Everything You Need To Know About Aux Heat On Your Thermostat

You might have looked at your thermostat on a cold New Jersey morning and suddenly noticed ‘aux heat’ or ‘heat on auxiliary’ flashing back at you. 

And if you’re like most homeowners, your first reaction would probably be: 

Is something wrong with my heat pump?

Why is this turning on?

Is this going to cost me a lot of money?

But you aren’t the only one with these questions. 

Aux heat confuses almost everyone the first time they see it. 

However, the truth is, aux heat is a totally normal and essential part of how your heat pump works. At least most of the time.

In this blog, I will walk you through what does aux heat mean exactly in homes. You’ll know: 

• What is auxiliary heat
• Why it turns on
• When it’s normal and when it’s a problem
• How it affects your energy bill
• How to reduce your aux heat use
• When you should call in a professional for home HVAC maintenance

So, let’s get started. 

Key takeaways  

• Aux heat is your heat pump’s backup heater.
• It turns on automatically when extra heat is needed.
• Occasional aux heat is normal in cold weather.
• Constant aux heat or use in mild weather signals a problem.
• Emergency heat is manual and only for breakdowns.
• Aux heat uses more energy and costs more in utility than your heat pump.
• Common causes of excess aux heat use are dirty filters, airflow issues, duct leaks, or a weak heat pump.  
• Call a professional if aux heat runs all the time or your home doesn’t heat properly.  

What does aux heat mean on a thermostat?

Aux heat stands for auxiliary heat and it is your heat pump’s backup heating system.

Think of it like this: Yur heat pump is your main heater. But Aux heat is a support heater that kicks in only when your heat pump needs extra support.

It will turn on automatically when:

• It’s extremely cold outside
• Your heat pump needs help catching up
• Your system is in defrost mode
• Your thermostat senses the heat pump can’t reach the set temperature fast enough

Aux heat usually comes from electric resistance heat strips, which are like giant versions of toaster coils. Also, it can come from a backup gas furnace if you have a dual-fuel system.

And here’s the important part. 

If you’ve been told aux heat is bad, that’s not true as this mode is just a normal part of your heating system.

It only becomes a problem if it’s turning on when it shouldn’t or running constantly.

We’ll talk about it later in the blog.

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How does aux heat work?

First starters, here’s your heat pump normally works:

1. It absorbs heat from the outside air
2. It compresses that heat
3. It pumps the warm air into your home

But when it’s really cold outside or your indoor temperature drops too fast, the heat pump can’t keep up.

So your thermostat activates electric heat strips or backup furnace if you’ve got a dual-fuel HVAC system.  

This is aux heat for you. It is more powerful, but also more expensive to run.

Why do homes in New Jersey see aux mode more often? 

New Jersey winters are cold, windy, and humid. Here, heat pumps work best when the temperature outside is above 35-40 degrees fahrenheit. 

But when temps drop lower, the outdoor coils get too cold, the heat pump loses efficiency, and its struggles to pull heat from outside air

This is when your thermostat turns aux mode on. 

So your thermostat is actually doing exactly what it’s supposed to do. And if you see aux heat on for short periods, especially when the temperature is too low, that’s totally normal.

Want to avoid unnecessary aux mode usage?

Read my blog on winter HVAC maintenance tips where I have shared practical ways to keep your heat pump working normally and efficiently. 

Why is auxiliary heat on? Here are all the possible scenarios 

Aux heat doesn’t turn on randomly, and it doesn’t mean your system is failing. 

You see, your thermostat is constantly watching how fast your home is heating, how cold it is outside, and whether your heat pump can keep up. 

And when it decides the heat pump needs help, it automatically turns on aux heat. 

Here are some of the most common reasons why this happens in New Jersey homes: 

1. It’s too cold outside

Heat pumps work by pulling heat from the outdoor air. But the colder it gets, the harder your heat pump has to work. 

Once temperatures drop into the mid-30s and below, the system often needs backup to maintain comfort.

In New Jersey, winter temperatures frequently fall into this range, especially overnight and during cold snaps. 

And when this happens, aux heat turns on for short periods to help your heat pump keep your home warm. This is completely normal and expected during colder NJ weather.

Here’s what you can expect: 

• Temperature below 35-40 degrees fahrenheit: Aux heat might run occasionally
• Temperature below 30 degrees fahrenheit: Aux heat runs more often
• Temperature below 25 degrees fahrenheit: Aux heat runs frequently

2. You raised your thermostat too quickly

Your thermostat is designed to keep your home comfortable. So, if you crank the temperature up several degrees at once, the system assumes you want fast heat. 

Now, instead of waiting for the heat pump to slowly catch up, the thermostat brings in aux heat to speed things along.

For instance, if you crank the thermostat from 65 to 74 degrees fahrenheit, it tells the system that you need a lot of heat right now. 

And most thermostats automatically turn on aux when the indoor temperature is more than 2–3 degrees below the set point. This isn’t a malfunction, it’s just how the system is programmed to work.

3. Heat pump is running a defrost cycle

This one confuses a lot of homeowners. 

In cold and damp winter weather, frost can build up on the outdoor unit. And when that happens, the heat pump briefly switches into defrost mode to melt the ice and protect itself.

During this short cycle, the heat pump temporarily stops heating your home and focuses on warming the outdoor coils instead. 

But to prevent cold air from blowing inside, aux heat turns on automatically and keeps your home comfortable. 

You might not even notice it happening. However, this process is normal, automatic, and essential for proper heat pump operation.

4. Heat pump needs help with heating demand

Sometimes aux heat turns on not because it’s freezing outside, but because something is limiting your heat pump’s performance. 

For instance, if the HVAC system can’t produce heat fast enough, the thermostat brings in backup heat to make up the difference.

Common causes of this include restricted airflow, dirty air filters, leaky ductwork, low refrigerant, or simply an older or undersized system. 

And when it happens, aux heat steps in to help maintain your set temperature and keep your home comfortable.

5. Thermostat sensors detected slow heating

Thermostats rely on sensors. And sensors aren’t always perfect as they can give incorrect temperature readings. 

So, if your thermostat is installed in a poor location, it might think your home is colder than it actually is. On top of that, drafts, sunlight, nearby vents, or exterior walls can all affect its readings.

Now, when the thermostat senses that the temperature is rising too slowly, it might activate aux heat even when it’s not really needed. 

In short, here’s why auxiliary is heat on: 

Is aux heat bad?

Short answer: No, aux heat is not bad. 

And seeing it on your thermostat doesn’t mean your system is broken, failing, or doing something wrong. 

Aux heat is a built-in feature designed to keep your home comfortable when your heat pump needs a little help. It is normal, automatic, and expected during cold weather, big temperature changes, or defrost cycles.

That said, aux heat is more expensive to run. It consumes much more electricity than your heat pump. 

So, when aux heat runs occasionally, it’s no big deal. But if it’s running constantly or for long periods, that’s when your energy bills start to climb. 

Aux heat vs emergency heat: What’s the difference?

AUX heat and emergency heat are not the same thing, even though they both use backup heat.

AUX heat turns on automatically when your heat pump needs help. The heat pump is still running, and the system is simply adding extra heat to keep your home comfortable. 

This is normal, expected, and something most NJ homeowners will see during winter.

Emergency heat, on the other hand, is something you turn on manually. 

When you use emergency heat, the heat pump shuts off completely and your system runs only on backup heat. 

This uses a lot of energy and should only be used if your heat pump is broken, frozen solid, or not working at all. 

Here are all the differences between aux heat and emergency heat: 

When does aux heat mean something is wrong?

Aux heat is helpful, but only when it runs briefly and at the right times. It becomes a warning sign when it seems to be running all the time or in mild weather.

For instance, it might be a problem if:

• Aux heat runs for hours at a time
• Aux stays on when it’s above 40-45 degrees farenhight outside
• Aux turns on every time your system heats
• Aux runs even when your home already feels warm
• Aux activates during mild weather
• Your energy bills suddenly jump
• Your home takes forever to warm up
• Your thermostat struggles to reach the set temperature

If you’re noticing any of these, your heat pump is likely underperforming and relying too heavily on backup heat. 

 Here are all the signs of aux heat indicating something is wrong: 

Bonus: Here are some common reasons why aux heat is running too much

Schedule Your Free Estimate Today

Contact us to improve your indoor air
quality with our expert cleaning services in NJ.

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Final words 

By now, I am sure what does aux heat mean exactly, how it works, and when it’s normal versus when it could signal a problem.

Simply put, occasional aux heat is completely normal and constant aux heat is a warning sign. Also, emergency heat is only for system breakdowns.

Got more questions or want to get your HVAC system cleaned for efficient winter performance?

You can get in touch with us!

At Clean Air Technologies, we’ve been helping New Jersey homes stay warm, comfortable, and efficient for 30 years now. And we’ve got all your HVAC maintenance needs covered on a budget. 

FAQs about aux heat on thermostat 

Is aux heat supposed to come on?

Yes, absolutely. 

Aux heat is designed to turn on automatically when your heat pump needs extra help. This often happens when outdoor temperatures drop below about 35-40 degrees fahrenheit, during defrost cycles, or when your home needs heat quickly. 

Why does aux heat keep turning on?

If aux heat turns on frequently, it usually means your heat pump is struggling to keep up. 

This could be due to cold outdoor temperatures, a big thermostat adjustment, or performance issues like dirty filters, airflow problems, or duct leaks.

To fix this, you can do simple things like cleaning dirty air ducts, or call in an HVAC professional to fix leaks in the ductwork. 

Does aux heat use more electricity? 

Yes, aux heat costs significantly more to run than your heat pump alone. It uses electric heat strips that consume much more electricity. 

How much electricity does aux heat use? 

Heat pumps are very efficient, but aux heat is not. 

A typical heat pump might use around 1-3 kilowatts per hour, while AUX heat strips can use 5-15 kilowatts per hour. 

Is aux heat the same as emergency heat?

No, they are very different. 

Aux heat turns on automatically and works alongside your heat pump. 

Emergency heat is something you turn on manually and shuts the heat pump off completely. 

Can aux heat damage my system?

No, aux heat will not damage your heat pump or HVAC system. It’s designed to work safely with your equipment. 

However, if aux heat runs too often, it can put extra strain on electrical components and lead to higher energy costs over time.