Understanding Auxiliary Heat: When Should It Turn On?

If you’ve ever glanced at your thermostat and noticed the words “auxiliary heat,” you’re not alone in wondering what’s going on.

A lot of homeowners search “why is my aux heat on” the moment they see that light pop up. It sounds serious. Sometimes it even feels like something’s wrong. But here’s the thing. Auxiliary heat is often doing exactly what it’s designed to do, and there’s probably no need to worry. 

Let’s break down what auxiliary heat actually means, when it should turn on, and when it might signal a heating troubleshooting issue.

What Is Auxiliary Heat?

Auxiliary heat is a secondary heating system that works alongside your heat pump.

Most heat pumps pull warmth from the outside air, even when it’s cold. But when outdoor temperatures drop too low, your heat pump may need backup support. That backup is your auxiliary heat.

In most systems, this comes from electric resistance heat strips inside the air handler.

Think of it as your system’s safety net.

When Should Auxiliary Heat Turn On?

In a properly working system, auxiliary heat turns on under specific conditions:

• Very cold outdoor temperatures: When the heat pump can’t extract enough warmth from the air.
• Large temperature adjustments: If you raise your thermostat several degrees at once.
• Defrost mode: When the heat pump temporarily switches to cooling mode to melt ice buildup.

During these situations, auxiliary heat helps your home maintain a comfortable indoor temperature.

In colder climates, it may come on fairly often. In milder regions, it may only run during occasional cold snaps.

Why Is My Aux Heat On When It’s Not That Cold?

This is one of the most common heating troubleshooting questions homeowners ask.

If your auxiliary heat is running when outdoor temperatures aren’t extreme, a few things could be happening:

• Dirty air filter: Restricted airflow from a dirty air filter makes the system work harder.
• Thermostat issues: Incorrect thermostat settings or faulty sensors can trigger backup heat.
• Refrigerant problems: Low refrigerant levels reduce heat pump efficiency.
• Undersized system: The unit may struggle to keep up with your home’s heating demands.

If your energy bills suddenly spike and the aux heat light stays on constantly, that’s usually when it’s time to look deeper.

Auxiliary Heat vs Emergency Heat: What’s the Difference?

Homeowners often confuse auxiliary heat vs. emergency heat. They’re related, but they’re not the same thing.

Here’s a simple breakdown:

Auxiliary heat: Automatic backup that works with the heat pump.

Emergency heat: Manual setting used when the heat pump isn’t working properly.

Emergency heat is typically used if the outdoor unit is damaged or frozen solid. Running it long term can significantly increase energy costs.

Does Auxiliary Heat Mean Something Is Wrong?

Not necessarily. Auxiliary heat is built into the system for a reason. Seeing it occasionally during very cold weather is completely normal.

However, it can signal a problem if:

It runs constantly: Even in moderate temperatures.

Your home never reaches the set temperature: Despite extended run times.

Your electric bill jumps dramatically: Without a clear weather-related reason.

At that point, heating troubleshooting is worth considering. It could be something minor like airflow restriction. Or it could involve deeper mechanical issues.

How to Reduce Auxiliary Heat Usage

If you want to minimize how often your auxiliary heat runs, a few simple habits help:

Avoid large thermostat jumps: Increase temperature gradually.

Change air filters regularly: Clean airflow improves efficiency.

Schedule seasonal maintenance: A tuned system performs better in cold weather.

Seal air leaks: Drafts force your system to work harder.

For homeowners who suspect system performance issues, a diagnostic visit can help determine if you need heat pump repair.

When to Bring in a Pro

If your auxiliary heat stays on for long periods or your heating bills don’t make sense, it may help to have a professional take a look.

Glenmont Heating, Air Conditioning & Refrigeration can walk you through what your system is doing and whether it’s operating the way it should. If you’re in the area and want a second opinion, you can reach us at (227) 250-5844 or schedule an appointment online. 

FAQs About Auxiliary Heat

Is it expensive when auxiliary heat runs?

It can be. Electric resistance heat uses more energy than a heat pump alone. Short cycles during very cold weather are normal. Constant operation can increase utility costs.

Should I turn off auxiliary heat?

No. Auxiliary heat turns on automatically when needed. Manually switching to emergency heat without a reason can actually cost more.

How long should auxiliary heat stay on?

It depends on outdoor temperatures and your thermostat setting. During extreme cold, it may run longer. In mild conditions, it should cycle off once the set temperature is reached.

Why does auxiliary heat feel hotter?

Electric heat strips produce warmer supply air than a heat pump alone. That stronger blast of heat is often why homeowners notice it right away.