De-mystifying Heat Pumps

(save 50% on heating and cooling costs and get a $500 rebate from Efficiency Maine

Office of Sustainability, Unity College

December 15, 2014

What is a heat pump? Your refrigerator is a heat pump. It moves heat from inside the unit to the outside, thus cooling the interior (and warming the room – hence the fan and the recommendation to clean your coils periodically). If you could reverse the direction of heat flow, it would be a heat pump heating your refrigerator just like it was your home.

 There are two main types for residential and commercial applications:

1) Air source heat pumps extract heat from the outside air down to about -15 degrees (yes, minus 15 degrees F) and pump it into your house;

2) Ground-source heat pumps (often erroneously called ‘geothermal heat pumps’) extract heat from water in the ground (groundwater in Maine has a temperature typically of 45 to 55 degrees) and pump it into your house (the term ‘geothermal’ is best used to describe the water from geologic activity in Finland or Yellowstone National Park).

Both types of heat pumps are 200% to 300% efficient compared to fossil-fuel fired units. This efficiency is because heat pumps move heat from placed to place rather than creating heat.

Air source heat pumps only need air to heat your home; ground source heat pumps require a water well (or wells) to function. Air source heat pumps work silently, ground source heat pumps are typically noisy. All air source heat pumps (and some ground source heat pumps) also provide air conditioning as a side benefit.

Cost: Air source heat pumps cost $1,500 to $4,000 and one or two of them can heat a well-insulated typical home. Ground source heat pumps cost $20,000 to $40,000 and can heat an average size home, not counting the cost of drilling wells.

Both Unity House and Terrahaus have air source heat pumps as their main source of heat. Air source heat pumps are being considered for Unity 2, the (new) new residence hall.


  1. Low-cost heat – The cost of heating with a heat pump is about half the cost of heating with oil, kerosene, electric baseboard or propane.

2.  Comfort – With advances in controls, heat pumps can maintain very constant temperatures.

3. Safety – Because heat pumps are electrically powered, there is no risk of combustion gas leaks.

4. Air quality – Heat pumps filter air as they heat or cool.

The only consideration for homeowners is that the units become less efficient as the outside temperatures drop. For example, a unit that delivers four units of heat for every unit of electricity at 50°F, may only deliver two units of heat for every unit of electricity at -15°F. If the temperature drops low enough, the system may turn off completely. For this reason, air-source heat pumps are often coupled with your existing source of heat or with a back-up wood stove.


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