What Does Aux Heat Mean?

Imagine waking up on a cold morning, checking your thermostat, and seeing “aux heat” on the display. What is auxiliary heat, and why is it showing up on your thermostat?

Auxiliary heat or aux heat is a heat pump setting that refers to the fact a secondary heat source has been activated. It’s your backup heating source. It sometimes kicks on if your heat pump can’t produce enough heat to keep your home warm.

Auxiliary heat is sometimes confused with emergency heat, but it’s not the same. If you’re curious about why auxiliary heat kicks on, what it means for you, and whether you should worry or not, read on. We’re covering everything you need to know about auxiliary heating.


How Does a Heat Pump Work?

Before we dive into the reasons your aux heat might kick on, it’s important to understand how heat pumps work.

Heat pumps heat and cool your home. They have both indoor and outdoor units. The units contain coils and refrigerants that absorb heat energy from the air.

When your heat pump is in heating mode, it takes the warm outdoor air and pumps it into your house. When it’s cooling your home, it takes warm indoor air and pushes it outside.

Unlike furnaces and boilers, heat pumps move heat. They’re cheaper to install and more energy-efficient, making them quite popular. In 2020, there were nearly 180 million heat pumps being used.

Heat pumps are common in newly built homes.

One of the disadvantages of heat pumps is that they don’t work as well when the temperatures drop below 30°F. Thankfully, lows in Ventura typically average 46°F.


What Is Auxiliary Heat and Why Does It Turn On?

First, you’ve probably noticed the setting on your thermostat says “aux heat.” Aux is short for auxiliary which means supplementary or additional.

As we mentioned, your auxiliary heat is your backup heating system. It kicks on when your heat pump can’t keep your home warm enough by itself. The aux heat indicator will either light up or flash on your thermostat, depending on the model you have.

Auxiliary heat activates when the indoor temperature is 2-3 degrees colder than the set temperature. Aux heat should turn off once the thermostat reaches the goal temperature again.

Many homeowners want to know if their backup heating turning on should be cause for concern. There are two reasons that you might see aux heat flashing on your thermostat.


A System in Defrost Mode

Your heat pump might enter defrost mode if the outside temperatures are very cold. This mode prevents the unit from freezing over. Thankfully, that’s not too common here in California.

Systems in defrost mode turn to cooling mode to let the coils inside the system warm up. Some signs a system is in defrost mode include steam coming from the system and the fan ceasing to circulate. Certain models might also have a light that indicates the unit is in defrost mode.

If this happens, your heating system will move to auxiliary heating and your heat pump should kick back on in a few minutes.


The Outside Temperature Is Below Freezing

Heat pumps make a highly efficient choice for homes in regions that don’t reach below freezing. They don’t operate well in temperatures below 32°F.

When temperatures outside reach below freezing, your system simply can’t get enough heat from the air outside and moves to the auxiliary mode for help.

If it’s an unusually cold day, this is one reason you might see “aux heat” flashing on your thermostat.


The Heat Pump Isn’t Producing Enough Heat

If your heat pump is struggling to maintain or reach the set temperature on the thermostat, aux heat can help give your system the boost it needs to warm your house.

Your aux heat should turn off once the internal temperature reaches the set temperature on your thermostat. If you notice aux heat turning on more often, it could be due to the temperature settings. Raising the temperature by more than 2°-4°F at a time can cause your system to turn on auxiliary heat.


What Is Emergency Heat?

The emergency heat setting (EM heat) exists for when your primary heating system fails. It’s also a backup heating mode, but it’s not the same as aux heat.

Emergency heat is for emergency situations only. You should only use it when there’s an issue with your first-stage heat.

Emergency heat needs to be turned on manually. It’s also more expensive to run, so it should only be used in situations where your first-stage heat has failed.


How to Prevent Your System from Entering Aux Heat Too Often

Aux heat is meant to help support your heat pump, but you may want to save on your heating bills by lowering the chances it will turn on. Here are some tips you can utilize.


Lower Your Set Temperature

Cranking up the heat setting on your thermostat can cause your heat pump to turn on aux heat. As we mentioned, we don’t recommend turning your thermostat up by more than 2°-4°F at a time. Otherwise, you’re asking your system to work too hard.

Set your thermostat to a comfortable range to prevent aux heat from activating and to save more money on your energy bills. The recommended temperature for winter is 68°F.

If you find that temperature to be too uncomfortable, you can experiment by raising the heat one or two degrees at a time. Raising it too high can cause the heat pump to energize the heat strip inside your backup heating source to reach the set temperature more quickly.


Close Unused Rooms

If you have any unused rooms in your home, closing them off can help prevent the aux heat from activating. For instance, maybe you have a spare bedroom or office that you’re not using at the moment.

Close those rooms and shut the vent to prevent warm air from pumping into unused areas.


Warm Your Home in Other Ways

We’re lucky that temperatures don’t drop too far here in Ventura County, but there is always a chance that a cold day might make your home feel chilly.

There are other ways you can warm your home up that might prevent your heating system from energizing the aux heat too often.

Opening your window blinds or curtains in the morning lets more sunlight inside, which can help warm your home naturally.

You also might want to search for drafts and block them off to keep your home toasty and warm. Close cracks along the window frames, reverse the direction of your ceiling fan, use draft stoppers on doors, add rugs to your floors, use outlet insulators to block drafts from outlets, and invest in some warm blankets and bedding.

Wearing warmer clothing, even inside, can also help you feel cozier without turning the thermostat up.


Is Your Heating System Stuck on Aux Heat?

What if it doesn’t seem like your system should be in aux heat mode? There are times when a failing or faulty heating and cooling system can get stuck on aux heat.

You may have noticed signs your system isn’t running as well as it used to, that it frequently enters aux heat mode, or that your heating bill is higher than it should be. When should you worry and call for a repair service?

If it seems your system can’t keep up with the set temperature, even if you follow the tips outlined above, it’s likely an issue with your unit. If you see aux heat turning on constantly, or if it’s turning on when it’s warm outside, you need to call a heating and cooling professional. Your auxiliary heat shouldn’t activate if it’s 40°F or warmer outside, so if it’s turning on, you might need to have it repaired.

Does your home seem too cold or too warm? Is your heating bill sky high suddenly? You might need to repair or replace your heat pump or thermostat.

Setting up a maintenance plan can put your mind at ease and ensure your heat pump remains working as it should. It’s also a good idea to schedule one to make sure your system runs efficiently so that you save more money on your heating bills.


When to Call for HVAC Repair

We often hear homeowners ask, “What is auxiliary heat, and should I be worried?” Hopefully, this article told you more about aux heat and whether you should be concerned or not. In most cases, seeing aux heat on your thermostat isn’t an issue.

However, if you see aux heat more often than you’d like and it seems like your system isn’t running efficiently, give Kaiser Air Conditioning and Heating a call. We’ll be happy to come out and fix your heat pump so that your home remains comfortable all year long.