What Is Emergency Heat? When to Use It and What It Costs

Emergency heat is a thermostat setting on heat pump systems that shuts off the heat pump and runs only the backup heat, which in most homes is a bank of electric resistance strips. Use it only when the heat pump is broken or damaged and you need heat until a technician can fix it. It keeps the house warm, but it costs far more to run than the heat pump, so switch back to Heat as soon as the repair is done.

What emergency heat does on a heat pump thermostat

A heat pump does not make heat. It moves heat from the outdoor air into your home. ENERGY STAR says a heat pump can deliver up to three times more heat energy than the electricity it uses. We explain the full cycle in how a heat pump works in winter.

Most heat pump systems also have a second heat source in the indoor unit. Lennox says that for most homes the backup is electric resistance heating, and some dual-fuel systems use a gas or oil furnace instead.

When you switch the thermostat to emergency heat (often labeled EM Heat, Em Ht or Emer. Heat), two things happen:

• The outdoor unit and its compressor stop running.
• The backup heat carries the whole house on its own.

Honeywell puts it plainly in its Aux Ht versus Em Ht explainer: in the Em Ht position the compressor does not run, and only the emergency heat runs. The setting stays on until you turn it off. Google says a Nest thermostat keeps using emergency heat until you switch it off manually.

Emergency heat vs auxiliary heat

Aux heat and emergency heat usually use the same equipment. Honeywell notes it is common for the two to share one heat source, such as electric heat strips. The difference is who turns it on and whether the heat pump keeps running.

Auxiliary heat is automatic. The thermostat brings it on to help the heat pump when the pump cannot keep up, and the heat pump keeps running alongside it. Emergency heat is manual. You select it, and it replaces the heat pump instead of helping it.

Carrier lists the usual triggers for aux heat on its auxiliary heat glossary page: outdoor temperatures below freezing, the defrost cycle, and raising the thermostat by more than 2 to 3 degrees at once. Carrier also says there is no single outdoor temperature at which every one of its heat pumps calls for aux heat. It depends on the point where your heat pump's output stops matching your home's heat loss.

So if your thermostat shows "Aux Heat On" on a cold morning, that is the system working as designed. It is not a reason to switch to emergency heat.

When to use emergency heat

Lennox gives three situations on its emergency heat guide:

1. Your HVAC technician tells you to turn it on while you wait for a repair or parts.
2. The heat pump has failed completely and you need heat until it is fixed or replaced.
3. The outdoor unit is frozen solid or damaged, for example by storm debris or ice, and cannot run safely until someone inspects it.

Carrier is stricter: switch to emergency heat only if the heat pump has broken down or malfunctioned. In practice the two agree. Emergency heat is for a heat pump that cannot or should not run.

A thin layer of frost on the outdoor coil is not the same as a unit frozen solid. Heat pumps run a defrost cycle to clear it, and aux heat helps during that cycle, according to Carrier. If you see ice or snow on the unit, Lennox says to clear away loose debris but not to force anything or chip at the ice.

When not to use emergency heat

Do not use emergency heat just because it is cold outside. If your system has aux heat, the thermostat already brings in the backup automatically when the heat pump needs help, and it keeps the heat pump running for the share of heat it can still deliver. Switching to EM Heat throws that share away.

Google's Nest emergency heat page says you should rarely need it, typically only in very cold weather, and that needing it regularly may mean the system needs a technician. That is the one place the manufacturers differ slightly on cold weather. None of them suggest it as an everyday cold-weather setting.

Do not leave it on for weeks either. Lennox warns that emergency heat parts are not designed to run as the primary heat source for long periods.

What emergency heat costs to run

Emergency heat costs more because of how the heat is made. The heat pump moves existing heat from outside, which is why ENERGY STAR credits it with delivering up to three times the heat energy it uses in electricity. Electric strips generate heat directly from the electricity they draw, so you get no such multiplier. Carrier states that electric resistance heating typically costs more to operate than heat pump heating.

Google gives the clearest ratio. Its Heat Pump Balance page says aux heat can cost about 2 to 5 times as much as running your heat pump. Emergency heat runs that same backup with no help from the heat pump, so the whole heating load lands on the expensive source. Google's emergency heat page calls running it "usually expensive and inefficient."

We do not give a dollar figure here, because it depends on the size of your heat strips, your electricity rate and how many hours they run. Our cost to run calculator has a preset for heat pump aux heat strips. Type in your own strip rating if you know it, pick your state, and it shows the cost per hour, day and month.

If your backup is a gas or oil furnace (a dual-fuel system), the cost picture is different. Our heat pump vs furnace comparison covers how those two compare.

How to turn emergency heat off

Turning it off means switching the thermostat back to its normal heating mode. Lennox says the mode should typically be Heat or Auto. Give the system a few minutes to restart the heat pump, and do not keep switching it on and off.

Thermostats with a mode button or switch

On Honeywell thermostats, and many others, emergency heat is one position of the system mode setting (Em Ht or EM Heat). Move it back to Heat. If the screen still shows an emergency heat message after a few minutes, check the manual for your model.

Google Nest thermostats

Google lists these paths on its support page, as of October 2026:

• Google Home app: touch and hold the thermostat tile, then Settings, Thermostat, Emergency Heat, and tap the switch.
• Earlier Nest Learning Thermostats and Nest Thermostat E: open Quick View, select Thermostat, then Turn Off.
• Nest Thermostat and Nest Learning Thermostat (4th gen): go to Settings, Emergency Heat, and choose Turn off.

Google adds that you only see the option if your system has emergency heat set up.

If you did not turn it on

Lennox notes that, depending on the thermostat, emergency heat may come on by itself. If that happens, check the mode first, then look for error codes or warning lights and note any odd sounds or smells. Trane says an HVAC technician has to configure emergency heat on its systems, so if the setting looks wrong, your installer is the person to call. If it keeps coming back, the heat pump needs a professional diagnosis.

How to keep backup heat to a minimum

You cannot avoid aux heat in very cold weather, but you can stop triggering it when you do not need it. Carrier suggests these habits:

• Avoid dropping the thermostat a lot at night.
• Avoid raising the setpoint by more than 2 to 3 degrees at once.
• Keep the filter clean, since dirty filters can push the system onto backup heat.

Carrier adds that smart thermostats can ramp the temperature up slowly to avoid calling for aux heat. Nest thermostats have a setting for this called Heat Pump Balance, which lets you pick between comfort and savings, and the thermostat then decides how often aux heat runs. If you are shopping for one, our guide to the best heat pump thermostats covers the models we recommend.