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When To Turn On Emergency Heat, Explained

Only use emergency heat activation if the heat pump isn’t working properly and needs repair or maintenance. This means that you should only use emergency heat temporarily. Fortunately, emergency heat does not activate automatically, giving you greater control. 

The general opinion is that you can turn on emergency heating when the temperature drops significantly. However, as a devoted enthusiast of heating, ventilation, and air conditioning (HVAC), I strongly advise against it. My research and conversations with HVAC professionals have taught me much about emergency heating. 

This article explains the differences between emergency and primary heating and when to turn on emergency heat in heat pumps. I also go over using the HVAC system’s backup heat. Read to the end for answers to frequently asked questions. 

Emergency Heat vs. Primary Heat

Heat pump (Image by global energy systems on Pixabay).

Emergency heat is a secondary heating system that keeps your home warm when the primary heat system fails. The heat pump is usually the primary heat system in most homes. 

Let’s look at the distinct features between direct and primary heat. 

Emergency Heat

The emergency heat system is a secondary heating source that includes hot water, electricity, oil, or gas. You should only use emergency heat when the primary heating system fails, not when temperatures drop drastically. 

It’s important to note that emergency heat is not the same as auxiliary heat. Emergency heat comes from an independent heat source, while auxiliary heat is a backup heating system within your heat pump. 

Another name for auxiliary heat is supplemental heat. In most cases, you must manually turn on the emergency heating system. The auxiliary system automatically switches on when the heat pump becomes inefficient in colder temperatures.

In freezing temperatures, the supplementary (auxiliary) heat turns on at about 35°F (1.7°C). An electric resistance heating element is an auxiliary heating system. The auxiliary offers extra heat when the heat pump can no longer cover the heating demand.

Casteel Air, an HVAC, plumbing, and electrical services provider, shows us How To Switch To Emergency Heat Mode on their YouTube channel.

Primary Heat

The primary heating source in most households is a heat pump. Heat is transferred indoors via a heat pump, which draws hot outdoor air. The heat pump moves hot air from the indoors to the outdoors during the warmer months. Usually, heat pumps run on electricity. 

Unlike furnaces, heat pumps do not burn fossil fuel. 

In cold weather, heat pumps cannot draw heat from outdoor air. Therefore, the heat pump requires a secondary or supplementary power source.

There are two heat sources that you can use as supplemental heat sources, which I highlight in the table below. 

Secondary Heat SourcesFeatures
Boilers (hydronic systems)-Natural gas, electricity, propane, and heating oil are available as fuel alternatives. the most popular being natural gas and oil.
-Basically, water heaters use hot water to heat your home.
-The boiler in your home is run by your fuel choice to heat the water.
-The hot water is then transferred into radiators or other appliances in your home via a network of pipes.
-After the heat has left the water, the cycle begins again with the cooler water returning to the boiler.
Furnaces-When fuel and air combine they burn, producing warm air.
-The heated air is subsequently dispersed throughout the rooms via the ducts.
-You can fuel your furnace with propane, heating oil, electricity, or natural gas, depending on your preferences and location.

When To Activate Emergency Heat

Emergency Heat Thermostat (image by Casteel Air).

You should turn on your emergency heat when there is an issue with the heat pump—the primary heating source. Resort to emergency heating only when your heat pump is unavailable. Turn on the emergency heat until a heating system repair expert fixes your system. 

Your heat pump can occasionally freeze and fail to thaw even with the heater. Then, while you wait for a professional to fix your primary heater, you can turn on the emergency heat. 

Emergency heat activation is expensive, especially with an all-electric heat pump. A heat pump consumes less energy than emergency heating. As a result, using emergency heating over time will result in high energy bills—an even greater justification for only using the emergency heat in an actual emergency. 

Emergency Heat in Heat Pumps

 Exterior Heat Pump (Image on Pixabay).

A heat pump cannot be defrosted by emergency heat. The emergency heat turns on the air handler’s electric heat strips. A defrost control board clears a heat pump’s cycle. The outdoor coil’s temperature sensor is connected to a timer that powers the control board. 

Your heat pump can freeze in the winter, stopping the heat supply. Regularly inspecting your exterior heat pump for signs of excessive ice or snow buildup would be ideal.  

The reversing valve shifts to the regular cool mode, temporarily turning on the pump and heating the outdoor coil. The interior electric heat strips will activate simultaneously to stop cold air from escaping through the vents. 

Once the defrost cycle is complete, the reversing valve is switched back to the heating position, and the strip heat is shut off.  

In the YouTube channel American Veteran Air-conditioning (A US veteran-owned company), Shawn McGraw demonstrates What to do if your heat pump ices up in the winter.

Using Backup Heat In HVAC

Your primary heat source absorbs heat from the earth or the outside air, typically your heat pump. However, when the temperature outdoors drops below freezing, the heat pump can no longer extract warm air. As a result, to keep the heat on inside, you’ll need a backup heat source. 

Specific heat pumps can be fitted with an auxiliary or backup system to provide more heat. When the heat pump cannot draw heat from the environment, electrical resistance heating coils come on. For this reason, the heat pump will contain a thermostat to turn on the auxiliary heat feature. 

Use high-quality fireplaces or electric baseboards as backup choices if your property is without ductwork. Central heat pumps are a suitable fit for natural gas. You must make a few modifications to ensure the natural gas heating system works effectively with your home heating system. 

In specific locations, heat pumps are backed up by wood heating. 

A furnace will do if your heat pump doesn’t have supplementary heating. A gas-forced air furnace is suitable for areas with milder winters. Heat pumps, however, can act subsequently as an auxiliary system.

Speak with an expert who can advise you on how the backup system will integrate with your heat pump before deciding on a backup heating system. 

Emergency Heating System Guidelines

Frigid weather is not enough reason to turn on the emergency heating system in your home. Use emergency heating only if your heat pump stops functioning properly. You can use the emergency heating system guidelines below. 

  • Emergency heat setting: A red light indicator is displayed when you turn on your emergency heat setting. The signal will remain on until you manually turn off your emergency heat system. Simply put, the light is there to let you know that you’re in emergency mode. 
  • Emergency mode: You must rely on secondary source heating when your heat pump ceases functioning. Your heat pump is having trouble if the light is on even though you did not turn it on. The outdoor heat pump unit has likely turned off and signaled the thermostat.
  • Emergency heating duration: You should restrict the use of emergency heating only to the time it takes for a qualified HVAC expert to fix the heat pump. 
  • Emergency heating maintenance: Remember that your emergency heating system may not operate as intended. Make it a routine to examine your HVAC system, or have a professional do so. 


The heat pump is the primary heat source in many homes. However, emergency heat activation is necessary when the heat pump fails, is being repaired, or during maintenance. You have to switch on the emergency heating manually. 

However, because emergency heating uses a lot of energy, you should only use it briefly. The heat pump automatically activates its backup heat source when the temperature drops considerably. 

Frequently Asked Questions

  1. Why won’t my emergency heat turn off?

Your emergency heat won’t go off because of a malfunctioning thermostat. Failures might occur in the wiring, switches, or heating sensors. You’ll then need to replace the thermostat. 

  1. Can I use emergency heat for a week?

Your heating system may be overworked during an emergency, not to mention the energy costs involved. Therefore, while using emergency heat for a week is technically possible, I would strongly advise against it. 

  1. When should I activate emergency heat? 

Only during emergencies should you activate your emergency heating system. Emergencies include situations where the heat pump is broken or requires repairs or maintenance. 

As a result, using emergency heating is not necessary in icy conditions. Alternatively, use supplementary heating sources such as oil, boilers, and furnaces.


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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