Hybrid Heating and Cooling Systems
A hybrid heating and cooling system combines an electric space-conditioning heat pump with a fossil fuel fired furnace. When providing cooling a hybrid system operates like a conventional air conditioning system: using the heat pump to chill the air. When heating both the furnace and heat pump have roles, depending on the outdoor temperature. The heating capacity of a fossil fired furnace is relatively independent of outdoor temperature, but it’s efficiency is higher at low ambient temperatures, because of less on/off cycling. Conversely the heating capacity and efficiency of an air-source heat pump are highest at higher outdoor temperatures and decrease as the outdoor temperature drops,until, at some point, supplemental heat is required to keep the house warm. This is considered the heat pumps’ thermal balance point temperature.
In a conventional heat pump, the supplemental heat is provided by electric resistance strip heaters. In hybrid systems, the supplemental heat is provided by the furnace. Pairing the two technologies allows each to operate at maximum efficiency.
The change over point(when the system switches from air source heat pump to fossil fired furnace) is negotiated by the control/thermostat which is installed with your system. This change over point is set based on the balance between energy savings and desired comfort level. Since a heat pump produces 90 degree air, as opposed to a furnace which produces 120 degree air, each customer must find their desired comfort level(typically between 30-40 degrees, although with better insulated homes it may go lower). Some consumers will also take into account the fossil fuel pricing to determine the change over point. Using a programmable thermostat, the change over point can be adjusted readily allowing consumers to stay in tune with market trends.
With the historical instability in fossil fuel prices and the general stability in historical electrical prices a dual-fuel system has many cost benefits. In fact according to Carrier, a hybrid heating and air conditioning system can save you over $700 per year.
Hybrid heating systems also have various comfort benefits such as:
Fossil fired furnaces are sized to keep a home warm in the most extreme cold. This causes much on/off cycling at moderate ambient temperatures, which is experienced as hot blasts from the registers followed by no heating at all. A heat pump cycles less at moderate temperatures, providing more heat flow.
The discharge temperature of a heat pump at moderate temperatures is less than that of a furnace(comfortably above skin temperature but not excessively hot). This results in a more pleasant relative humidity level, which is less drying to the skin.
Stratification: the effect of cold air migrating to the floor while warm air travels to the ceiling, contributes to “zonal heat trapping” in which the hot air trapped at the ceiling cannot disperse evenly from room-to-room. As a result, some rooms seem to not get enough heat,while others are too hot, even with properly designed ducts. Again the heat pumps’ more moderate discharge temperatures allows more uniform room-to-room comfort.