Aux Heat vs. Emergency Heat on a Heat Pump: When to Use Each

Your thermostat says “aux heat” or “emergency heat,” and you are not sure which one is normal or whether you should change anything. Both are part of how a heat pump keeps your home warm through a Bucks County winter, but they do very different jobs.

Quick Answer: Auxiliary (aux) heat comes on by itself when your heat pump needs help keeping up, and the heat pump keeps running. Emergency heat is a setting you choose by hand. It shuts the heat pump off and runs only the backup heat. Use emergency heat only when the heat pump has stopped working, the outdoor unit is damaged or iced solid, or a technician tells you to.

How a Heat Pump Uses Backup Heat

A heat pump does not burn fuel to make heat. It pulls warmth from the outdoor air and moves it inside. That works well in most weather, but on the coldest days there is less heat outside to collect, and the heat pump has to work harder.

That is why heat pumps come with backup heat. In many homes the backup is a set of electric resistance heating elements, often called heat strips, inside the indoor air handler. They warm the air directly, much like a space heater. Other homes have a “dual-fuel” system, where a gas or oil furnace is the backup instead of electric strips.

Aux heat and emergency heat both use that backup.

Aux Heat: The Automatic Helper

When your thermostat shows “aux heat,” it means the system decided the heat pump needed a boost and brought the backup heat on by itself. You do not have to do anything.

Aux heat commonly comes on:

• On very cold days, when the heat pump alone cannot keep the house at your setting.
• After a big jump on the thermostat. Raising the temperature several degrees at once can call for backup heat to catch up. Smaller changes let the heat pump do more of the work.
• During a defrost cycle. In cold, damp weather, frost builds up on the outdoor unit. The heat pump runs a short defrost cycle to melt it, and backup heat keeps cool air from blowing inside for those few minutes.

With electric heat strips, aux heat runs alongside the heat pump. With a dual-fuel system, the furnace usually takes over and the heat pump pauses until conditions change, because a furnace and heat pump generally do not heat at the same time. Either way, the system makes the switch for you.

Seeing aux heat now and then in cold weather is normal. It is not a sign that something is broken.

Emergency Heat: The Manual Backup

Emergency heat (often “EM Heat” or “Emergency” on the thermostat) is a mode you select yourself. It turns the heat pump off completely and runs only the backup heat.

It is meant for times when the heat pump cannot or should not run:

• The heat pump has stopped working and you need heat until it is repaired.
• The outdoor unit is damaged, or it is covered in ice and not clearing on its own.
• A technician has told you to use emergency heat until a repair is done.

In those cases, emergency heat keeps your home warm while you wait for service.

When not to use it: Do not switch to emergency heat just because it is cold outside or because you saw “aux heat” on the screen. Aux heat already handles cold weather automatically. Running emergency heat on a heat pump that works shuts off your most efficient source of heat for no reason.

Aux Heat vs. Emergency Heat at a Glance

What Running Emergency Heat Too Long Does to Your Bill

Electric heat strips turn one unit of electricity into one unit of heat. A working heat pump moves more heat than that for the same electricity, which is why it is the efficient choice in most weather. When emergency heat runs on electric strips for days or weeks, your electric bill can climb fast, and the backup equipment is not designed to be your main heat for long stretches.

If your backup is a gas or oil furnace, emergency heat runs the furnace instead, so the cost shows up on your fuel bill rather than your electric bill.

A few habits help:

• Switch back to normal heat as soon as the heat pump is working again.
• Do not use emergency heat as a shortcut to warm the house faster.
• Make small thermostat changes so the heat pump does most of the work.
• Have the system checked if aux heat seems to run far more than it used to.

Signs Something Needs a Closer Look

Some patterns are worth a service call:

• The thermostat has been on emergency heat for more than a day or two, but nothing is wrong with the heat pump.
• Aux heat runs almost all the time, even in mild weather.
• The outdoor unit sits still during normal cold weather while the house still gets some heat.
• Cool air blows from the vents for long stretches, not just during a short defrost cycle.
• The electric bill jumps and you cannot explain why.

These can point to a thermostat setting, a heat pump problem or a defrost issue. A technician can tell you which. For more on related symptoms, see why a heat pump blows cool air in heating mode and signs a heat pump’s reversing valve is not working properly.

If your home also has a gas furnace as backup, keeping it in good shape matters too. See Service First’s furnace repair services.

Not Sure Your Heat Pump Is Working Right?

If you are unsure whether your heat pump is behaving normally, keep the thermostat on its normal heat setting and have it checked rather than guessing. Service First HVAC has served Bucks County homeowners since 2009 and offers heat pump repair and service. Call (215) 876-0486 to schedule a visit. If your heat pump stops working completely, 24/7 emergency service is available.

Frequently Asked Questions