December 2, 2016

Hot Water Can Heat Homes

Using a reverse cycle chiller with your home’s heat pump can be a less costly alternative to supplemental resistance heating.

Q: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?

A: 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 that 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 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 asked about 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 output capacity of a typical heat pump is sized for the cooling Btu requirements of the house. In most parts of the U.S., 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 heat pump also drops just as the heating needs of your house increase.

You might think you could install a larger capacity heat pump to provide enough heat for your house even on very cold days. This would be possible with an air-source heat pump, but it would not work well in the air-conditioning mode. An oversized air conditioner results in short cycles, indoor temperature swings, and poor dehumidification.

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.

It may be more efficient and comfortable in some rooms to use radiant floor heating. For this heating method, the hot water flows through pipes in the floor. This is one of the most efficient heating methods because you can feel comfortable at a lower room air temperature. This option is more feasible for new construction, but it can be used if you have a basement providing access to the underside of the floor above.

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.