Emergency heat kicks in when your heat pump cannot pull enough warmth from outside air

Emergency heat, also called auxiliary heat or backup heat, is a second heating system built into most heat pumps. It activates automatically when outdoor temperatures drop so low that the heat pump becomes inefficient — typically around 35°F or below, depending on your system. Instead of forcing the heat pump to work harder and use more electricity, the backup system takes over and heats your home directly.

The backup is usually electric resistance heating, which works like a space heater or toaster: electricity passes through a wire coil and produces heat. This heat goes into your ductwork and warms your home the same way the heat pump does. You do not have to do anything to trigger it; your thermostat senses the outdoor temperature and switches over on its own.

Emergency heat is not the same as the emergency heat mode you can manually select on your thermostat. That manual setting forces the backup system to run even when the heat pump could still work, and it uses much more electricity. Most homeowners should never touch that button except in a true emergency.

Key Takeaways

  • Emergency heat activates automatically when outdoor air becomes too cold for the heat pump to work efficiently, usually below 35°F.
  • The backup system is electric resistance heating that produces warmth directly, rather than extracting it from outside air like the heat pump does.
  • Automatic emergency heat is normal and necessary in cold climates; manual emergency heat mode uses significantly more electricity and should be avoided.
  • Your heating bill rises when emergency heat runs because resistance heating costs more per unit of warmth than a heat pump does.
  • If emergency heat runs constantly or your system switches between heat pump and backup repeatedly, a technician should inspect the heat pump for problems.

Why heat pumps need backup heat in cold weather

A heat pump works by moving heat from one place to another using refrigerant and a compressor. In heating mode, it pulls warmth from outside air and moves it indoors. But as outdoor temperature drops, there is less heat available to extract, so the heat pump has to work much harder and run longer to reach your target temperature. Below a certain point — usually 35°F to 40°F depending on the model — the system becomes so inefficient that it costs more to run than to use the backup heating.

At that threshold, the thermostat automatically switches to emergency heat. The backup takes the load off the heat pump, and your home heats up faster and more cheaply. Once outdoor temperatures rise again, the heat pump resumes its normal job and the backup shuts off.

In mild climates where temperatures rarely drop below freezing, emergency heat may run only a few times per year or not at all. In cold climates, it can run for weeks or months during winter. This is why heat pumps are more common in the South and Southeast than in the Upper Midwest or Northeast — in those colder regions, the backup system would run so often that the cost advantage of the heat pump shrinks.

How to tell if emergency heat is running

Most thermostats display which heating mode is active. On a digital thermostat, you may see a label like "Heat Pump" or "Aux Heat" on the screen, or a light that indicates which system is running. On an older dial thermostat, you might not see any indicator at all.

The most reliable way to know is to check your electric meter or smart meter data. Emergency heat uses significantly more electricity than the heat pump alone. If your heating bill spikes on a cold day, emergency heat was likely running. You can also call an HVAC technician to check your system's runtime data — most modern heat pumps log which mode they operated in and for how long.

If you want to monitor it yourself, some smart thermostats (like Ecobee or Nest) show heating mode in their app or on a daily report. You can see exactly when the system switched to backup and how long it ran.

The difference between automatic and manual emergency heat

Your thermostat has two emergency heat settings. The first is automatic, which is the default. The system monitors outdoor temperature and switches to backup heat only when it needs to. This is the setting you want.

The second is manual, usually labeled "Emergency Heat" or "E-Heat" on the thermostat dial or menu. If you select this mode, the backup system runs all the time, and the heat pump shuts off completely. This uses far more electricity because resistance heating is less efficient than a heat pump, even in cold weather. You should only use manual emergency heat if the heat pump itself has failed and you need heat while waiting for a repair.

Many homeowners accidentally switch to manual emergency heat thinking it will heat their home faster or better. It will heat faster, but the electric bill will be two to three times higher. If you find your thermostat in manual emergency heat mode and you did not put it there, switch it back to "Heat" or "Auto" when ready.

What emergency heat costs compared to regular heating

Electric resistance heating costs roughly 2 to 3 times more per unit of warmth than a heat pump does. If your heat pump produces one unit of heat using one unit of electricity, resistance heating needs one unit of electricity to produce roughly one-third of a unit of heat. The difference is that a heat pump moves existing heat; resistance heating creates heat from scratch.

In a mild winter where emergency heat runs only occasionally, the cost difference is small — maybe $20 to $50 extra on your heating bill. In a harsh winter where temperatures stay below 35°F for weeks, emergency heat can add $200 to $400 or more to your bill, depending on how often it runs and how large your home is.

This is why heat pump owners in cold climates often see higher winter bills than they expected. The system is still more efficient than a traditional furnace over the whole heating season, but the months when emergency heat runs heavily will cost more than the months when the heat pump can handle the load alone.

When emergency heat runs too often or not at all

If your thermostat shows emergency heat running on days when outdoor temperature is above 40°F, or if it switches back and forth between heat pump and backup repeatedly, something is wrong. The most common cause is a refrigerant leak, which reduces the heat pump's ability to extract warmth from outside air. A dirty outdoor coil can have the same effect — leaves, dust, and ice block airflow and make the system less efficient.

Another possibility is a faulty outdoor temperature sensor. If the thermostat cannot read the outdoor temperature accurately, it may switch to emergency heat at the wrong time. A technician can test the sensor and replace it if needed.

If emergency heat never runs even on very cold days, the backup system itself may have failed. This is less common, but a blown heating element or a failed contactor (the switch that turns the backup on) can prevent it from working. You will not notice this until a day when the heat pump alone cannot keep up and your home gets cold.

How to reduce emergency heat use

You cannot prevent emergency heat from running when outdoor temperatures drop — it is a necessary part of how heat pumps work in cold climates. But you can reduce how often it runs by improving your home's insulation and air sealing. Better insulation means your home loses heat more slowly, so the heat pump can keep up longer before the backup is needed.

Sealing air leaks around windows, doors, and ductwork also helps. Leaks force the heating system to work harder and run longer. Weatherstripping, caulk, and foam sealant are inexpensive ways to reduce them.

You can also adjust your thermostat. Lowering your target temperature by just 2°F can noticeably reduce emergency heat runtime, especially on borderline cold days when the system is switching back and forth. Programmable thermostats let you set lower temperatures at night or when you are away, which saves money without sacrificing comfort when you are home.

Frequently Asked Questions

Is it bad for my heat pump if emergency heat runs a lot?

No. Emergency heat running frequently is normal in cold climates and does not damage the heat pump. The system is designed to switch to backup heat automatically. However, if emergency heat runs constantly even on mild days, have a technician check for a refrigerant leak or sensor problem.

Should I switch to manual emergency heat to save money?

No. Manual emergency heat uses more electricity, not less. It should only be used if your heat pump has failed and you need temporary heating while waiting for repair. Keep your thermostat in automatic mode.

Why does my heating bill spike on cold days?

Emergency heat is running. Resistance heating costs more per unit of warmth than a heat pump, so your bill rises when outdoor temperatures drop below the point where the heat pump becomes inefficient. This is normal and expected.

Can I turn off emergency heat to lower my bill?

No. Emergency heat is automatic and cannot be disabled without disabling the entire backup system. If you want lower bills, improve insulation, seal air leaks, or lower your thermostat setting.

What temperature does emergency heat turn on?

Most systems switch to emergency heat around 35°F to 40°F outdoor temperature, but this varies by model. Check your heat pump manual or call the manufacturer to find the exact threshold for your system.