Efficient Heating: Duel-Fuel Heat Pump

Overview

Conventional heat pumps have been heating and cooling homes for decades. In fact, about one in three homes in the United States already uses one. However, there aren't many north of the Mason-Dixon Line because they don't work efficiently in subfreezing temperatures. These heat pumps are great at pumping heat in or out of a house in moderate temperature swings like those found in the Sun Belt. But they're notoriously inefficient and expensive to run in cold Northern winters.

“Dual-fuel” heat pumps are different. Attached to your existing furnace, this system looks (and works, during the summer) like a high-efficiency central air conditioner. However, in those mild spring and fall months in the snowbelt, they provide cheap heat just as well as they do in the South. As the temperatures drop, the pump shuts off and tells your furnace to take over.

A dual-fuel heat pump, such as Lennox's XP-14, will cost about 20 to 25 percent more (including installation) than an A/C. Depending on your region and fuel costs, however, it can pay for itself in five or six years.

Pumping cheap heat out of thin air

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Figure A: Cool-Period Operation

During cooler seasons like fall and spring, the heat pump handles the heating duties. The dual-fuel system still incorporates the furnace, but without using any burners, just its air distribution features. The heat pump sends hot “refrigerant” through the air conditioning coil within the furnace. The furnace fan draws air from the home's cold-air returns and blows that air over the warm coil and then sends the warmed air throughout the house. In the summer, the heat pump reverses the refrigerant flow so cold liquid flows over the coil and cools your home just like a conventional air conditioner.

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Figure B: Cold-Period Operation

During cold periods, the heat pump slumbers while the furnace takes over, supplying the heat by operating like the conventional furnace it is. The burners ignite the fuel (oil, propane or natural gas) and the furnace fan blows air drawn from the cold-air returns and warms it by blowing it over the heat exchanger and on throughout the home.

As shown in Figure A, an air-source heat pump is basically a hybrid air conditioner. Both have a compressor (a high-pressure pump) that circulates refrigerant (a volatile gas) through indoor and outdoor coils, a network of tubes designed to facilitate the capture and release of heat. But while an air conditioner can move refrigerant in only one direction, a heat pump can force refrigerant in either direction, for heating one way and cooling the other. The pump does this by means of an extra diverting device called a switchover valve.

Run the numbers

To figure out whether a heat pump is practical for your home, you'll need to contact a heating contractor and ask a few questions:

1. Start with a Heating and Cooling Load Analysis. Don't trust the label on the old furnace; ask your installer to show you the math. According to some reports, there's a good chance that your system may not have been sized correctly in the first place. Even if it was sized properly originally, subsequent home improvement projects (new insulation, new windows or an addition) can change your heating and cooling needs.

2. Check the numbers. Manufacturers use different technologies, but one number can provide an apples-to-apples comparison. For cooling efficiency, check the Seasonal Efficiency Ratio (SEER). The higher the SEER, the more efficient the unit is. Of course, units with higher ratios cost more, but every two points can reduce cooling costs by about 15 percent. (Energy Star–rated pumps are at least 8 percent more efficient than standard models.)

3. Conduct a comparative cost analysis. If you live in an area with lower-priced natural gas and sky-high electrical rates, a heat pump will not pay itself off as quickly. Your installer can factor in local energy rates (including peak and off-peak electrical rates) to calculate your potential savings and payback.

4. Ask about compatibility. Dual-fuel heat pumps are designed to work as a straightforward A/C replacement, but older furnaces probably won't work with a new switch-hitting system. You'll probably have to upgrade to a brand new furnace to have this system—adding a sizable chunk to the cost of the project.