Heat Pumps1

Air to Air and Add-On Heat Pumps 

A heat pump is an extremely efficient, year- round heating and cooling system that runs on electricity. During the summer, it "pumps" heat inside your home to the outside to cool and dehumidify your home. In the winter, it "pumps" heat from outside to the inside to heat your home.

How does it work? 

The scientific principle that makes heat pumps work is that heat always attempts to equalize temperature by moving from warm to cool. The heat pump takes advantage of this principle to provide heating and cooling in an extremely efficient manner. In the summer, the compressor moves freon through a coil in the ductwork where it absorbs heat in the air. It then compresses the freon to make it even warmer. The freon is then pumped to the coils outside where a fan blows air across the coils, removing heat and cooling the freon before it returns inside to absorb more heat.

In the winter, the flow of freon is reversed. It flows through the outside unit where it absorbs heat from the outside air. From there, it goes to the inside coil where it radiates it's heat to the inside, cooler air. It is then returned to the outside unit again to absorb more heat energy.

Why is it so efficient? 

Most heating systems convert energy from one form to another to create heat, (gas to a flame, liquid to a flame and electric energy to heat energy). Therefore, they can only give out as much heat energy as the original form contained (most systems cannot convert at 100% efficiency because heat escapes up the flue with hot exhaust gases). A heat pump does not change energy forms to create heat. It simply moves free heat from the outside to the inside. The only energy it needs is in the compressor motor which pumps the freon through the system. Because it is moving heat and not creating it, a heat pump is from 170% to 330% efficient, depending on the unit and the outside air temperature.

As the temperature difference between the air inside your home and the air outside increases, the amount of heat energy (BTU's) needed inside also increases. Conversely, as the outside air temperature decreases, the amount of heat energy (BTU's) that the heat pump can absorb from it decreases. Where these two rates are equal is called the balance point. Below this, the heat pump cannot supply enough heat to maintain the inside air temperature. This is when some type of auxiliary heating system is required. This can be from either a fossil fueled furnace or electric resistance heat. You could oversize the heat pump to provide 100% of the heating needs, but in the summer air conditioning mode, the unit would not run enough to remove the humidity from the house. As relative humidity is a large factor in cooling comfort, a heat pump should be sized to the cooling load requirement.

What is the difference between a heat pump and an add-on heat pump? 

When installing a conventional heat pump, the contractor will install a complete heating system consisting of an outside condenser unit, an inside evaporator coil and an air handier. The air handler contains the auxiliary electric resistance heat strips and the fan that moves the air through the duct system.

An add-on heat pump installation differs from a conventional one in that the contractor can use the home's existing furnace for the auxiliary heat source and fan. He will simply add the outside condenser unit and the inside evaporator coil to the central heating system. The add-on heat pump can be added to an existing heating system or it can be installed in conjunction with a new central heating system. The add-on heat pump will work with any type of auxiliary system, be it electric, natural gas, fuel oil or propane. When the outside temperature falls below the balance point, (when the home's heat loss and the heat pump's output are equal) the auxiliary heat system will begin to work in conjunction with the heat pump in order to maintain the thermostat setting. If the auxiliary heat is provided by a natural gas, fuel oil or propane furnace, as opposed to electric resistance heating strips, the electrical demand on the power supply system will be reduced substantially.

Such an add-on heat pump adds little, if any, to the electrical system's peak demand during the peak heating times and consequently the cooperative does not incur additional wholesale power demand charges from its power supplier. That is why a rebate is being offered for add-on heat pumps that are installed in conjunction with a fossil fuel auxiliary heating system.

How much can I save?  

Your actual savings will depend on your home and the heat pump installed. Normal savings range from 10% to 30%. Your cooperative has computer programs that can calculate the proper size equipment for your home and the savings that you might expect. Additional Information   If you would like additional information on heat pumps, download the Heat Pump Reference Guide. This 74 page document has been prepared by the Cooperative Research Network for use by our consumer-members.