Emergency heat on a thermostat is a manual setting, found only on heat pump systems, that shuts off the heat pump's outdoor unit and heats the house with backup heat alone. In most homes that backup is electric air handler heat strips. In a dual fuel system it's the furnace. Use emergency heat only when the heat pump is broken, iced over or damaged, because electric strips cost far more to run than the heat pump.
Aux heat is different. It's the same backup heat, but the thermostat turns it on automatically to help a working heat pump, during defrost or on very cold days. Seeing "aux" on your display is often normal. Seeing "emergency" usually means someone selected it.
Emergency Heat vs Aux Heat
| Aux heat (auxiliary) | Emergency heat | |
|---|---|---|
| Who turns it on | The thermostat, automatically | You, manually |
| Heat pump running? | Yes, backup adds to it | No, heat pump is locked out |
| Heat source | Strips or furnace plus heat pump | Strips or furnace only |
| When it's normal | Defrost, after a big setpoint increase, very cold weather | When the heat pump is broken or iced and waiting for repair |
| Running cost | Higher than heat pump alone, briefly | Highest with electric strips, for as long as it's on |
| Display label | Aux, Aux heat, Backup heat, Heat stage 2 | Em heat, Emer, Emergency, sometimes Aux heat only |
Both settings use the same backup equipment. Some thermostats only offer one of the two, and labels vary by brand, so check your thermostat's manual if yours looks different.
When to Use Emergency Heat
Switch to emergency heat when the heat pump can't or shouldn't run:
- The outdoor unit is encased in ice that hasn't cleared after several defrost cycles. Emergency heat stops the outdoor unit so the ice can melt without more stress on the coil and fan.
- The outdoor unit is damaged, for example by a fallen branch, a storm or a vehicle.
- The outdoor unit's breaker keeps tripping. Leave it off and use emergency heat until a technician looks at it.
- The heat pump runs but blows cold air in heat mode and you've confirmed the thermostat is set to Heat.
- A technician tells you to, while you wait for a part.
When you switch it on, schedule a repair. Emergency heat is a bridge, not a way to run the house all winter. The heat pump repair guide covers what usually causes each of these.
When Not to Use It
- Just because it's cold outside. Your thermostat brings on aux heat automatically when the heat pump needs help. Forcing emergency heat throws away the heat pump's efficiency.
- When you see frost and steam on the outdoor unit. That's defrost mode, and it's normal.
- To warm the house up faster. Strips do add heat quickly, but your thermostat's aux staging already handles recovery. A smaller setback overnight works better with a heat pump.
Leaving emergency heat on by accident is one of the most common reasons for a surprise winter electric bill in a heat pump home. If your bill jumped, check the mode first.
What Emergency Heat Costs to Run
Resistance strips turn each kWh into 3,412 BTU of heat. A heat pump moving two to four times as much heat per kWh is that much cheaper per unit of heat. So in mild weather, running on strips can cost two to several times as much as the heat pump would.
To estimate it yourself, multiply the strip size in kW by your price per kWh. A 10 kW strip heater uses about 10 kWh for every hour it runs. On a cold day it may run most of the hour. Your air handler's nameplate or the installation paperwork lists the strip size, commonly in the 5 to 20 kW range.
In a dual fuel system, emergency heat runs the gas or propane furnace instead. That may cost about the same as or less than the heat pump in cold weather, depending on your rates. The dual fuel heat pump guide explains how that changeover works.
Heat Pump Defrost Mode: What's Normal
Defrost is the behavior most often mistaken for a breakdown, and it's the most common reason aux heat lights up for a few minutes.
In heating mode, the outdoor coil runs colder than the outside air. When it's near or below freezing and the air is damp, frost builds on the coil and blocks airflow. The defrost control periodically flips the system into cooling mode for a few minutes. Hot refrigerant flows through the outdoor coil and melts the frost.
What you'll see and hear during defrost:
- The outdoor fan stops while the compressor keeps running.
- A loud whoosh as the reversing valve shifts, then another when defrost ends.
- Steam or vapor rising off the outdoor unit. It's water vapor, not smoke.
- Water dripping and draining from under the unit.
- Aux heat comes on indoors so the vents don't blow cold air.
How long and how often. A defrost cycle usually lasts a few minutes. Most controls end it once the coil warms past a set temperature, with a maximum time commonly around 10 to 15 minutes. Older time-temperature controls check for defrost at fixed intervals of run time, often selectable at 30, 60 or 90 minutes. Newer demand defrost controls use sensors to defrost only when frost has actually built up.
| Normal defrost | Not normal |
|---|---|
| Light frost that clears after a cycle | A solid block of ice that stays through several cycles |
| Steam for a few minutes | Ice building on top of the unit and the fan grille |
| Fan stops briefly, then restarts | Fan never restarts, or never stops |
| Whoosh at the start and end | Grinding, screeching or loud banging |
| Water and slush on the pad | Ice building up around the base so the unit sits in it |
If the ice stays, switch to emergency heat and call for service. Don't chip at the ice, because the fins and tubes puncture easily. Water dripping from a gutter onto the unit causes a lot of these ice caps, so check above the unit too.
Why the Aux Heat Light Stays On
Aux heat that comes on briefly is normal. Aux heat that runs most of the time is a sign something isn't right, especially in mild weather.
| When you see aux heat | Likely reason | What to do |
|---|---|---|
| During defrost, a few minutes at a time | Normal | Nothing |
| After raising the setpoint 2 or more degrees | Normal staging for recovery | Use smaller setpoint changes |
| Very cold weather, most of the day | Normal for many standard heat pumps below their balance point | Consider whether a cold climate heat pump fits at replacement time |
| Mild weather, constantly | Outdoor unit not running or not heating | Check the outdoor breaker and look at the unit; call for service |
| Outdoor unit runs but air is cool | Low refrigerant, stuck reversing valve | Technician |
| Right after a new thermostat install | Wrong O/B setting, aux lockout or staging misconfigured | Review the setup or have the installer check it |
| Emergency label instead of aux | Emergency heat selected | Switch back to Heat |
A thermostat setting called an aux heat lockout or balance point prevents strips from running above a chosen outdoor temperature. Another, sometimes called aux heat droop or temperature differential, sets how far below setpoint the house must fall before strips come on. If these are set too aggressively, strips run more than they need to.
How Emergency Heat Is Wired
On a heat pump thermostat, these terminals control the backup:
- W2 or AUX: auxiliary heat, energized automatically by the thermostat as a second stage.
- E: emergency heat, energized when you select emergency mode. Many installers jumper E and W2 together at the air handler so either call runs the same strips.
- O/B: the reversing valve. A wrong O/B setting can make the heat pump cool when you call for heat, which then looks like a need for emergency heat.
- Y: the compressor.
Some thermostats don't have a separate E terminal and handle emergency heat through the W2 or AUX wire with the compressor locked out in software. See thermostat wiring terminals for the full letter chart.
How to Turn Emergency Heat On and Off
- Open the system mode menu on your thermostat. On many models it's a Mode or System button.
- Choose the mode labeled Em Heat, Emer or Emergency. Some smart thermostats call it Aux or Aux heat only.
- Most thermostats show a light or icon while it's active.
- To turn it off, go back to the same menu and choose Heat.
If your thermostat has no emergency mode, your system may not have one, or it may have been disabled during setup.
Check Your Backup Heat Before You Need It
Backup heat runs so rarely that a failed strip or tripped breaker can go unnoticed until the heat pump breaks. In the fall, switch to emergency heat for a few minutes and check that warm air comes from the vents. Air handlers with strips often have their own breaker or breakers, separate from the outdoor unit. A technician can test each element and sequencer during heat pump maintenance.
Safety: don't open the air handler's electrical compartment. Heat strip circuits are 240 volts at high current, and the thermostat doesn't cut power to them.
Frequently Asked Questions
Should I turn on emergency heat when it's really cold?
Usually no. Cold weather alone isn't a reason to use emergency heat. Your system brings on auxiliary heat automatically when the heat pump needs help. Switch to emergency heat only when the heat pump itself has a problem, such as an outdoor unit encased in ice, a tripped outdoor breaker or visible damage, and arrange a repair.
What does aux heat mean on my thermostat?
Aux heat means the thermostat has turned on your backup heat, usually electric strips in the air handler, to help the heat pump. It's normal during defrost, after you raise the setpoint several degrees and in very cold weather. In a dual fuel system the backup is the furnace.
Is emergency heat expensive to run?
With electric strips, yes. Resistance heat turns one unit of electricity into one unit of heat, while a heat pump moves two to four in mild weather. So emergency heat can cost two to several times as much per hour of heat. In a dual fuel system, emergency heat runs the furnace, which may not cost more.
How long does heat pump defrost mode last?
Usually a few minutes. Most defrost controls end the cycle once the outdoor coil warms past a set temperature, and they also have a maximum time, commonly around 10 to 15 minutes. How often it runs depends on the control and the weather, and it's more frequent in damp air near or just below freezing.
Why does my thermostat say aux heat all the time?
If it's very cold, that can be normal for a standard heat pump. In mild weather it usually means the outdoor unit isn't doing its share: a failed capacitor or contactor, low refrigerant, a stuck reversing valve, a tripped breaker or a thermostat setting such as an aux heat lockout set too high or the wrong O/B setting. Have it checked.
Will using emergency heat damage my heat pump?
No. Emergency heat simply locks out the heat pump and runs the backup, so it can protect a damaged or iced outdoor unit. The risk is cost: leaving it on for days or weeks when the heat pump works fine can make your electric bill climb fast.