What is Emergency Heat?

What is “EM heat” on a thermostat next to the “heat” and “cool” settings?

In brief, em heat refers to “emergency heat,” which is a set of heat strips that heat up in emergency situations in place of an air-source heat pump.

The two most common em heat uses are when the heat pump has mechanical failure or freeze up and when the outside weather is too cold for the heat pump to adequately warm your home.

But such a brief explanation doesn’t cover all the details concerning this significant heating feature. If you are unaware of what em heat does for your home and how to manage it, use our navigation guide to help find the answers:

Heat Pumps: How They Work

There are three types of heat pumps used in a home—air-sourced, water-sourced, and geothermal. The most common of these types is the air-source heat pump, which is likely what you have for your central heating system.

Air-source heat pumps use a small amount of energy to transfer heat from one location to another. In other words, they don’t burn fuel to function or even create heat. They move the existing heat in the air.

Because of this transferring process, heat pumps are useful as both a heating system in the winter (drawing in heat) and an air conditioning system in the summer (sending it out).

Heat pumps are most efficient and typically used in moderately cold climates. This doesn’t mean they cannot be provided for homes that experience harsher winters, but a heat pump cannot extract enough heat to warm the home in weather that ranges between 33- and 40-degrees Fahrenheit. It requires assistance, which usually comes from auxiliary heat (aux heat), a heat pump-furnace combo system, and/or remote heating items, such as portable indoor space heaters.

Em Heat vs. Heat: What Is Em Heat?

What is emergency heat on thermostat? If emergency heat isn’t an assisting heat source, what is it? The answer is quite frankly in the name: it is an emergency backup.

When the outside temperature drops, your heat pump will be forced to work harder to absorb heat from the outside air. Even with the assistance of other heat sources, the pump becomes colder the longer it tries to produce heat. The pump has a “defrost” setting that sets in when it starts to freeze, but if the weather is at freezing temperature (32F) or below, the “defrost” setting may not adequately thaw the heat pump, which could lead to the pump becoming damaged or irreparably broken.

Em heat acts as a safeguard for these colder weather conditions. When em heat is turned on, it will simultaneously shut off your heat pump so that the pump will not freeze over, and it will provide your home with enough heat so that you won’t need to evacuate.

Keep in mind that your thermostat won’t automatically switch your system to em heat. You will need to manually turn it on. This can be done by moving a switch or through the press of a button, depending on the kind of thermostat you have. We’re asked how to turn off emergency heat. You can turn the em heat off through this same control.

EM Heat Thermostats
EM Heat

As a rule of thumb, em heating should not be used in excess. While the modern heat pump can reduce your electricity consumption for heating by as much as 50%, em heat doesn’t utilize the same heat processing as heat pumps.

Em heat is a set of radiant heat strips in your air-handling unit. They use electric resistance heating (ERH), which means that 100% of the available electrical energy used for heating is converted into heat. This may sound financially efficient, but don’t be deceived: it actually means that you are using a lot of energy to power the heat strips, and this will increase energy use significantly.

You shouldn’t avoid using em heat during emergencies, but you should make sure to reserve it for emergencies only.

In summary, the major differences for em heat vs. standard heat are:

  • When to use emergency heat. Your central heating system can be used anytime your house needs to be warmed up, but em heat should only be used if the weather is 32F or below or if your heat pump is damaged and awaiting repairs.
  • Efficiency. Heat pumps reduce energy usage by about 50%, but em heat requires a lot of electricity to run.
  • Cost. Heat pumps save you money in the long run since they don’t require burning fuel to function. Em heat costs much more money to use than a heat pump.

Em Heat vs. Aux Heat 

Thermostats connected to a heat pump have a light—and sometimes a switch—that reads “aux heat” next to it. Aux heat is a supplementary heating component for your heat pump in 33F-40F weather,

Many homeowners believe that aux heat and em heat are the same thing when, in fact, they are not. Of course, the mix-up is understandable since both aux and em heat utilize the same radiant heat strips to warm the home. However, they serve different purposes that do happen at the same time. Aux heat is used in conjunction with the heat pump as a supplement, so the heat strips don’t take over completely, while em heat is used in place of the heat pump.

In other words, when em heat is on, aux heat is off—and the reverse.

Not every heat pump system includes aux heat. Do you know if yours does? For more aux heat know-how, see our article What is Auxiliary Heat on a Heat Pump.

Heat Strip Sizes

It may relieve you to know that it is up to HVAC professionals to figure out the proper heat strips size for your home. But, if you’re hoping to be more informed on some of the behind-the-scenes work that determines size, HVAC professionals follow two criteria:

  1. The size of your current heat pump. If you own a larger central heating system, your heat strips will have to be larger. Alternatively, a smaller heat pump equates to a smaller set of heat strips.
  2. Home details. The size of your heat pump, and therefore your heat strips, is determined by the industry-standard Manual J load calculation, which assesses the configuration, construction, climate, and size of your house, among other house particulars.

Radiant heat strips (also known as electric heat strips) come in various sizes between 3 kilowatts (kW) and 25kW for residential air handlers. The most commonly used sizes are 5kW, 7.5kW, 15kW, 20kW, and 25kW.

The load calculation finds what British Thermal Unit (BTU) your heat pump and heat strips should be, which have their equivalent kilowatts. Refer to the chart below for the common heat strip sizes. Note that the average cost for heat strips is anything between $100 and $400 depending on the size.

Heat Strip Sizes (kW) BTUs
3 kW 10,000 – 15,000
5 kW 15,000 – 20,000
7.5 kW 20,000 – 30,000
10 kW 30,000 – 40,000
15 kW 50,000 – 55,000
20 kW 65,000 – 70,000
25 kW 85,000 – 90,000

The larger the kW or BTU number, the larger the heat strips will be. And, of course, a larger set of heat strips means more produced heat and more electricity for that heat.

The goal is to have a perfectly fitted set of heat strips for your heat pump and home. You wouldn’t want strips that are too small, or else you won’t receive the amount of heat needed when you use your em heat. Strips too large will overheat your home and cost you even more money on utilities.

When an HVAC professional is applying the load calculation for your heat strips, make sure to inform him about any recently updated features, such as new windows or insulation, as they will factor into the calculation.

Pro Help When Heat Pump Breaks

Our hope is that you will never find yourself using em heat because of a broken heat pump, yet we know circumstances like that can happen. If your heat pump does break down, you should know that fixing it is not a DIY-friendly project. A non-expert repair or installation may lead to even more headaches down the road. And it will likely void your warranty. Most residential heat pump warranties exclude claims made on units not installed and repaired by certified contractors.

We advise you call your most reliable, licensed HVAC company to do the job. If you don’t have a contractor in mind, use our Free Local Quote phone number or convenient form to get advice, have questions answered at receive estimates on repairs or heat pump replacement.

It’s free, and you have no obligation to accept an estimate.

While you wait for your pump to be fixed or newly installed, use the em heat to keep your home warm in the meantime. This said, make sure you call your HVAC company as soon as possible to avoid crazy high utility bills.

In order to avoid damaged heat pumps or worst-case scenarios in the future, we also suggest having your HVAC company check over your heat pump every fall to iron out possible kinks before the weather gets colder. As an incentive, many companies provide pre-season specials on inspections during their slow seasons.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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