Centrally ducted heat pump systems utilize the addition of an indoor furnace, fan coil or air handler which connects to a central ducting system and distributes heating and cooling to all areas of your home. There are two main types of ducted heat pumps. The first, and the most commonly known are air source heat pumps (ASHP). ASHP’s have a furnace inside connected to a heat pump condenser located outside. The second type of heat pump system is the Geothermal or “ground source” which have a series of pipes underground that absorbs the heat from the earth and then rejecting it inside your home through central ducting.
For more information on Geothermal Heat Pumps click here
When it comes to the operating ability of an air source heat pump, they will provide the majority of your homes heating needs until the outdoor ambient temperature drops down to what is known as the heat pumps “balance point”
The balance point is based on the system capacity and current load demand. The capacity of a heat pump is how much heating and cooling it is capable of producing. The load is how much heating or cooling is needed for your home. The capacity of your system is rated by the manufacturer under controlled conditions.
With oil or gas furnaces, electric baseboard or hot water boilers, the ouput capacity should not change as the outdoor temperature drops.
Air-source heat pumps on the other hand use outside air as the heat source. As the outdoor temperature drops, the heat pump BTU output decreases. The homes heating demand also increases as the temperature drops. The balance point is when the system capacity and total heating demand are equal.
When the outdoor temperature drops below your heat pump systems balance point it needs supplemental or auxilliary heat.
The balance point for air source heat pumps varies. The balance point of an energy star heat pump is lower than that of non energy star models and the greater the efficiency of the heat pump the lower the balance point which means that the heat pump willl provide more of your homes heating demand at lower temperatures before your supplemental or auxilliary engages.
With a fully electric heat pump system there is an electric heating element located inside the fan coil or air handling unit (AHU) that acts as the supplemental heat to boost the system when the temperature is below the balance point. The heat pump will still operate and provide as much of the heating demand as it can and the two work together to maintain the heating set point.
Supplemental heat is not auxiliary heat.
When a heat pump is combined with an oil or gas furnace the two systems will not operate at the same time as with an electric supplemental system. This type of combination is known as a “hybrid heat” or “add on” system. These systems do not operate the heat pump when the gas or oil furnace is engaged, making this type of system much less efficient than a fully electric.
Many customers that have existing forced air oil or natural gas furnaces can have a heat pump added to their already installed furnace to be utilized as their primary heating system during the shoulder seasons and milder times through the winter and central air conditioning in the summer. If the main electical service that supplies a home with power is at least 200AMP the exsiting gas or oil system can be replaced with a fully electric heat pump system, but in many cases customers that have oil or gas a limited with a smaller electrical panel that would need to be upgraded in order to rid themselves of gas or oil.
An “add on” or “hybrid heat” system is the best option for homeowners with existing forced air furnaces that don’t have the budget for a full system replacement.