How do heat pumps work?
A heat pump is a mechanical-compression cycle refrigeration system, which runs on electricity and can either heat or cool a space. An indoor unit called an air handler is installed, and a unit similar to a central air conditioner (referred to as a heat pump) is mounted outside. A compressor moves refrigerant inside and outside between these two units, absorbing or releasing heat. When it’s cold outside, a heat pump extracts outside heat and moves it indoors. When it’s hot outside, it reverses the process and acts like an air conditioner, moving heat from indoors outside. They are highly efficient between 20 and 80 degrees.
How are heat pumps powered?
Heat pumps are wired into the electrical system of the house, and are powered like any other household appliance. Most electricity is generated in power plants by massive generators, and these are often powered by fossil fuels like natural gas or coal. Sometimes, these plants generate electricity through wind, nuclear or hydropower. During peak electric usage time, when there is high demand for air conditioning or heating, many oil-fired, electric-generating plants are called on to supplement the power grid. The best efficiency for transferring electricity from a generating plant to the home is 40%. Most plants and transmission lines lose more than 60% of their created energy by the time it reaches your house.
In some cases, heat pumps can be a great option for—especially in mild, dry weather—and demand for heat pumps has been increasing. A heat pump will turn on and off less often than a gas furnace and most systems have eliminated the blowing of cold air through the vents that used to occur when the system temporarily switched into reverse to defrost the coils.
But in colder climates like the Northeast, heat pumps have not typically been popular. In Maine, most homeowners would need to supplement their heat pumps with an oil and/or propane furnace during the winter months. So, for those who are interested in heat pumps because they may be “greener,” keep in mind that the electricity they require is typically generated by the burning of fossil fuels, rendering them no greener than the furnace in your basement. Heat pump technology has advanced in recent years (with some manufacturers selling heat pumps that stand up to the coldest New England weather), but most homeowners can’t rely on heat pumps as their only source unless their house is extremely well insulated.
Other draw backs worth considering:
- Small wall mounted cassettes within a heat pump system deliver reduced temperature uniformity in the home (think air conditioning at only one end of the house), increasing the potential for frozen and broken pipes.
- Oil-fired boilers have their highest efficiency under high load conditions; extended operation under low-load, heat pump backup service can lead to strong efficiency degradation.
- The outside cabinet of air-source heat pumps requires attention to make sure nothing blocks free airflow, such as debris, leaves, snow and ice (whereas a furnace and boiler are completely protected inside the home).
Before deciding on a heat pump, request a home energy audit to learn what will work best for you.
Modeled Energy and Economic Performance Assessment of a Portland, ME Home with and Existing Oil-Fired Non-Condensing Boiler and a Two-Head Mini-Split 38,000 BTU Heat-Pump Operating in Three Modes. Dr. Tom Butcher. July 2015. Brookhaven National Laboratory.