What Is Auxiliary Heat?

If your home has a heat pump, you’ve probably seen the words “Aux Heat” pop up on your thermostat at some point — especially during those rare but sharp Hill Country cold snaps. Auxiliary heat (often called “aux heat”) is a built-in backup heating system designed to help your heat pump when outdoor temperatures drop low enough that pulling heat from the air becomes less efficient.

A heat pump normally heats your home by transferring warmth from the outdoor air into your house. This works great most of the winter in Central Texas, because our temperatures usually stay mild. But when the thermometer dips near freezing, there’s less heat available outside. That’s when auxiliary heat steps in to give your system an extra boost.

In most homes, auxiliary heat is electric resistance heat — basically a set of heating coils inside your air handler that warm the air directly. Think of it like a very large space heater built into your HVAC system. It warms your home quickly and reliably, but it uses more electricity than your heat pump alone. The good news is it only runs when it truly needs to, and it turns off automatically once outdoor conditions improve.

You might also notice a different smell when auxiliary heat runs for the first time of the season. Many homeowners describe it as a light “dusty,” “toasty,” or even slightly “burning” smell. This is usually just dust burning off the heating coils after months of not being used — and it’s completely normal for short periods. If that smell lingers for hours or is very strong, though, it’s a sign to call for service.

Auxiliary heat helps protect your comfort during sudden freezes, keeps your home from feeling chilly, and ensures your heat pump doesn’t have to work overtime trying to do something it wasn’t designed to do on its own.

How to Tell If You Have Auxiliary Heat

Here are a few simple ways to know:

• Your thermostat displays “Aux Heat” or “Emergency Heat”

• You feel noticeably warmer air from the vents during very cold weather

• Your electric bill jumps during freezing weeks

• You notice a brief dusty or warm “burn-off” smell early in the heating season

• You don’t have a gas furnace but still get strong heat output

Signs Your Auxiliary Heat Is Working Correctly

• It only runs during colder weather

• The smell goes away after a short time

• Your home warms up steadily without long cold periods

• Your system doesn’t run constantly when temperatures return to normal

Auxiliary heat is one of those behind-the-scenes features that you don’t think about much — until you really need it. In Central Texas, it’s what helps your home stay comfortable when winter decides to surprise us.