What You Should Know About Heat Pumps and Emergency Heat

Heat pumps are very efficient at heating a home when temperatures are above 30 degrees. However, even in moderate climates, you can expect some freezing nights. Heat pumps have emergency heat to help keep up with demand in freezing temperatures. Emergency heat is expensive to use and shouldn’t be your primary heat source.

All heat pumps have a backup heat source known as emergency heat or ancillary heat. When the outside temperature is below 30 degrees, heat pumps auto-engage emergency heat to keep up with demand from the interior thermostat.

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Heat pumps use emergency heat if you raise your thermostat temperature by more than 2 degrees. Typically, heat pumps will temporarily engage the emergency heat to reach the desired temperature as quickly as possible.

Below, I will walk you through how emergency heat works and many of the common questions surrounding it. Through these explanations, you will better equip yourself to operate the heat pump system within your home.

Electric emergency heat is used to help the heat pump warm the home to the setting on the thermostat. Heat pumps automatically engage the emergency heat when needed. You do not need to set your thermostat to emergency heat.

Setting your thermostat on emergency heat forces your heat pump to use this option no matter what the outside temperature is. Emergency heat (aka ancillary heat) is more expensive because more electricity is needed to power the emergency heat coil.

The below table outlines the cost differences of normal heat pump use a=in 30 degrees F weather for one week versus one week of use in 10 degrees F weather with the emergency heat in use for 12 hours per day to keep up with demand.

Cost BreakdownNormal Operation
at 30 Degrees
for 1 week
Emergency Heat
Operation at 10
Degrees for 1 week
Watts per hour:3000 watts3000 watts + 15000 watts
Hours use per day:12 hours24 hours + 12 hours
Number of days:7 days7 days
Total kWH:252 kWH252 kWH + 1260 kWH
Cost per kWH:$0.13$0.13
Total Cost per Week:$32.76$196.56

As you can see, there is a severe increase in the cost of $163.80 per week when the emergency heat is in use in extremely cost weather.

How Emergency Heat Works

Heat pumps work by using heat transfer from warmer outside air blowing across the outside condensing coil. Heat pumps cool the refrigerant oil to a temperature much colder than the outside air.

As the warmer outside air passes over the condensing coil, heat from the outside air is transferred to the refrigerant oil, heating it and passing it to the evaporator coil where cool interior air from the air return passes over the evaporator coil, heating it and then distributing it back through the branch ductwork.

Emergency heat is an electrical backup heating coil within your heat pump that assists in warming the air to provide heat. It works by using electricity to heat coils to warm the air going into the ductwork.  

At What Temperature Does Emergency Heat Turn On?

Emergency heat will turn on automatically when the outside temperature gets below 30 degrees Fahrenheit. The true cause of the emergency heat turning on is the heat pump’s inability to warm the home to the temperature set on the thermostat.

For example, let’s say that the outside temperature is 17 degree Fahrenheit, but the interior thermostat is set to 74 degrees. The heat pump will not be able to keep up with this demand and will engage the emergency heat to assist in reaching and maintaining the desired interior temperature.

Thankfully, modern heat pumps are able to detect when it is necessary to have the emergency heat turn on. The best part of this is the energy it saves and the user’s peace of mind. This means that your heat pump is automatically running as efficiently as possible but only activating the emergency heat when necessary. 1

In our article, Will a Heat Pump Work in Subzero Temperatures? we detail how heat pumps operate in subfreezing temperatures.

When Should You Use Emergency Heat?

You should not need to use the emergency heat setting on your thermostat, even when it’s frigid cold. The emergency heat will automatically turn on when needed to heat your home to the desired temperature.

The only time to use this setting is if your heat pump stops working. This setting essentially runs your backup heat constantly, which is incredibly inefficient and costly. Typically, if you increase your thermostat by more than 2 degrees, the emergency heat will kick on automatically to reach the desired temperature as quickly as possible.

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Why Home Inspections Are Important

In 16 Reasons Your Heat Pump Doesn’t Blow Hot Air, we discuss emergency heat and 16 reasons why your heat pump may not be blowing hot air.

The “emergency heat” option is a setting that is only to be used to hold you over until a repairman can get to your home in the case that your heat pump has stopped working properly. Due to the way that backup heat works are much less efficient than your heat pump, so using it sparingly is best.

Heat pump should switch to emergency heat if necessary, and will not change to this less energy-efficient method of heating your home if it is not required. If you have any questions regarding your heat pump’s functionality, you may always contact a repairman with any questions.

Is Emergency Heat Expensive to Run?

Emergency heat is expensive to run. Electric backup heating strips will take 1kw of energy to produce 1kw of heat which is much less efficient and cost a lot more than heat pumps. This is one reason why you should not use your emergency heat unless necessary. 

In dual fuel heat pumps, the emergency heat runs on propane or natural gas. The cost may or may not cost more. Usually, it will still be more expensive, but sometimes it can run in a fashion that does not require more fuel than the standard heat pump. 

Therefore, it is a safer bet to use your home’s thermostat’s regular heat setting – not its emergency heat availability- to heat your home in most cases. You will save energy (electricity or gas), thus saving money in heating your home regardless of the weather conditions.

Auxiliary Heat vs. Emergency Heat – Are They the Same?

Auxiliary and Emergency heat are the same heat source. If you have both on your thermostat, the only difference is the name and purpose of use. Auxiliary heat turns on automatically to supplement the heat pump when the temperature drops too low; emergency heat is manually selected on your thermostat and causes the backup heat to provide all heating. 

Emergency heat is often referred to as auxiliary heat, backup heat, or even heat strips. In general, it is safe to assume that these all mean the same thing. It would help if you allowed your heat pump to do what it needs to unless something goes wrong. When the heat pump controls the auxiliary heat, you would be saving more energy (and thus money) than if you set your thermostat to emergency heating.

If you are confused, consider the location of where you will see the terms “auxiliary heat” and “emergency heat.” Typically, the term “emergency heat” is a setting option on your thermostat.

You can select it when necessary. However, “auxiliary heat” is not as commonly seen as a setting option in modern thermostats. It is the automatic function that your heat pump will switch on, thus not needing manual involvement to turn on. 

Why My Emergency Heat is Not Working

The most common reason that your emergency heat is not working is due to a lack of electricity. This can be due to a tripped circuit breaker or a faulty heating element.

If this is not the case, you can check inside the unit to ensure that the sequencer is working correctly. A sequencer is a small electrical circuit used to control the energy flow of the heat pump by shutting on/off varying switches depending on the electrical setting. It puts different heating elements in order, aka “sequencing” them, to work in their respective turn.

The distribution of energy prevents the breaker from being tripped or a fuse from being blown- a beneficial function for a heating system that is used regularly.

If you suspect that your heat pump’s sequencer is not working correctly, you may test it with an electrical test meter, being sure to turn off the power before doing so. If you are unsure how to do this safely, it is recommended to contact a local HVAC technician.

If you are confident that your sequencer is not the issue, you will want to check the electricity source- the most common reason your emergency heat would not be working. 

To do this, the first thing you should do if you notice that your emergency heat is not working is to check the circuit breakers. If they are flipped and working properly, then you can check the fuses. If you have an electric current checker, it is good to get enough volts to the unit.

You can verify this by looking up your specific unit and checking what its voltage should be. 

If all of this is good, it is time to inspect the machine. The sequencer inside of the box should be able to turn the emergency or auxiliary heat on. If this unit is bad, you will want to replace it.

Replacing any part of your heating system will inevitably create lasting benefits to your home- particularly as it will impact your comfort and the assurance of your home’s interior remaining at an appropriate temperature.

Emergency heat strips can also burn out, requiring replacement. Heat pumps will have two heat strips. In the event one fails, there is still one heat strip to provide heat to the home.

One heat strip will not keep up with the demand for an extended time. It’ll also cause the system to run longer, consuming more energy.

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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