If you have spotted “EMH” on your thermostat and wondered what it does, here is the short answer. The EMH meaning on thermostat displays is simple: it stands for emergency heat, a backup mode found on systems that run a heat pump. When you switch to EMH, your thermostat tells the system to stop using the outdoor heat pump compressor and instead heat your home with a secondary, backup heat source. It is a feature designed for trouble, not for everyday comfort, and using it incorrectly can send your electric bill climbing fast.
What a Heat Pump Normally Does
To understand emergency heat, you first need to understand how a heat pump heats. Unlike a furnace that burns fuel to create warmth, a heat pump moves heat. In winter it pulls heat energy out of the outdoor air, even cold air, and transfers it indoors using a refrigerant cycle and the outdoor compressor. This is remarkably efficient. A typical air-source heat pump delivers two to four units of heat for every unit of electricity it consumes, which is why heat pumps cost so little to run compared to electric baseboard or resistance heaters.
That efficiency drops as the temperature outside falls. Around 35 to 40 degrees Fahrenheit, most standard heat pumps start to struggle to keep up. Below roughly 25 to 30 degrees, there simply is not enough heat in the outdoor air for the compressor to extract economically. That is where backup heat comes in, and where the EMH and AUX settings on your thermostat enter the picture.
What EMH (Emergency Heat) Actually Switches To
When you select EMH, the system bypasses the heat pump compressor entirely. Instead, it powers the backup heat source. In the vast majority of homes, that backup is a bank of electric resistance heat strips mounted inside the air handler, much like a giant toaster element the air blows across. In some installations, particularly dual-fuel or hybrid systems, the backup is a gas or oil furnace instead.
Either way, emergency heat will keep your house warm. Electric heat strips produce reliable, steady heat regardless of how cold it is outside. The catch is cost. Resistance heat delivers only one unit of heat per unit of electricity, so running EMH can cost two to three times more per hour than running the heat pump normally. A weekend stuck on emergency heat can add $50 to $150 or more to a monthly bill, depending on your rates and climate.
EMH Versus AUX: An Important Difference
Many homeowners confuse EMH with AUX, and they are not the same thing. AUX stands for auxiliary heat, and it is automatic. Your thermostat brings on the auxiliary heat strips by itself when the heat pump cannot keep up, usually on very cold days or during a quick recovery from a deep setback. In AUX mode, the heat pump and the strips run together, so you still get the efficiency of the compressor plus a boost.
EMH, by contrast, is manual and exclusive. When you turn it on, only the backup source runs and the heat pump compressor stays off completely. Think of it this way:
- AUX: Automatic, the thermostat decides, heat pump plus strips work together. Normal and expected on cold days.
- EMH: Manual, you decide, compressor off, strips only. For genuine emergencies.
This is why seeing AUX light up on a 20-degree morning is no cause for alarm, but deliberately flipping to EMH should be a conscious choice for a specific reason.
When You Should Use Emergency Heat
Reserve EMH for situations where the heat pump itself is the problem. The clearest case is a failed or damaged outdoor unit. If the outdoor compressor has stopped working, is making grinding or screeching noises, or the fan has quit, switching to emergency heat lets the indoor backup keep your family warm until a technician can come out.
Another valid case is a thick coating of ice on the outdoor coil that the unit cannot defrost on its own, or an outdoor unit buried in snow or a fallen branch. If you see a solid block of ice that is not clearing during normal defrost cycles, moving to EMH protects the compressor from running under a heavy load while you address the obstruction. Use it as a stopgap, then call for service rather than treating EMH as a permanent fix.
When You Should Not Use It
Do not use emergency heat just because it is cold outside. That is the most common and most expensive mistake. Your system is built to handle cold weather through normal heating and automatic AUX staging. Manually forcing EMH on a chilly but otherwise normal day only shuts off your efficient compressor and runs the costly strips around the clock.
Avoid using EMH to warm the house faster after a temperature setback as well. The thermostat will stage AUX in automatically if it needs a boost, and it will shut that boost off once you are comfortable. Leaving the system on emergency heat overnight by accident is a classic way to wake up to a shockingly high usage spike. If your heat pump is running fine, leave the thermostat on the standard Heat setting and let it do its job.
The Red Warning Light and Stuck EMH
Most thermostats display a red or amber light, or an “EM Heat” indicator on the screen, whenever emergency heat is active. That light is intentional. It is a reminder that you are paying premium rates so you do not forget you left the system in this mode. If you ever notice that indicator and did not turn it on, switch back to normal Heat right away.
If your thermostat seems stuck on EMH and will not return to regular heating, work through a few checks before panicking. Confirm the mode setting actually shows Heat, not Emergency Heat. Replace the thermostat batteries if it uses them, since low power can scramble settings. Check whether the outdoor unit is running at all, because some thermostats default to backup heat when they lose communication with the outdoor compressor. If the compressor is dead, the system may fall back to emergency heat automatically until repairs are made. Persistent EMH with no obvious cause usually points to a wiring, control board, or sensor fault that warrants a service call.
Quick Reference for Homeowners
Keep these takeaways in mind so the EMH meaning on your thermostat never costs you more than it should:
- EMH means emergency heat, a manual backup mode that runs your secondary heat source instead of the heat pump.
- It is less efficient and can cost two to three times more per hour than normal heating.
- Use it only when the heat pump or outdoor unit has failed, iced over, or is otherwise unable to run safely.
- Let AUX handle ordinary cold weather automatically. Do not force EMH just because temperatures dropped.
- Watch for the red EMH indicator light and switch back to standard Heat as soon as the problem is resolved.
Understanding this single setting can save you real money and keep your family comfortable during a heating emergency. When in doubt, leave your thermostat on Heat, and reserve emergency heat for the moments your heat pump genuinely needs the help.
