asking, “can I save even more in energy costs if I set my thermostat back at night and when I’m away from home?” The answer, when you have a heat pump, is maybe, maybe not. Most heat pumps have an auxiliary heat source that is used for several scenarios. The auxiliary heat source is often an electric resistance heating element that is more expensive to operate than the heat pump compressor. Minimizing use of the auxiliary heat source is important for keeping energy use and costs down. There are times when the auxiliary heat source is a necessity, for example when the outdoor temperature drops to a point that the heat pump cannot keep up. This is referred to as the “balance point” and varies with the heat pump model.

Another scenario that can cause the auxiliary heat to kick on is when the temperature in your home fluctuates more than, typically, two degrees below the “set point” of your thermostat. This can be a problem when you set your thermostat to 58° at night, then turn it up to 68° in the morning. In some heat pumps, it can signal the more expensive auxiliary heat source to kick in to help bring the temperature up to the new desired level.

Some electronic programmable thermostats are designed for use with heat pumps, optimizing efficiency for various conditions. If you have a heat pump, but don’t have a thermostat that is designed to do this, keeping your thermostat at a constant temperature (about 68° in winter) is a more energy efficient approach.

If you are considering adding a programmable thermostat to your heat pump system, be certain the one you choose is designed to work with your system. Also, choose one that is easy-to-program; you don’t want to dig out your manual every time you need to make a change. Some have directions on the inside of the thermostat door. For additional useful information about thermostats and other home improvement topics, visit
http://www.hometips.com/cs-protected/guides/thermostats/thermostat_types.html . Several utility companies also provide useful information on their websites. One example is www.dom.com/customer/efficiency/bus/hc_thermo.jsp.

By Edna O. Schack, MSU Professor on sabbatical with Frontier Housing,
Fall 2008

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