What is EM Heat? When Should You Turn it On?


EM heat, short for “Emergency Heat,” is a feature found in heat pump systems, especially in regions with colder climates. Here’s a summary of what EM heat is and when you should turn it on:

What Is EM Heat?

  • EM heat is a backup or auxiliary heating mode in a heat pump system.
  • Heat pumps typically rely on the transfer of heat from the outdoor air to provide warmth during the heating season. However, when the outdoor temperature drops significantly, the heat pump’s efficiency decreases.
  • EM heat activates an electric resistance heating element (also known as heat strips) inside the indoor unit to provide supplemental heating when the heat pump alone is not sufficient to maintain the desired indoor temperature.

When Should You Turn On EM Heat?

EM heat should be used in specific situations, such as:

  • Extreme Cold: When the outdoor temperature drops significantly below freezing, the heat pump’s efficiency decreases. Turning on EM heat provides faster and more reliable heating during extreme cold spells.
  • Heat Pump Malfunctions: If the heat pump is not operating correctly or has a malfunction, you can switch to EM heat temporarily while waiting for repairs.
  • Defrost Cycles: During defrost cycles, which occur periodically in cold weather, the heat pump temporarily switches to cooling mode to remove ice buildup on the outdoor coil. EM heat can be used to maintain indoor comfort during these cycles.
  • Faster Heating: In some cases, homeowners may choose to use EM heat when they want to heat their home quickly or when they need rapid temperature adjustments.

How a Heat Pump Works

Unless you know where to look, a heat pump looks exactly like a central air conditioner. So, what is the difference between them?

An air conditioner can only cool a house (during summer) while a heat pump will cool the house during summer and heat it during the cold months of winter. For that reason, air conditioners are always paired up with furnaces for heating purposes.

Heat pumps have a reversing valve inside the outside/condensing unit which as its name implies reverses the direction of flow of the refrigerant. But how is that important?

In thermodynamics (heat transfer), heat always moves from a point of high concentration to a point of low concentration.

In a cooling cycle, the refrigerant moves from the compressor to the condenser coil and then to the evaporator (indoor unit). The reverse happens during the heating cycle.

When the heat pump is used to cool the house, the refrigerant enters the compressor from the indoor unit as a low pressure gas. After being compressed, its pressure and temperature dramatically increases, and that is how it is able to dissipate the heat to the surrounding (inside the condenser coil).

When the heat pump is then used for heating purposes, the high-pressure superheated refrigerant gas goes to the evaporator coil inside the house and not condenser coil. That way, the heat inside the refrigerant is dissipated throughout the house.

Instead of losing heat at the condenser coil, the refrigerant extracts heat from the surrounding and uses it to heat the house. Check out this guide on how a condenser coil works.

The heat pump can comfortably extract heat from outside as long as temperatures don’t dip below 35 degrees.

For that reason, heat pumps have a secondary source of heat designed to keep the system running when the temperatures outside fall below 35 degrees. Usually, the secondary or back-up heat is in the form of electric resistance heating but could also be natural gas, oil or hot water.

The second-stage source of heating as it is called provides the emergency heat, or simply EM heat. It is named as such since it should only be used during emergencies.

How Does the EM Heat Work?

When you turn on the EM heat on your thermostat you will notice that a light will come on. That is also what happens when the system automatically turns it on.

There is however a difference between you turning the emergency heat manually, and the system doing it. But why?

When you turn on the EM heat manually, you are telling the system to turn off the heat pump (bypass it) and draw heat 100% from the secondary source. At that time, you cannot hear the outside unit running.

On the other hand, when the system automatically turns on the EM heat, the 2 sources of energy will work together, complementing each other. When the temperature outside increases, the system will turn off the EM heat and solely depend on the primary heat source.

As you can see, your HVAC system will work seamlessly on its own and you don’t need to mess up with the EM heat settings unless the heat pump has failed.

Tip: Keep an eye on your heat pump during the cold months of winter. Build-up of snow on the unit can badly damage it necessitating a replacement. Also, make sure there are no weak tree branches that are likely to fall and crash it.

When Should You Use Emergency Heat?

As I have mentioned, your HVAC system will know when to use the primary source of heat and when to turn to the back-up heat. So, when should you turn on the EM heat on your thermostat?

If you are freezing inside the house, it is a sign that the heat pump is not working. That could be caused by a damaged compressor among other things like snow buildup or even a tree crashing on the unit.

As you wait for an HVAC technician to come over and have a look, you can turn on the EM heat on the thermostat. That will keep your house warm until the heat pump has been fixed.

Heat pumps are very efficient when cooling (SEER rating) and even when heating (HSPF rating). Their efficiency is however drastically reduced when the emergency heat is activated.

Is Emergency Heating More Expensive to Run?

The not-so-simple answer to this question is “it depends”. It depends on where you live and the type of secondary source of heat you have.

In some areas and in some times of the year, electricity is more expensive than natural gas. Sometimes natural gas is way expensive compared to electricity.

If therefore your heat pump uses natural gas (in a furnace) as its secondary source of energy and at a time when the price of natural gas is at an all-time high, it will definitely be more expensive to run the emergency heat.

The opposite will also happen if the prices of natural gas or oil are at an all-time low.

However, if you have an electric heater as your secondary heat source, the answer is a definite YES! It will be more expensive to run your HVAC system on EM heat.

Emergency Heat vs Auxiliary Heat

Your thermostat will have both EM heat and Aux heat. What is the difference between the two, or are they one and the same thing?

Emergency heating works independent of the heat pump while auxiliary heating works hand in hand with the heat pump. As I have mentioned, when you turn on the EM heat, the heat pump turns off and the secondary heat source takes over 100% until you turn it off.

Auxiliary heating is fully automated and typically kicks in when the outside temperature dips suddenly. The Aux heat kicks in when the heat pump is on, but due to a minor difference between the thermostat setting and the actual indoor temperature.

When the auxiliary heat is activated, the heap pump powers up a heat strip for supplemental heating. This typically runs for a few hours unlike in EM heat which runs until manually turned off.

That means that since auxiliary heating is a function of the heat pump, it will fail when the heat pump fails. And that is where emergency heating comes in.

Wrap Up

And basically that is everything you need to know about your thermostat’s emergency heating setting. I hope that this guide was helpful.

Since your heat pump runs throughout the year (unlike an air conditioner which gets a break during winter), you need to have scheduled maintenance to ensure it does not fail when you most need it.