What Is EM Heat on a Thermostat?

Discover what the EM Heat setting does and when to use it.

thermostat digital Programmable on wall
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There are many different appliances and systems that help to keep the home comfortable and running efficiently, but most people don't know a lot about these systems. The good news is that as long as they are working properly, there is no need to touch them, and if something goes wrong, you can call a professional to address your concerns. That is definitely one way of approaching home maintenance, but issues can arise when a little bit of unnecessary information falls into the mix.

The EM Heat setting on a thermostat is a great example of this. Some people who don't really have an understanding of how a HVAC system functions or what the EM Heat setting actually does may infer from the description that it increases the heat to the home. So, they turn the EM Heat setting on when the weather gets cold, assuming that this will heat up the home faster than the normal setting. However, this actually reduces the heating efficiency of the system and increases heating bills. Read on to avoid this situation by discovering exactly what EM Heat is, how it works, and when it should be used.

EM Heat Is a Backup System for Heating Emergencies

A home thermostat is typically used to control the temperature for the entire home, though some homes may have more than one thermostat, depending on the size of the property or the heating system installed. If you look at your thermostat and spot a setting that reads EM Heat, this refers to the emergency backup heating system that can be turned on if the primary heating system for the home fails.

By pressing a button or flicking a switch on the thermostat, you can manually switch over to the emergency heat mode, allowing the heat pump to bypass the standard operating process and rely solely on the auxiliary heat system. However, if the primary system is working correctly, then switching to emergency heating is a bad idea because the cost of heating the home will increase rapidly and the actual heat produced by the system will be significantly limited.

Some thermostats will have an Aux Heat setting instead of an EM Heat mode, but the Aux Heat setting is essentially the same. By switching to Aux Heat, you activate the auxiliary heating system and turn off the primary heating system.

Secondary Heating and Emergency Heating Functions

Thermostats that have an EM Heat or Aux Heat setting usually rely on a heat pump for the primary heating of the home. Heat pumps operate by extracting heat from the outdoors and pulling it into the home, but when temperatures drop, the air becomes too cold for this to be an effective method of heating the home. At this point, the secondary heat source, like a gas furnace or electric heat strip, is activated to provide heat to the home.

EM Heat or Aux Heat can also be automatically activated when the inside temperature is 3 degrees or more below the thermostat setting. If you arrive home and raise the temperature on the thermostat several degrees, EM Heat or Aux Heat may be activated to reach the desired temperature more quickly.

Normally, the two systems work in tandem throughout the coldest months of the year and when the temperatures outside begin to warm up, the heat pump alone is enough to keep the home comfortable.

However, when the EM Heat mode is activated, the heat pump is essentially turned off. In this mode, only the secondary heating system can provide heat to the home, resulting in higher bills and less efficient heating because, instead of heating the home with a primary and secondary system, you are now relying solely on the secondary system.

EM Heat Should Only Be Used in Emergencies

The reason that the EM Heat setting results in the heating system working less efficiently is that the primary system actually relies on the secondary or auxiliary heating system to help boost heat production when the temperature is below 35°F. However, when the thermostat is set to EM Heat, the primary system is bypassed entirely, so instead of having both systems working in tandem to generate excess heat, you are only able to use the secondary heating system. 

The heating and cooling system typically has a heat pump as a primary method of heating the home and will use a gas furnace or electric heat strip to provide auxiliary heat when it's necessary to boost the system. If the heat pump fails, then switching over to EM Heat allows the home to keep producing heat, but this operation increases the cost of heating and takes a toll on the heating and cooling system, so it's recommended to contact a licensed HVAC company as soon as possible to repair the problem. 

If you accidentally turned on EM Heat, just switch it off. The heat pump will automatically activate the secondary heating if necessary.

Significantly Higher Heating Cost

One of the main reasons people turn on the EM Heat function is because they believe that it will heat the home up quicker. As noted above, this simply isn't true. Instead, the home will heat up at a slower rate while relying on gas or electricity for constant heat production. Anyone that has made this mistake has likely gotten a nasty surprise on their next gas or electric bill, because EM Heat mode typically uses about 2 to 3 times the amount of gas or electricity than the heating and cooling system normally would. This also means that you will be responsible for paying about 2 to 3 times more on your bill.

If the heat pump does break down and you do need to switch to EM Heat mode to keep the home warm, then it's important to call an HVAC technician as soon as possible to resolve the problem. Keep in mind that if the temperature in the home gets too low, then the water pipes are vulnerable to freezing and bursting. A flood anytime is costly and time-consuming to clean up: A flood in the middle of winter without reliable heat in the home is a disaster.