Although we have written about heat pumps before, back in the spring, Winter Is Coming, and we figure that heating systems are on many people’s minds as they consider their optimum strategies for staying warm over the next 6 months! Also, as the climate crisis becomes more visible, many are trying to reduce their household carbon emissions in practical ways.
With the rebates currently available in Maine for high efficiency air source heat pumps, these systems will pay for themselves in savings within 3 to 5 years, and can reduce your heating system’s greenhouse gas emissions by 75%.
Heat pumps are a bit difficult to fathom if you are not an engineer or physicist. People tend to think ground source heat pumps tap into geothermal energy (like hot springs), and they tend to think that air source heat pumps only work if it isn’t very cold out.
In fact, ground source heat pumps (confusingly often referred to as geothermal) take heat from water running through pipes in ground that is 40-45 degrees Fahrenheit by cooling it – that is, extracting its heat with a refrigerant, a compressor, and a condenser. Likewise air source heat pumps use a refrigerant to extract heat from air running through a heat exchanger similar to your car radiator – in any outdoor temperature – and put it into your house.
This is the same thing that happens with refrigerators, but inside out – in refrigeration, we are taking the heat out of a space (the inside of the refrigerator) and dumping it out through those radiator fins on the back side of it. (Which brings up the fact that heat pumps can also provide air conditioning – as well as dehumidifying – in the summer!)
True, the colder it is outside, the smaller the fraction of your household heat that can be provided by either type of heat pump. In Maine, we need some kind of automatic back-up heating, whether a furnace, a boiler, or an automatic pellet heater, to kick in when the weather is too cold. Meanwhile the heat pump is still helping – it does not stop working or shut down, as many people seem to believe.
Now it is true that heat pumps use electricity to run. But the increase in your electricity usage falls far short of the reduction in the combustion fuel it is displacing. A high efficiency oil furnace will provide about 125,000 Btus from a gallon of heating oil (and waste 15,000 Btu’s). That 125,000 Btu’s, if it is in the form of electricity (which then would be 37 kilowatt-hours) and used to run a heat pump, will provide the same amount of heat as 3 to 5 gallons of oil.
Put another way, you can get a gallon’s worth of heat for less than 12 kWH of electricity. The increased heating performance is due to the (free) heat from the air or ground which the heat pump will be extracting.
Reductions in greenhouse gas emissions attributable to your home heating will always depend on the ultimate sources of your electricity, but in Maine we have a large proportion of renewables in our mix; and again, the overall energy use with a heat pump is substantially lower than any other system. Burning a gallon of fuel oil produces about 20 pounds of CO2, while generating those 12 kWh of electricity produces about 5 pounds of CO2, a 75% reduction.
The installation cost of air source heat pumps will vary depending on the construction of your home and the size of the space to be heated. A typical installation is around $2500 for an 18,000 Btu-per-hour unit. Maine has a program that will provide rebates of $1,000 for installing your first high-efficiency heat pump, and $500 for the second one, so now is a good time to act.
Efficiency Maine is running the rebate program. You can either visit their website (https://www.efficiencymaine.com/at-home/ductless-heat-pumps/) or call them at 866-376-2463 to learn more about the program, which heat pumps qualify for a rebate, and how to find a contractor.
Incidentally, if you are wondering whether a ground source heat pump is for you, the question is quite complicated. Our system in Chesterville is a horizontal pipe system that is 3600 linear feet of piping buried 6 feet deep and this provides about as much heat as about 3 mini-splits; however, the installation of our system would be many, many times as expensive. So, air source is probably the way to go, unless you are an amazing DIY-er.
Feel free to contact the email address below if you would like clarification on anything in this or any of our articles.
Happy Heating Season!

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side. He can be reached at pauls@maine.edu. Cynthia Stancioff is an amateur naturalist who likes to write. Previous columns can be found at https://paulandcynthiaenergymatters.blogspot.com/

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