When the outdoor temperature heads below 32º F (freezing) a heat pump won’t work well and becomes more inefficient the colder it gets. You have to understand how a heat pump functions to know why this happens. It does not generate heat. Instead, a heat pump absorbs heat in one location and moves it to another. 

    The system absorbs indoor heat in the summer and exhausts it outside, then reverses itself to move outdoor heat indoors during the winter. As the temperature moves below freezing, there is less  heat to grab and the system has to work much harder to pull out any heat.

     Luckily, heat pump systems also have a supplemental (backup) heat source to turn to when it gets really cold, usually an electric resistance heat coil— or sometimes a gas furnace. The system will sense when it needs to switch to the alternate heat source and do it automatically. It is not necessary or recommended to turn the thermostat setting to “EM HEAT” (emergency heat) on very cold days.

    This issue does not apply to ground source (geothermal) systems. There are not too many of them out there, but they use underground piping to tap into the relatively constant temperature of the soil a few feet below grade. Most systems are “air source heat pumps."

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.