Electric Heat: Heat Pump Electric Back-up Heating

There are different types of electric heating available in homes but probably the most common use for electric heat other than portable electric heating units is electric heat strips used for back-up heat in heat pumps. Sizing the electric heat strips for heat pump back-up heating applications will depend on many different variables which include the size of the space to be heated, the insulation values of the whole structure including doors and windows, and the geographical location of the dwelling. Typically, heat pumps are more common in southern climate regions where the temperatures in the winter are mild. When the temperature outdoors drops below 38 degrees Fahrenheit the outside heat pump units starts lagging behind on demand to heat the structure so a back-up source of heat is needed to provide heating when the temperature outside drops below this temperature.

Staging Electric Heat Strips for Heat Pump Back-up Heat

As the temperature outside falls the ability of the outside heat pump condensing unit to provide declines. A heat pump condenser absorbs heat from the outside air and moves this heat indoors. Even though the temperature outside is 40 or 45 degrees Fahrenheit there is still heat present in the air. However, as the temperature drops the mechanical process used to move this heat to the indoors declines and it becomes necessary to provide back-up heat for the structure. This is sometimes done in stages to minimize energy usage. When a heat pump system with back up heat strips are properly controlled only the amount of energy necessary to provide heat for the demand is utilized. This is down with a heat pump thermostat. As the temperature outside drops and the outside unit loses its ability to provide heat the thermostat ambient temperature reading drops. Put simply, the unit cannot keep up with demand and the temperature in the structure drops. Depending on the thermostat, staging set points or the settings of the stage set points (in some heat pump thermostats the temperature set point for staging is manually set on installation. These are usually top of the line digital heat pump thermostats.). A heat pump system with two stages of heat will have temperature set point differentials of 2 to 4 degrees Fahrenheit to initiate staging of the heat strips. So when the temperature drops below 2 to 4 degrees Fahrenheit of the manual set point stage one will turn on providing back-up heat for the system. If the temperature were to drop 2 to 4 degrees Fahrenheit further below the manual set point adjustment then the second stage of electric heat strips will engage providing additional heat for the structure.

Electric Heat Strips and Heat Pump Condenser Defrost

Every air to air heat pump system needs a defrost cycle to defrost the coils of frost and ice. The heat pump condenser almost always operates below the dew point when in the heating mode and when the heat pump condenser is operating below the dew point moisture in the air will cause frost or ice to build up on the outside heat pump condenser unit. It becomes necessary to defrost the heat pump condenser coils on a periodic basis to prevent blockage of the heat pump condenser coils from ice and frost build-up. This controlled by a defrost timer control board (printed circuit board). The outside heat pump condenser unit will change from the heating cycle to the cooling cycle. In the cooling cycle the heat pump condenser unit will begin to heat up so that the ice and frost will defrost. When this happens, cold air can be introduced into the structure and needs to be countered. The defrost timer control board, when it calls for the heat pump condenser unit to go into the defrost mode, will also engage the electric heat strips to counter the cold air produced by the heat pump condenser when it goes into defrost mode.

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