Air Source Heat Pumps

Posted on: December 7, 2012

An air source heat pump, attached to a traditional forced air system, is very similar to attaching air conditioning to such a system. These are often called split systems since the air handler (i.e. large metal box containing a blower, heating or cooling elements) or furnace and evaporator coil are within the house while the condenser is outside. Both systems comprise of similar components: furnace or air handler, an inside coil, outside condenser and the piping connecting the two components. On heat pumps, the outside condenser incorporates a reversing valve, which reverses the flow of refrigerant and therefore reverses the movement of energy. In cooling mode, heat is moved from inside the home to the outside air thus lowering the inside air temperature. During heating, heat is extracted from the outside air and moved into the home thus raising the inside air temperature. Of course, it takes energy to move energy, however it takes much less than using the combustion process or electric resistance to produce heat. Geothermal systems do the same thing except energy is either moved into the ground or a body of water for cooling or energy is extracted from the ground or a body of water when in heating mode.

Although air source heat pumps use virtually the same technology as ground source heat pumps (i.e. geothermal systems) they do not get the credit they deserve. In fact, currently the federal government provides a 30% tax credit with no upper limit for geothermal installations and no  incentives for air source heat pump systems. Luckily, in some states such as Montana, tax credits are available for either system. For example, installing a  geothermal system in Montana can qualify a married couple with up to a $1,500 tax credit where an air source heat pump can qualify a married couple for 25% of the installation cost with an upper limit of a $1,000 tax credit.

Due to the large capital investment required and the usage of a tax credit as the primary offset for this expenditure, it is often not financially beneficial for the average homeowner to install a geothermal system. However, there are many situations where people who can not financially swing the investment for a geothermal system, could benefit from the simple air source heat pump. Air source heat pumps can definitely reduce a user’s energy costs and carbon footprint.  For example, take an existing home with a conventional (properly sized) forced air system that utilizes propane and compare the retrofit of an air source heat pump vs. a ground source heat pump to the existing system.
Here are some of the specifics for this example:

  • Location Helena, MT for climate data
  • Cost of propane per gallon $2.16 (Energy Information Admin Rocky Mtn Region Feb 2011 Avg) cost of electricity $.105 per Killowatt-hour (Northwestern Energy Power Bill Jan 2012 Bozeman, MT)
  • Home design heating load 50,000 Btu/hr (approx. 1600 sf house)
  • Estimate to add 3 ton (36,000 Btu/hr) air source heat pump – 9.2 HSPF (Heating Seasonal Performance Factor) to existing 80% AFUE (Annual Fuel Utilization Efficiency)
  • Propane furnace $5,000 – cost when tax incentives mentioned above are applied $4,000
  • Estimate to add 3 ton (36,000 Btu/hr) ground source heat pump – 4.5 COP (Coefficient of Performance) $15,000 – cost when tax incentives mentioned above are applied $9,000
  • Annual heating cost before retrofit $3,261
  • Annual savings air source heat pump $1,562 (payback period approx 2-1/2 yrs utilizing tax incenctives)
  • Annual savings ground source heat pump $2,500 (payback period approx 3-1/2 yrs utilizing tax incentives)

As you can see the annual savings for a geothermal system is 1.7 times greater than the savings for an air source heat pump, however we are still able to reduce the energy costs using an air source heat pump by approximately 50%. When trying to attain cost savings, that’s a substantial number for a minimal investment. The payback period for this air source heat pump system is a little shorter than a geothermal system even though it does not qualify for the large federal tax credit available for geothermal systems. In some cases the main reason geothermal systems are able to compete with the air source heat pumps systems is due to the federal tax subsidy. Another item of concern is maintenance. As a mechanical contractor experienced in maintenance for both types of systems, I can say in my experience geothermal systems cost more to maintain and repair than an air source heat pump systems. The water side of geothermal systems is just more involved than the air side of air source heat pump systems, therefore geothermal systems inherently cost more when repairs are required. Although it is often overlooked, the repair and maintenance of these systems should not be ignored when selecting a system.

Both systems have their advantages and disadvantages. It is important to look at all the options and utilize what is best for your situation.

Mike Campbell is a mechanical engineer, master plumber, and owns a mechanical contracting firm.