When temperatures drop into the teens or single digits, many homeowners start asking the same question:
“Why is my heat pump running nonstop and what is AUX heat?”

If you have a heat pump system, this behavior is normal during extreme cold weather. Understanding how your system works can help you stay comfortable, avoid unnecessary worry, and prevent high energy bills.

How a Heat Pump Works

Unlike a traditional furnace that creates heat, a heat pump moves heat from the outside air into your home. Even when it’s cold outside, there is still heat energy in the air. The heat pump captures that heat and transfers it indoors.

Heat pumps are extremely efficient in mild to moderate temperatures, which is why they’re popular in Virginia and surrounding areas. However, as outdoor temperatures drop, there is less heat available to extract, and the system has to work harder.

What Is Auxiliary (AUX) Heat?

Auxiliary heat, often shown as “AUX” or “EM Heat” on your thermostat, is a backup heat source designed to help your heat pump when it can’t keep up on its own.

In most systems, auxiliary heat is:

Electric heat strips, or

A gas or oil furnace (in dual-fuel systems)

When outdoor temperatures fall too low or your home temperature drops several degrees below the thermostat setting, the system automatically activates AUX heat to maintain comfort.

This is not a malfunction; it’s your system doing exactly what it was designed to do.

Why AUX Heat Runs More in Extreme Cold

During extreme cold:

The heat pump becomes less efficient

It takes longer to warm your home

The system may struggle to maintain set temperature

When this happens, auxiliary heat turns on to:

Help recover temperature faster

Prevent your home from feeling cold

Protect the heat pump from excessive strain

Because auxiliary heat uses more energy, it can cause higher electric bills during cold snaps. This is normal and temporary.

Is AUX Heat Bad or Something to Avoid?

No but it should be used strategically.

AUX heat is not meant to run constantly under normal conditions. If you see it frequently when temperatures are mild, it could indicate:

Poor insulation or air leaks

A system that’s undersized

Dirty filters or lack of maintenance

Thermostat settings causing unnecessary activation

In extreme cold, however, occasional or even frequent AUX heat is expected.

Tips to Reduce AUX Heat Usage

Here are a few homeowner-approved tips to keep your system running efficiently:

Avoid Big Thermostat Adjustments

Raising your thermostat several degrees at once can trigger AUX heat. Instead, make small adjustments.

Change Your Air Filter

A dirty filter restricts airflow and makes your system work harder.

Seal Drafts and Insulate

Cold air leaks force your system to rely more on backup heat.

Schedule Preventive Maintenance

A well-maintained heat pump performs better in cold weather and reduces unnecessary AUX usage.

Trust the System

Heat pumps provide steady, even heat, not hot blasts like furnaces. This is normal operation.

When Should You Call a Professional?

You should contact an HVAC professional if:

AUX heat runs constantly in mild weather

Your home never reaches the set temperature

You notice unusual noises or smells

Your energy bills spike unexpectedly

These could be signs of a system issue, not just cold weather.

Trusted Local Heat Pump Experts

At D & L Mechanical Services, we’ve spent over 25 years servicing heat pumps in Fredericksburg and the surrounding areas. Our approach is honest, educational, and focused on what’s best for your home not upsells.

If you have questions about your heat pump, AUX heat, or cold-weather performance, we’re here to help.

Call D & L Mechanical Services today for expert heat pump service you can trust.