Auxiliary Heat vs. Emergency Heat: What is the Difference?

Kenneth Wilson

You might have noticed that your thermostat displays the term ‘AUX’ every once in a while, and wondered what on earth it means. And as we explain in this article, AUX stands for auxiliary heat and is a feature of most home heating systems.

But it’s important to recognize that auxiliary heat is very different from emergency heat, and the two are typically used in contrasting circumstances.

To help you understand the differences, we introduce you to what auxiliary heat is and why it’s beneficial, before explaining what to do if you need to operate your home’s heating system in its emergency heat mode.

What is Auxiliary Heat on a Thermostat?

As soon as the temperature within a building drops below the desired temperature as set on the thermostat (usually 2-3 degrees lower), your auxiliary heating boots up and helps to warm your property more quickly. In most instances, you won’t even notice that your auxiliary heat has switched on unless you look at the thermostat during the time that it has kicked in and see the words ‘AUX’ displayed.

How Does Auxiliary Heating Work in Practice?

Your auxiliary heating kicks in automatically when temperatures fall below a certain level. It supports your heat pump to maintain the desired internal temperature within your building, and the auxiliary heat comes from a secondary source, which is usually gas or electric.

It is activated by the thermostat, which deploys the secondary heat source when the temperature is below what is expected. At most properties, auxiliary heating kicks in when the outside temperature is between 35-40 degrees.

Just as it is deployed automatically, your auxiliary heating turns itself off once it has helped your home reach the required temperature. So, while it works similarly to emergency heating, it’s not actually the same thing, as we explain below.

What is Emergency Heat?

Emergency heat refers to the supplementary heat source that is installed alongside your heat pump. Depending on your installation, your emergency heat might be electric resistance heating within an indoor unit, gas, oil, or even a backup hot water heating system. You should only need to use your emergency heat source when there is a problem with your heat pump.

You will need to turn on the emergency heat source yourself, and it should only really be used when your primary heat pump is down and is being repaired, as it is typically more expensive to run. (Related: When Should You Use Emergency Heat?)

How is Auxiliary Heating Different from Emergency Heating?

While auxiliary and emergency heat rely on the same backup source for the heat they supply to your system, they’re not the same thing. The fundamental difference is that auxiliary heating kicks in and shuts off automatically, whereas emergency heating needs to be initiated manually. Below is a concise overview of the differences between the two types of heating:

Auxiliary Heating Overview

  • It kicks in automatically when the temperatures drop
  • It extracts as much heat as possible from the primary heat pump
  • It also draws additional heat from the secondary heat source
  • Auxiliary heating switches off automatically when it is no longer required

Emergency Heating Overview

  • It should only be turned on in an emergency – i.e., when your primary heat source fails
  • It must be manually selected as and when it is required
  • Your system will continue operating in emergency mode until you switch it off
  • Emergency heat drawn from the secondary heating source is more expensive than heat from a standard pump
  • You should schedule a service ASAP if you need to put your heating into emergency mode

So, the bottom line here is that auxiliary heating and emergency heating are very different and should not be confused. While auxiliary and emergency heating both draw on your secondary source of heat, they have different primary purposes, as explained above.

What Should I Do if I Need to Use Emergency Heat?

If you need to put your heating into emergency mode in order to heat your property, you need to get the pump serviced. The fact that it’s not heating your building indicates that there is a problem with the coils or the pump itself, so you need to get it checked out by a professional.

Should you continue heating your home via emergency heat provided by your secondary heat source, it will cost you considerably more to heat your property and shouldn’t be relied upon as a long-term solution for heating your property.

To prevent this situation from arising, it’s a good idea to service your HVAC system every year so that any potential issues with the pump or coils can be resolved before they cause the system to malfunction.

Kenneth Wilson
December 12, 2021

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

Ask The Author Your Question In The Comments!

  • I have a new HVAC heat pump and air handler, installed last summer. For a few winter nights here in Central Georgia the lows will get below freezing, even into the low 20s, and the system struggles to maintain the set temperature. My installer recommends that I switch to emergency heat (which is electric heat strips) until the outside temp rises back above freezing, usually by 10 a.m. the next day. What do you recommend? Thanks.

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