Heat pumps heat and cool by moving heat from one place to another. In heating mode, heat pumps are essentially an air conditioner running in reverse. Latent heat energy in outdoor winter air is extracted by refrigerant circulating through the outside coil. The heat is concentrated as the refrigerant passes through the compressor and then conveyed inside to a coil installed in the indoor air handler. As the refrigerant rapidly depressurizes in the coil, it releases its load of concentrated heat energy. That heat is dispersed through your ductwork by the blower and effectively warms your house — except when it doesn’t.
The Balance Point
Many people are surprised to learn that there’s sufficient heat in cold winter air to warm a home. But it’s true only to a point. Depending on the efficiency of the unit, there’s a specific outdoor temperature — called the balance point — where the heat pump can no longer extract enough heat energy to get the job done. For most residential heat pumps today, that’s somewhere around 35 degrees Fahrenheit. Below that level, most units incorporate electric resistance coils mounted in the air handler that automatically energize to supply the supplemental heat necessary to keep the house toasty.
An Electric Drawback
So what’s the downside of a heat pump when weather turns very cold? Electric supplemental heating required below 35 degrees is substantially more expensive than either heat from a heat pump or gas-fired heating. If outdoor temperatures frequently drop below the minimum, homeowners may discover that savings gained by utilizing the efficient heat pump are negated by the high operating costs of frequent reliance on supplemental electric heat.
Dual Is Better Than One
A dual-fuel furnace takes on the problem with a hybrid approach that combines a heat pump and a backup gas-fired heater. As long as outdoor temperatures remain above the balance point, the heat pump handles the job of keeping the house warm with typical heat pump energy-efficiency and low operating costs. Meanwhile, an outside sensor and thermostat continuously monitor the outdoor air temperature. If it drops to the balance point, the thermostat controller automatically shuts down the heat pump and actuates the backup furnace. When outdoor temps again rise above the balance point, the gas-fired unit shuts down and the heat pump kicks in again. Because natural gas is significantly less costly than electricity, the household heating budget takes a considerably smaller hit when the system utilizes the gas backup furnace.
More Good News
Many installed gas-fired furnaces can be configured as the backup heater in a dual-fuel arrangement. It isn’t always necessary to buy a new, separate furnace. Therefore, a homeowner with an existing split central air conditioner/furnace system may choose to upgrade only the A/C to a high-efficiency heat pump that delivers both cooling and heat, while retaining the installed gas furnace to provide cost-effective supplemental heat whenever low temperatures demand. An HVAC contractor can easily install the outdoor sensors and thermostat controller required to link and actuate the respective units.
"The Hybrid" of an HVAC System
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