Dear Jim: We have an old, inefficient electric resistance furnace. I have heard there are some 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? — Carter N.
Dear Carter: Although electric resistance heating can be relatively expensive to operate, it is 100-percent efficient — that means all 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 your house for each 1 Btu on your electric bill. This is because the heat pump does not actually create heat directly. It uses a compressor, coils and other equipment to draw heat from the outdoor air and pump it into your house.
The heat pump system you refer to is called a reverse cycle chiller. It basically 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 small (20- to 40-gallon) insulated tank. The water then flows through the indoor coil. The entire system will cost 15 percent to 20 percent more to install than a standard heat pump/electric furnace combination.
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, this large heat pump cooling capacity chills the water in the insulated tank to 40 degrees or so. 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 is coming 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.
During summer, an optional refrigeration heat reclaimer (similar to a geothermal desuperheater) can be used. Instead of the heat pump exhausting the heat to the outdoor air in the cooling mode, it can be used to heat your domestic hot water for free. During winter, the heat pump can be used to heat your domestic hot water in addition to the house. This produces hot water at a COP (coefficient of performance) of between 1.5 and 3.0, as compared to a standard water heater at only 1.0.
The following companies offer reverse cycle chiller systems: Aqua Products, 800-849-4264, www.aquaproducts.us; Multiaqua, 864-850-8990, www.multiaqua.com; and Unico System, 800-527-0896, www.unicosystem.com.
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Send inquiries to James Dulley, The Tennessee Magazine, 6906 Royalgreen Drive, Cincinnati, OH 45244, or visit www.dulley.com.