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AC 4 Life - FAQ's - Sizing Your System - Heat Pumps

How does the supplemental heat feature of a heat pump work?

This page is designed to provide anyone looking for a new HVAC system with a better understanding of frequently asked questions about the selection, AC and furnace sizing and other important considerations in making the best choices for your home or office.

This information guide offers the following categories of information pertaining to proper AC and heating system sizing as well as important guides to ensure you make informed decisions:

Because the heat pump is located outdoors and is subject to the temperature conditions around it, the heating capacity of a heat pump will diminish as the outdoor temperature drops. The heat pump will measure the demand you are placing on it at the moment (your indoor temperature setting) and compare this to the outdoor temperature. 

If the spread is high it will automatically energize the supplemental heating element to maintain the indoor thermostat setting you have selected. 

I've heard that heat pumps don't work below a certain temperature. Is this true and if so, what is this temperature? 
There is no certain temperature that a heat pump ceases to produce heat. The heating capacity of all heat pumps is rated at a standard 47 degrees. As the outdoor temperature drops, a heat pumps' ability to produce heat decreases in a linear fashion. 

For example, let's use a 3 ton heat pump with a heating capacity of 35,000 Btu's for discussion. At 47 degrees outside temperature this heat pump will produce it's rated capacity of 35,000 Btu's. At 27 degrees outside temperature, it will produce about 22,500 Btu's. 

When the outside temperature drops to 17 degrees it now produces about 18,000 Btu's. At this point the supplemental heating element will have energized automatically to restore the heat pump's ability to heat your home. 

You can see that there is really isn't one temperature point at which it can be said the heat pump is no longer useful for heating. Instead, this decline is gradual and relative to the outdoor temperature. Speaking in general terms however, it can be said that in very cold climates a heat pump may not be the best choice for heating.