What is Auxiliary Heat and How Does it Affect HVAC Performance?
If you have a heat pump heating and cooling system, you may have noticed the words AUX Heat displayed on your thermostat in the winter. Learn more about what that means and when you should worry.
By Anne Fonda
What does Aux Heat mean? Aux is short for auxiliary, which means supplemental or backup. Every heat pump (HP) should have an auxiliary heat source - both for your continuous comfort and as a safety net.
In this article, we’ll go over the details of what you need to know about auxiliary heat and your heat pump system.
What is auxiliary heat and how does it work?
A heat pump’s auxiliary heat system is a backup heating system. Your thermostat will signal either the HP or the backup system to provide the heat for your home. The majority of the time, your heat pump will be the primary heat source. The thermostat will choose the most energy-efficient heat source at any given time.
In general, there are two types of auxiliary heat systems for your heat pump. Which one you have or which one you should get with a new system will depend on how harsh your winters are.
Gas furnace (dual-fuel system)
Pairing your heat pump with a gas furnace in a dual-fuel system is recommended for homeowners in the northern part of the country where frigid winter temperatures are common. The HP will heat for most of the fall and spring, but the furnace keeps things toasty when the temperatures drop in the colder months.
If there’s a January thaw or a particularly balmy winter, the heat pump will take over the heating responsibilities again. Your thermostat will signal the appropriate heating unit automatically.
Electric resistance heat strips
Heat pump heat strips, or electric heat strips, are installed in the indoor air handling unit as a backup heat source. Almost all dealers will automatically install heating strips when installing a heat pump – it’s considered best practice. They might not install them if there is a code or rebate they are using that specifically requires heat strips to not be installed.
If your heat pump is sized correctly for your home, the heating strips may seldom operate. But it’s worth having the extra protection against frozen pipes or other risks if the unit fails.
When should auxiliary heat come on?
Your auxiliary heat, or secondary heat source, should come on when your heat pump is struggling to heat your home effectively.
The thermostat will trigger auxiliary heat when:
- The outside temperature is below 40 degrees or a set aux heat trigger temperature.
- The heat pump is in defrost mode.
- If you raise the heating temperature by more than 3-4 degrees.
That last one is because your heat pump system can take a while to raise the temperature in your home. If you suddenly raise the temperature by quite a bit, the thermostat will trigger aux heat to help reach the new set temperature faster.
Is it bad if the auxiliary heat comes on?
Not necessarily - in fact, it can be a good thing. Aux heat should come on when it’s too cold outside for the heat pump to efficiently heat your home to the set temperature, or when the heat pump is in defrost mode.
It’s only a bad thing if it comes on and stays on when temperatures are above 40 degrees - that can indicate there is a problem with your heat pump or thermostat, and you should call for service.
How often should the aux heat come on?
There is no set answer to this. If it’s extremely cold out, the auxiliary heat may come on more often. If it comes on frequently because the heat pump is entering the defrost cycle more frequently, you may want to call an HVAC professional to check things out.
Does auxiliary heat cost more?
Auxiliary heating with electric heating strips is less efficient and will use more energy than your heat pump, but it will also bring the temperature back up to where it should be in much less time. Having your furnace kick in may or may not cost more, depending on the price of natural gas vs electricity costs. Assuming you have an efficient furnace, heating performance will not be affected.
Once the house is up to temperature, the thermostat will exit auxiliary mode as long as your heat pump continues working effectively.
Auxiliary heat vs. emergency heat: understanding the difference
It’s pretty simple - auxiliary heat is supplementary heat to assist your heat pump in getting to the set temperature in your home. Your thermostat controls this automatically.
Emergency heat is a heat pump thermostat setting that you should only use in a true emergency - as in, your heat pump has failed and it’s really cold outside. You can manually turn on the emergency heat. Doing so will shut down your heat pump and the auxiliary system will become the main heating system. If you engage the emergency setting, call for HVAC service right away.
Troubleshooting heat pump aux heat issues
Unless it’s very cold outside, the aux heat system shouldn’t need to run too long. If it’s above 40 degrees and your thermostat shows AUX HEAT for more than an hour or two, you might want to investigate.
Check the outdoor unit
Go outside and make sure the heat pump isn’t covered with or surrounded by snow. That can prevent it from running effectively. If the heat pump is running and the thermostat is in Aux mode, that means the heat pump is defrosting.
When the heat pump is defrosting, you can see frost or ice on the outdoor unit. If it keeps running but hasn’t defrosted after an hour, you may need to call for heat pump repair. If the heat pump won’t turn on at all, you also need to call for service.
Check the thermostat
If you lower the indoor set temperature by several degrees, the Aux Heat indicator should disappear and the heat pump should continue running. If the Aux Heat indicator stays on for extensive periods when it’s not that cold outside, the system may not be switching between primary and auxiliary heating modes like it should.
Check your thermostat settings to ensure proper temperature settings. If you think your thermostat is the problem, you can try resetting it. See our thermostat troubleshooting tips for more guidance.
Maximizing efficiency: Tips for minimizing aux heat
If the temperature outside is below freezing, your heat pump’s auxiliary heat system will come on automatically - whether that’s a gas furnace or electric heat strips.
But there are some things you can do to minimize the need for aux heat.
- Make sure your heat pump is correctly sized for your home.
- Use a programmable or smart thermostat.
- Keep the temperature around 68 degrees.
- Wear a sweater instead of a T-shirt around the house.
- Bump up the temperature only a degree or two at a time.
- Open curtains and blinds to let the sunshine in.
- Seal windows and doors to prevent heat loss.
- Schedule regular heat pump maintenance.
Follow these tips and you’ll see the words AUX HEAT less frequently. If your thermostat is stuck on AUX HEAT and it’s above 40 degrees, you may need to call for service.
Having heat pump or thermostat issues?
Is your auxiliary heat running when it shouldn’t be? Are you having thermostat issues? If troubleshooting doesn’t help you resolve the problem, contact your local Trane dealer to schedule HVAC repair. Or, if you have the Trane Home App, consider opting into Trane Diagnostics and Dealer Remote Access.
They may be able to resolve the issue remotely, saving you time and money.
Anne Fonda
Content Writer, Trane Technologies
A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.
She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism.
When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.
Mark Woodruff, Senior Product Manager, Ducted Outdoor
