Guide to Auxiliary Heat

  • December 10, 2021
  • By MR COOL

If you have a heat pump system warming your home, it likely has an auxiliary heat function. You may have noticed auxiliary, aux, or emergency options on your thermostat. So what is auxiliary heat? And, what does it mean when the auxiliary heat is on? Auxiliary heat is an important function of any heat pump-based heating system, and it can help keep your home at your desired comfortable temperature all year round. If you’ve ever wondered just what the auxiliary heat does or wanted to troubleshoot auxiliary heating issues, this is the guide for you.

What Is Auxiliary Heat?

So what is aux heat? Auxiliary (aux) heat is an additional heating system that supplements a heat pump-based system and provides extra heat to warm up a home a little faster than what the heat pump could provide by itself. There are a few situations when a heat pump may not be able to warm up a building quickly enough, and the auxiliary heat provides the extra boost to speed it up. These situations include when the temperature drops more than two degrees below the number set by the thermostat or if someone adjusts the thermostat by more than three degrees at once.

In a heat pump system, there are two main components — an outdoor unit and an indoor one. The two units work together to transfer heat back and forth to ensure a consistent temperature inside the home, according to the thermostat settings that the homeowner sets. The inside unit houses the auxiliary heat system and will kick on automatically when the temps are below freezing outside. When the outdoor temp dips below about 32 degrees, the outdoor unit will be very cold and will take a long time to heat the home on its own. In this case, the auxiliary heat can warm the home much faster. This is common in the early morning hours of cold winter days.

How Does Auxiliary Heat Work?

When we discuss how auxiliary heat works, it’s important to note that it is a part of a heat pump system and to understand how heat pumps themselves work first. Heat pumps work differently than furnaces because they don’t generate their own heat. Instead, they transfer heat from one area to another to keep a home comfortable. When heating the home, a heat pump is like the reverse of an air conditioner, pulling in warm air from outside. It can also act as an air conditioner in the warmer months, moving warm air from the inside of the house to the outside.

When the outdoor temps are between 35 and 40 degrees or colder, the heat pump may struggle to pull enough warmth from the air outside. This is where the heat pump auxiliary heat system comes in. The auxiliary system often uses electricity as its main source of power, but auxiliary systems can also use gas or oil. Electric auxiliary heating systems, which are the most common, have coils similar to what’s inside a toaster. These coils help heat the air more quickly. The auxiliary heater is a supplement to the heat pump, not a replacement, and helps the heat pump run more efficiently.

Why Is the Auxiliary Heat Coming On?

If you see your thermostat has the “aux” setting on, there’s no need for concern. This is a normal and necessary aspect of using a heat pump system to heat your home. The auxiliary heat comes on to give the heat pump a little boost of extra heat. If someone in the home turns up the thermostat more than three degrees, you may notice the auxiliary heat come on to help your house warm up a little faster.

The most common time for auxiliary heat to be used is in the morning hours of a colder day. Many people opt to turn their thermostats down a few degrees at night because it makes for a comfortable sleeping temperature and can save a little on energy costs while everyone sleeps. However, in the morning, the temperature should be boosted back up to normal room temperature for comfortable living during the day.

This boost in temperature may be too much for the heat pump to handle on its own, plus the early morning hours are often the coldest part of the day outdoors, so it may be even harder for the outdoor unit to provide enough heat. When the auxiliary heat kicks in at these times, you get a more comfortable home in a much shorter time.

Why Is Auxiliary Heat Important?

Auxiliary heat is an essential component of a heat pump system when the heat pump can’t warm the house on its own. An auxiliary heating unit wouldn’t be a very sustainable option by itself, but when the heat pump and auxiliary system work together, the homeowner gets the best benefits of both. Auxiliary heat helps keep the home comfortable no matter the time of day or weather conditions. The auxiliary heat unit can also function as an emergency heating system when needed.

Auxiliary Heat vs. Emergency Heat

Many people use the terms auxiliary heat and emergency heat interchangeably. Depending on the type of heat pump thermostat you have, you might notice one or the other or both on your thermostat. In many cases, the auxiliary heat and emergency heat may be housed in the same unit or share the same electric elements. But just what is the difference between these two heating functions, and why do we need both? How do you know when to use emergency heat? These heat settings can be confusing if you’re not sure what they mean.

Auxiliary heat kicks on to assist the heat pump when the outdoor temperatures drop or if you raise your thermostat by more than three degrees at a time. This is a necessary part of the whole heating system and should turn on automatically when the thermostat detects that it is needed.

On the other hand, emergency heat needs to be turned on manually and should only be used in emergencies, as the name implies. If your outdoor heat pump unit is not working and heat is necessary, you should turn on your emergency heat source. Turning on the emergency heat means that only the indoor unit will be running, and the system will not try to draw heat from the outdoor heat pump at all.

Using the emergency heat system will cost more money to run on its own, so you’ll only want to use it when it’s absolutely necessary. You shouldn’t need to turn on the emergency heat if the temperature drops — your auxiliary heat should automatically take care of this. Only turn on the emergency heat if the heat pump is not working. And any time you need to use the emergency heat, you should call in the professionals to service the outdoor unit. If you get the heat pump fixed promptly, you likely won’t even notice the spike in energy usage from running the emergency heating system.

When Is Auxiliary Heat Bad? When You Don’t Want Auxiliary Heat On

Is auxiliary heat bad? In short, no. As a necessary component of heat pump systems, auxiliary heat helps the whole system work efficiently and keep your home comfortable. So, when the auxiliary heat kicks in, it isn’t necessarily a bad thing.

However, the auxiliary heat is far less energy efficient than the outdoor heat pump unit, so you don’t want your home to rely on the auxiliary function too much. Generally, heat pump systems are installed in homes located in areas with mild climates. Because these heating systems rely on the auxiliary system when the temperatures dip down below freezing, they are not efficient choices for homes in much colder climates.

In areas where heat pump systems are more common, the auxiliary heat should only come on during cold nights and mornings or during an occasional snowstorm. If you find that the weather is warmer and the auxiliary heating is still on, or it seems to be on more than it should be, there may be a problem with the system. In temperatures above freezing, your outdoor heat pump unit should be able to handle warming the home and is far more energy-efficient. If your auxiliary heat is turning on when the temps are above freezing, you may need to contact a professional to check your heating system and make sure everything is alright.

Does Auxiliary Heat Cost More?

Auxiliary heat is meant to be a supplement to a heat pump system, not a heating system on its own. Heat pump systems are incredibly efficient and affordable for certain mild-climate areas of the country. But when the auxiliary heat is being used, it means lower energy efficiency and higher energy costs.

Using the auxiliary heat as an additional heat source to the outdoor heat pump unit provides a great balance between a system that keeps the home warm and comfortable year-round and one that runs efficiently with lower energy costs. But in general, the auxiliary heat will cost you more to run than the outdoor heat pump, so you want to be sure that the auxiliary heat source isn’t running too often.

One way to help keep costs down is to install a smart thermostat in your home. A smart thermostat can help control when the auxiliary heating system is used and ensure it only comes on when necessary. This makes maintaining your home climate easier than ever and can help keep energy costs down. You may also be able to integrate your smart thermostat with other smart home systems, making it even easier to control and keep an eye on the settings.

Aux Heat Will Not Turn On

Why isn’t my heat working? If the aux heat is on, but no heat is coming out of the vents, or the aux heat doesn’t come on at all, there are some steps you can take to troubleshoot the issue. One of the most common causes of a heat pump system or auxiliary heat not turning on is a blown fuse or tripped circuit breaker. Start by checking your fuse box or circuit breakers.

Try these additional steps when the heat won’t come on:

  • Double-check the thermostat, ensuring it’s set to “heat” and the desired temperature.
  • Check for a power switch near the heat pump system. Not every system has one, but if you do, make sure it’s turned on.
  • Check the heating elements in the auxiliary heater. There may be circuit breakers or fuses inside your system’s indoor cabinet that may need to be reset.
  • Make sure none of the air return registers are blocked, and check your filter for clogs. Ensure the whole system allows for clear airflow.

If you’ve tried all of these tips, and your auxiliary heat still isn’t turning on, it may be time to check in with an HVAC professional.

Aux Heat Will Not Turn Off

If you’ve got the opposite problem and the aux heat won’t turn off or keeps turning on, there may be an issue with the outdoor heat pump unit. The auxiliary heat should only turn on if the temperature outside has dropped below freezing or you’ve adjusted your thermostat by more than three degrees. However, if the temps outside are mild and warm, you haven’t adjusted the thermostat and the aux heat is still kicking on or won’t turn off, something is likely malfunctioning with the heat pump outside.

Running the aux heat function more than usual will run up your energy bills, so it’s an issue you’ll want to take care of as soon as you can. Unless you have HVAC repair experience, there isn’t much you can do to troubleshoot this issue. If the heat pump isn’t working and the aux heat function runs too much, you’ll want an experienced HVAC professional to look at the whole system.

Have More Questions About Heat Pumps and Auxiliary Heat?

Heat pump systems are efficient, affordable heating options for any homeowner who lives in a mild climate. If you are considering a heat pump system for your home, or have any additional questions about heat pumps and auxiliary heat, contact MRCOOL today. We are your HVAC experts, offering the most innovative solutions for central heating and air conditioning. With so many quality HVAC options to choose from, and our helpful, knowledgeable staff ready to help, you’re sure to find what you need. The experts at MRCOOL are happy to take your questions and help you choose the right system for your home.

33 thoughts on “Guide to Auxiliary Heat”

  1. What outside temp should trigger aux heat with the Mr Cool Universal? I know it’ll keep my house at 70 when is 20 outside (I disconnected the aux heat) but my tstat keeps calling for aux heat and I can only assume it’s a built in algorithm is doing it because the Mr Cool is taking too long to go from 69 to 70 when it’s 20 outside.

    1. While there are some thermostats which have a time sensitive auxiliary heat activation, most do not. Rather, they activate aux heat based on temperature: when the temp in the house falls 1F below setpoint, the condenser is activated- If the temperature falls an additional 1F the aux heat is also activated. When that occurs is the result of system capacity versus heat loss of the structure. If the heat loss is less than the capacity then it will never activate aux heat.

    2. I wish I knew how your universal was working so well. Mine won’t get the house above 65, even on a 35-45deg day. It blew hot when it first started to get colder, now its just blowing 66ish degree air. I’m going to have to buy an aux heater, despite the advertising that the universal was good to zero or below.

  2. So, do all Mr Cool heat pump systems have auxiliary heating elements? If so, why are their options to add them? Finally, it sounds like they don’t work down to 0 degrees F without auxiliary heat elements, correct?

    1. They do not. Auxiliary heat cannot be added to ductless mini-splits. Our single-zone DIY ductless mini-splits can heat down to 5° Fahrenheit. Auxiliary heat kits are available for our central heat pump systems.

  3. I installed an MDUO180224036 heat pump with the MDU118036E air handler a month ago, replacing an existing (over ten year old) heat pump system. It has worked perfectly from a heating perspective with only one exception…my electric bill ROSE by over $40 for this month compared to last year’s electric consumption. This month averaged about 4 degrees warmer than the comparison month last year.
    I was particularly cautious to ensure there were no air leaks in the ductwork and added insulation to the ductwork. I re-used the installed thermostat since it was already running a heat pump based heating system
    I did not install an auxiliary heat kit after being convinced by multiple postings that this model did not require one based on my climate zone here in Southwestern Virginia. After reading your article, I am questioning the wisdom of that. Any guidance would be greatly appreciated.

    1. You may want to give us a call at 270-366-0457 so we can help diagnose any issues with your unit, if you have any. The Universal can retain a 100% heating capacity in temperatures as low as -5° Fahrenheit, so in your area, you may not need an auxiliary heat kit. One can always be added, though, if you find that it is necessary.

  4. In regards to the universal 5 ton. My thermostat is capable of calling for aux and heat pump at the same time. Will both activate in that case or is there something in the air handler that will only allow one to be on at a time?

  5. I have the Universal DIY 3 ton indoor and outdoor unit installed. I am using a Sensi Touch thermostat.

    It appears that every time that the thermostat calls for heat the back up/aux heat comes on and stays on, even if I move the thermostat up by only 1 degree.

    I have called your tech support number and left numerous messages and never got a return phone call.

    My thought is that the thermostat is not wired correctly.

    How can this be resolved?

    1. We apologize greatly for your experience. We are currently experiencing a high volume of calls, and we’re working on getting back to everyone as quickly as we possibly can. If you’d like, you can also send us an email via our contact page.

  6. Getting ready to purchase the MDUO18048060 with MDUI18048060. I will also be adding the 15KW heat strip MHK15U. Was wondering if you can wire the heat strip to be dual stage? only use 7.5KW in stage 1 then use all 15 in stage 2. Have seen multiple other systems do this but cant find documentation on any website for MR. Cool.

  7. i just installed a 4 ton 2 stage geothermal unit. question is where does the back up connect to the dallas control board. the thermostat wiring is connected to the W connector on the dallas control board but where does the control wiring from the back up heat connect to control board.


  8. I just installed a Universal 2-3 ton system with a 5 kW auxiliary heat kit and used a Honeywell T5 (not the “smart” version) thermostat. The thermostat has a W/W1 terminal as well as an aux heat terminal. However, the system only has the W1 terminal. I hooked the W1 wire from the system to the W/W1 terminal on the thermostat. My question is: Is this correct? On the thermostat setup it asks how many stages my heat pump has and whether it has a backup heat stage. Do I enter 1 or 2 for heat stage? Do do I select the backup heat even though there is not a wire connected to the aux heat terminal? Does the Mr Cool unit itself switch on the aux heat when needed? Also, it asks how many cool/compressor stages my unit has. Is it 1 or 2? Any help with the setup settings for this specific thermostat would be appreciated. Here is a link to the pdf manual: . The settings I refer to are on page 9 numbers 220 and 221.

  9. I purchased and installed a 4-5 ton Universal ducted system in August 2022. All has worked flawlessly when cooling & heating until this week (Dec. 22, 2022) when the outside temperature went to -3 and wind chill of-24. The unit is not heating and maintaining heat at 55° while we are wintering in Fl. I do not have a Heat Strip in the unit, because when I spoke to a MrCool Rep., he stated there was no reason to have one – my house temp dropped to 42° and I am using space heaters to supplement the heat currently.. The outside unit coil is iced over and does not defrost, so obviously won’t heat. If I had some of the info from posts here, I would think I should have installed an auxiliary heat strip in the air handler. Am I assuming correctly???

  10. After reading many of these posts, I’m as confused as ever. According to the manual, the 20k strip heater is EMERGENCY. I just want to know what brings it on, when, and why,

    1. “Emergency” is just another term that people use to describe auxiliary heat strips. If the system sees that it is not meeting the set temperature for heat, and is unable to because of the limitations of the system, it will engage these heat strips to meet the temperature the thermostat is calling for.

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