Leonard Splaine Why does heat pumps Aux Heat keep coming on

Emerson Smart Thermostat

Have you ever seen the words “aux heat” on your digital thermostat and wondered what exactly it means?

If you’ve been running your heat pump in heat mode you may have noticed that your thermostat is reading “AUX heat.

The AUX heat setting on an HVAC unit usually turns on when you need heat fast. If this secondary heating system is constantly running, it may mean that the weather outside is very cold or that the thermostat is set higher than normal. It’s also possible that a part of the heat pump is malfunctioning.

However, AUX heat refers to your heat pump’s “backup” electric resistance heating. This heat setting should only kick on when the heat pump can’t produce enough heat to warm your home or if your heat pump goes into defrost mode.

What the AUX Heat Setting Does

The auxiliary heating feature is helpful for homes in cool areas. It allows the heating system to reach the desired temperature quickly. Auxiliary heating is often a mainstay of wintertime.

One of the biggest advantages of having a heat pump is that it delivers excellent temperature control for a fraction of the energy cost of other options. During warm weather, the HVAC system uses a reverse cycle to cool rooms instead. This keeps your family comfortable no matter what the weather is like outside.

However, all heat pumps have one small weakness: speed. They can take a while to get to the target temperature. That’s where auxiliary heat comes in. It’s a type of backup system that uses electricity to supplement heating when needed. Usually, AUX heat and the main heat pump work together to keep your electricity bill as low as possible.

When It’s Normal for the AUX Heat To Turn On 

There are times when it’s perfectly normal for the secondary heating system to run. Here are a few reasons why auxiliary heating can turn on:

1.  The temperature outside is below 35°

When outside temperatures drop close to freezing, it can be hard for heat pumps to keep up. Cold weather means HVAC systems need a little push from secondary heating, so AUX heat turns on automatically.

Heat usually pumps absorb heat from the outdoor air and then move that heat indoors. And when outdoor temperatures drop too low, there’s simply not enough heat in the outdoor air to keep your home warm.

However, temperatures of 50–60 degrees shouldn’t require AUX heat in a correctly functioning HVAC system.

2. You Increase the Thermostat Temperature By at Least Three Degrees

It’s normal for AUX heat to turn on when your heat pump needs to heat the home quickly.

Most heat pumps are designed to automatically switch to AUX heat when the indoor temperature is 3 degrees colder than the thermostat setting. 

For example, let’s say that you wake up to a cold home (62° F) and immediately raise the thermostat to 68°. Because the temperature rise is more than 3 degrees (it’s 6 degrees in this scenario), the system will automatically turn on the AUX heat to help your home meet the desired temperature faster.

So first, check to see if your thermostat only says AUX when you raise the thermostat 3 degrees or higher than the current temperature of your home. If so, that’s normal.

3.  The Heat Pump is Defrosting

Modern heat pumps have sensors that can tell when something isn’t working right. For example, if ice builds up around the condenser coils, the HVAC system can detect it. This causes the unit to go into defrost mode. During this cycle, hot air from inside the home is routed to the coil to melt the snow or ice.

Some heat pumps let you know this is happening with small indicator light. Usually, a defrost cycle only lasts a few minutes. While defrosting, the HVAC unit switches to backup heat to keep your home warm.

Signs that your heat pump is in defrost mode include:

  • Steam and/or water coming from the outdoor unit
  • The fan in the outdoor unit has stopped running
  • A blinking light on the unit to indicate it’s in defrost mode (only on some heat pump models)

Signs That AUX Heat Isn’t Working Correctly

How can you tell when it’s time to call a pro? Here are a few warning signs that something is going wrong with the auxiliary heat on your heat pump:

The electricity bill is much higher than normal:    After several months with a new heat pump, you should have a fairly good idea of the average energy cost for the unit. If you notice a sudden spike in electricity consumption, it may mean that AUX heat is running too often. Of course, it’s also important to choose an HVAC system that’s the right size for your home to maximize your energy savings.

AUX heat is running, but it’s hot outside:    Unless temperatures are 35 degrees Fahrenheit or below, it doesn’t make sense for AUX heat to run for a long time. With the rising high temps, there usually isn’t much need for secondary heating. This can change in places where it gets cooler overnight, but even then, AUX heat shouldn’t turn on unless you increase the thermostat temperature by several degrees.

AUX heat doesn’t respond to thermostat changes:   Turning your thermostat down to ambient temperature should cause AUX heat to turn off immediately. If you notice that secondary heating keeps going even it’s warm inside or the thermostat is turned off, there’s probably something wrong.

The home feels too warm:   If you notice indoor temperatures over 80 degrees and the AUX heat is still running, there’s likely a problem. Check the thermostat to make sure it hasn’t been bumped. If not, ask Siri to help you find “heating services near me” for assistance.

The defrost mode light stays on constantly:   First, the heat pump shouldn’t even go into defrost mode unless the outside unit is covered in snow or ice. Second, the process should only take a few minutes. If you notice the heat pump defrosting for an extended period of time, call a pro right away.

If you see something that seems like a problem, the easiest course of action is to give our team a quick call. We may need to replace a faulty sensor or check the thermostat.


The best way to prevent AUX heat problems is to take great care of the HVAC unit. Following these simple tips may keep everything working correctly:

  • Schedule preventative maintenance every six months.
  • Clean the areas around the outdoor HVAC unit regularly.
  • Keep ducts and heat vents free of clutter.
  • Don’t use the emergency heating setting for normal heating.
  • Clean or replace the heat pump filter every month or two.

If you notice any issues with the heat pump, it’s better to call a pro sooner rather than later. Sometimes poor original installation or damaged wiring is to blame. This can cause the secondary heat to turn on at the wrong time. If the home’s thermostat is malfunctioning, it can cause a lot of different problems with HVAC performance.

If you’re ready to have your air HVAC serviced by a pro, give a Leonard Splaine expert a call at (571) 410- 3555 or contact us. You’ll be pleased with your experience—we guarantee it!