What is the ideal outside temperature to set for the change between my air source heat pump and oil burning furnace?

Asked by Gary Marder
Kingston, NY

I have a Nest thermostat which will change system automatically. I'm in upstate NY and looking for what makes the heat pump most efficient.


Michael Holcomb

Answered by Michael Holcomb

Byron Center, MI

Alliance for Environmental Sustainability (Headquarters)

November 19, 2012


A heat pump may be an efficient method of cooling your home in summer and warming it in winter.  A heat pump works by moving heat

  • In the summer, a heat pump operates like a standard electronically driven air conditioner, collecting heat from the air in your home and expelling it outside.
  • In winter, the process is reversed, and the heat pump collects heat from outdoor air to warm the air inside your home.

Even the coldest winter air contains some heat. However, heat pumps work best at temperatures above 35 degrees Fahrenheit (2-degrees Celsius).

Below this temperature, an electrical resistance heater switches on (or in your case fuel oil furnace), if required, to supplement heat brought in from outside.


While many people find the winter operation of a heat pump the most difficult understand, it is during the heating cycle that the heat pump produces the most significant savings. 


  • Unlike a furnace, that turns fossil fuel or electricity into heat, the heat pump collects heat that already exists in the outdoor air by means of its refrigeration cycle.
  • Consequently, your heat pump will produce 2 to 3 times more heat than the energy it uses. The colder it is outdoors the less available heat and the lower the efficiency.

So, in your climate zone I would suggest you set the cutoff at 35-degrees Fahrenheit if the manufacturer hasn’t given a different temperature based on their equipment and your climate zone.


For more information:

Read "My apartment has a heat pump. How can I install a thermostat with a timer?" a Q&A answered by Sean Shanley.

Do you have a question about greening your home? GreenHomeGuide invites you to Ask A Pro. Let our network of experienced green building professionals – architects, designers, contractors, electricians, energy experts, landscapers, tile & stone specialists, and more – help you find the right solution.