If you need it, we can finance it. Check out our financing options.

Learn More

What Is the Difference Between Auxiliary Heat & Emergency Heat?

If you’ve taken even a slightly longer glance at your thermostat, you may have noticed it has a couple of extra heat settings: one called “auxiliary heat” and one called “emergency heat.” While most people know these two settings exist, few people actually know the difference between them. And that’s not necessarily a good thing. Both of these heat modes are designed to work in extremely cold temperatures and choosing the wrong one could have potentially bad consequences for your heater and for your home in general. So in this blog, the experts at Parks Heating, Cooling & Plumbing take a look at both of them and explain the important differences between the two.

What Is Emergency Heat?

Emergency heat also referred to as “EM heat’ is a setting in your heating system that is responsible for controlling the backup heating system of your home. Most heating systems will typically have a heat pump as their primary setting. A secondary heating system is usually activated when supplemental heating is needed, that being gas, oil, or electric heating.

In areas with colder climates, a secondary heating source is necessary for ensuring that you are receiving appropriate and efficient heating. That is because, during extreme temperatures, your heat pump is unable to receive sufficient warmth from the outdoor air, to adequately heat your home at your desired temperature.

This is what EM heat essentially is; supplemental heating that is activated when temperatures drop. To put it simply, a heat pump is “first stage” heating, and emergency heat is a “second stage” or “back up” heating source. When emergency heating is activated will depend on your thermostat system, as each system will have a different way of determining when emergency heat is needed to assist first stage heating.

Auxiliary Heat vs. Emergency Heat

Heat pump systems have two units – one outside the home and one inside the home. The unit outside your home is a heat pump and the unit inside the home is the auxiliary heating system. During freezing weather, the outdoor heat pump will be too cold to quickly heat your home, which is when the auxiliary heating system would kick on. Auxiliary heating turns on automatically to help heat your home more quickly if the temperature drops suddenly. The emergency heat setting has to be manually switched on and should only be used in temperatures below 30 degrees. When turned on, the heat pump shuts off entirely, allowing you to receive heat without damaging your outdoor heat pump system. This should only be run when absolutely necessary to avoid damage to your central heating system.

In order to further understand what these modes do, it’s important to know how a central heater works. Essentially, your central heater and air conditioner are the same systems, only the direction of refrigerant flow is reversed to heat your home as opposed to cooling it. This way, your compressor creates heat by pressurizing your refrigerant before sending it into your home, and then your expansion chamber cools it again when it’s sent back outside.

Can My Heater Freeze?

However, during cold weather, this cycle can actually build frost on your outdoor coil. The refrigerant that reaches your coil in this instance is extremely cold, which allows it to still find a way to absorb heat even when the temperature drops to near-freezing temperatures. When you have temperatures that cold, even the slightest amounts of water vapor in the air can condense and freeze, resulting in frost on your outdoor coil that can result in your entire heater freezing over. Yes, as ridiculous as it may sound, your heater can literally freeze, usually at temperatures of around 40 degrees Fahrenheit or below.

Thus, when your heater has been on for a few hours, your system will have to defrost itself in order to continue functioning. That means it has to reverse the flow of refrigerant again in order to allow the hot fluid to flow through your coil and melt off any condensed or frozen snow or ice. But what happens when you still need heat? Simple: during this period, your system turns on a series of electrical radiant heat strips that can still provide you with heat, but while consuming more energy.

Both “auxiliary” and “emergency” heat have to do with the operation of these electrical strips. While they do allow you to continue to get the heat you need in your home to stay both comfortable and healthy, they do cost quite a bit more to run for longer periods of time, which is why they’re a less desirable option.

What is Auxiliary Heat on a Thermostat?

When the temperature inside your home drops about two or three degrees below the temperature the thermostat is set to, auxiliary heating kicks on to help warm your home more quickly. Most of the time, you won’t even notice the switch has happened unless you look at your thermostat and see that the “AUX” light is turned on.

How Auxiliary Heat Works

When your system reverses itself to defrost the coils, it automatically turns on the radiation strips in order to keep providing your home with warm air. This is known as running on “auxiliary heat.” Generally, this can’t be controlled manually but is instead switched over to automatically when your system either senses that it has been running for too long, or too much ice has built upon the outdoor coil and it needs to begin a defrost cycle.

How Emergency Heat Works

Generally, at temperatures below 40 degrees, your system needs to run defrost cycles every few hours to operate. However, at temperatures in the 20s and 30s or below, your heat pump may not be able to produce heat at all because there simply won’t be enough in the ambient air to extract. At this point, refrigerant gets so cold outside that compressing it becomes nearly impossible, and thus it can’t absorb what little heat is left. Continuing to run a heater in these conditions could do tremendous damage to it, so you need to shut it off.

This is what the “emergency” heat setting is for. When turned on, your compressor and heat pump shut off entirely and your radiation heat strips in your system turn on. This allows you to continue to receive heat without the risk of damaging your outdoor heat pump system. However, bear in mind that you will be using a considerable amount of extra energy in doing so, thus the name “emergency” being attached to it. Only run your system in this mode when you truly need it to avoid damaging your central system.

When To Use Emergency Heat?

Emergency heat should only be used in emergency situations. When auxiliary heat is unable to sufficiently heat your home due to colder temperatures, your emergency heating system will kick in, to ensure that you and your family are receiving your desired home temperature. If you feel that your home is not being properly heated, you can switch to emergency heat. In cases where your heat pump has frozen, it is best to contact an HVAC professional to get the situation inspected and properly taken care of. In the meantime, just make sure to switch to EM heating.

Suggested Reading:

Call Parks Heating, Cooling & Plumbing today at (980) 357-5427 or contact us online to schedule a heating service with our team. We’re available 24/7/365!

Skip to content