What Is Emergency (Auxiliary) Heat?

If your home uses a heat pump, you’ve probably seen your thermostat display “AUX,” “Aux Heat,” or “Emergency Heat” at some point. These modes can be confusing, especially when temperatures drop and your system behaves differently than you expect. Understanding the difference between auxiliary heat and emergency heat can help you stay comfortable, avoid unnecessary energy costs, and know when to call for help.

What Is Auxiliary (AUX) Heat?

Auxiliary heat is a secondary heating source built into most heat pump systems. Heat pumps work by extracting heat from the outdoor air, even cold air contains heat energy. But when temperatures get very low or the heat pump needs a boost, auxiliary heat automatically turns on to help your home reach the thermostat setting.

Auxiliary heat usually consists of electric heating strips or sometimes a furnace that kicks in temporarily. You don’t need to touch anything, AUX heat activates on its own and turns off when it’s no longer needed.

Common Times Auxiliary Heat Turns On

Here are the most normal, expected times you’ll see Aux Heat running:

1. Extremely Cold Outdoor Temperatures

When it’s very cold, your heat pump may struggle to pull enough heat from the outdoor air. AUX heat assists so that your system can maintain your set temperature.

2. Defrost Cycles

In freezing weather, heat pumps can develop frost or ice on the outdoor unit. During a defrost cycle, the system temporarily reverses operation to melt the ice. AUX heat may come on to keep your home warm during this brief process.

3. Large Thermostat Changes

If you suddenly raise your thermostat several degrees, your heat pump may not be able to heat your home quickly enough on its own. AUX heat provides the extra power needed to warm things up faster.

4. Meeting High Heat Demand

Sometimes your system simply needs a boost to meet demand, for example, on an unusually cold morning or during back-to-back heating cycles.

Important: Aux Heat should not run constantly. If you see “AUX” for long periods when temperatures are not that cold, it may indicate a problem with your heat pump or thermostat.

What Is Emergency Heat?

Emergency Heat uses the same backup heating source as Auxiliary Heat, but with one major difference:

Emergency Heat is turned on manually by the homeowner.

When you switch your thermostat to “EM Heat,” the heat pump is completely shut off, and the backup heating source becomes your primary heat.

Emergency heat is not meant for everyday cold weather. It’s designed for situations where the heat pump cannot operate properly.

When You Should Use Emergency Heat

Use Emergency Heat only in situations like:

• Your heat pump isn’t working or has failed.

• The outdoor unit is frozen, damaged, or unsafe to operate.

• A technician has told you to switch to EM Heat while waiting for repair.

Emergency heat is less efficient and more expensive to run. That’s why it should only be used as a temporary solution until service is performed.

When You Should Not Use Emergency Heat

Avoid turning on EM Heat in these situations:

• It’s just a cold day and your heat pump is running normally.

• You want your home to heat up faster.

• You think EM Heat is the same as Aux Heat.

• You’re trying to “help” the system, it doesn’t need it.

Turning on EM Heat unnecessarily can lead to very high energy bills.

Why Understanding the Difference Matters

Knowing how and when your heat pump uses auxiliary or emergency heat can make a big difference in:

Comfort

Your system will keep you warm even during unusually cold weather or when defrost cycles occur.

Efficiency

Heat pumps operate far more efficiently than electric strips or backup heat sources. Avoiding unnecessary EM Heat keeps energy costs under control.

System Health

If your system frequently relies on AUX or EM Heat, it could point to an issue that needs attention, catching it early can prevent costly repairs.

Peace of Mind

Understanding these modes helps you stay prepared, especially during extreme winter weather or unexpected equipment failures.

How Family Heating & Cooling Can Help

At Family Heating & Cooling, we’re here to make sure your home stays safe and comfortable all winter long. We provide:

• Heat pump inspections and maintenance to keep your system operating efficiently.

• Fast diagnostic service if Aux Heat runs too often or your heat pump struggles in cold weather.

• 24/7 emergency service when your system stops working and you need help right away.

Whether your heat pump needs routine care or urgent repair, our team is ready to help you stay warm, no matter what the weather brings.