Q: We have an old, inefficient electric resistance furnace. I hear there are new types of heat pumps that use a big water tank and don’t need backup heat. How do these systems work, and are they efficient?
A: Although electric resistance heating can be relatively expensive to operate, it is 100 percent efficient—that means all of the electricity you pay for ends up heating your house. With a gas or oil furnace, you lose some heat out the flue. The problem with electric resistance heating is it costs more to produce 1 Btu from electricity than it does by burning fossil fuels.
A heat pump can produce 3 Btu of heat for each 1 Btu on your electric bill. This is because the heat pump does not create heat directly. It uses a compressor and coils to draw heat from outdoor air and pump into your house.
This heat-pump system is a reverse-cycle chiller. It uses a standard high-efficiency heat pump to produce heat during winter and cool air in the summer. A typical air-source heat pump heats or cools a refrigerant that flows directly through an indoor coil. Air blows over the coil to heat or cool your house. A reverse-cycle chiller heats or cools water in a 20- to 40-gallon insulated tank. The water then flows through the indoor coil. The entire system costs 15 percent to 20 percent more to install than a standard heat pump/electric furnace combination.
The output capacity of a typical heat pump is sized for the cooling Btu requirements of the house. In most parts of the country, the heating Btu requirements are greater. To make up the difference, a backup electric resistance furnace is required during very cold periods. As the outdoor temperature drops, the heat output of the pump also drops, just as the heating needs of your house increase.
The primary advantage of a reverse-cycle chiller is it transfers heat to an insulated water tank. This allows you to install a heat pump with an extra-large capacity for adequate heating, even in cold weather, without the associated summertime cooling issues. Many of the major HVAC manufacturers’ heat pumps can be used with a reverse-cycle chiller system.
During summer, the large heat pump’s cooling capacity chills the water in the insulated tank to about 40 degrees. The chilled water is run through a coil in the blower system, which cools and dehumidifies indoor air just like a standard heat pump. The heat pump can cycle on and off as needed to chill the water in the tank independently of the indoor blower. Therefore, the blower can run as long as needed to provide comfort and efficiency.
Another key advantage of having the heated water tank is its wintertime defrost mode. A heat pump regularly switches to the cooling mode to defrost ice that collects on the outdoor condenser coils. During this time, expensive electric resistance heat comes on or chilly air blows out the registers.
With a reverse-cycle chiller, the heat to defrost the coils comes from the heated water tank so warm air continues to blow out the registers.
During regular operation, the temperature of the air coming out the registers is also warmer than with a typical air-source heat pump.
In addition to eliminating or greatly reducing the use of backup resistance heating, a reverse cycle chiller provides options for efficient heating. Because the heat comes from the insulated water tank, you can select different types of heating for different rooms. The hot water can be piped through a heat exchanger (fan coil) and typical ductwork to produce heated air.
The following companies offer reverse-cycle chiller systems: