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What Is Emergency Heat and When Do You Need It?

If you know only a little bit about your thermostat, you may not know what the emergency heat function means, and why you should pay attention to it. We take a look.

What Is Emergency Heat and When Do You Need It?

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Roger Bakies


May 17th, 2022

That setting on your thermostat that says emergency heat… Ever wondered what it signifies? And is there a difference between emergency heat and auxiliary heat?

Part of our job as an HVAC company is to demystify terminology that we all use all of the time. Sometimes we take it for granted that customers understand us, and that they understand their equipment. That’s not always the case.

At Fire & Ice, we talk to thousands of homeowners, and we often get questions about their thermostat’s functions beyond the on and off buttons.

If you’d like to know a bit more about the emergency function, you’re reading the right article. We can help you understand the difference between emergency and auxiliary functions (there is a key difference)  with the hope that you can get through the winter heating season with peace of mind.

What Is Emergency Heat? 

Emergency heat is sometimes referred to as backup heat. 

When it’s activated, it usually means something’s wrong with your heat pump. Either it’s not working at all, or it has generated heat until the coil (located outside) might become encased in frost, which can cause damage. That triggers the heat pump to go into a “defrost” mode. During the defrost cycle, your heat pump will switch to cooling mode to warm up the coil and melt the ice. While in defrost mode, no heat will be transferred inside.

And if no heat is going inside, your house has no central heat. Until…

That’s when the emergency function kicks on. 

Heat strips located in the air handler or electric furnace turn on and glow red hot. That heat is then circulated by the blower through the ductwork. When one heating implement shuts down (the heat pump), another takes over.

The same process holds true if something catastrophic happens to your heat pump. If it shuts down for any reason, you can set your thermostat (some do it automatically) to emergency. That stops the signal going to the heat pump calling for heat, and turns on the heat strips. They will keep your house warm until you can get an HVAC contractor out to fix the problem.

How Does a Heat Pump Work in the Winter? 

If you have a heat pump combined with either an air handler or a furnace, have you ever had the impulse to shut down the heat pump when the temperature drops below freezing? Perhaps you know that a heat pump’s effectiveness starts to drop as the temperatures do likewise. If you own a heat pump, you’ll likely see this when the outdoor temperatures dive below 30 degrees or so. It’s a sign that something in your system can’t perform its job adequately.

The unit uses outdoor air and warms it before it transfers the warmth inside. If the temperature is too low, there is less warmth to extract. It still runs, but that air that goes inside will get cooler and cooler.

If it’s below freezing and the heat pump can only get your home to 60, that’s pretty chilly for most people. Heat pumps are efficient, but they cannot handle freezing weather well. 

Since we live in Columbus, where winter temperatures often dip below 30 degrees F, homeowners need a backup plan for when their heat pump can’t keep homes comfortable.

Depending on the model and features, heat pumps will lose their heating effectiveness somewhere between 35 and 0 degrees.

The balance point is where the heat pump will no longer provide enough warmth to keep your home comfortable. When the heat pump can’t squeeze any more warmth from the outside air, you need backup. That’s when auxiliary heat kicks in.

If there’s no backup plan in place, you will suffer from the cold, and you also need to worry about your water pipes freezing.

Auxiliary heat is like emergency heat in that it’s using heat strips (also known as coils, since they’re shaped like a tightly-wound coil). The difference between auxiliary is that your system is using heat strips in addition to your heat pump. Emergency is using only heat strips.

What Are Heat Strips?

Heat strips are wire elements in your electric furnace or air handler that are heated by electricity, which in turn heat the air that flows over them. Heat strips are similar to the inner workings of a toaster. They are pieces of conducting metal that get very hot. The official terminology is “electric resistance heat.”

They usually come in 5 kW increments. Common packaged sizes are 15 and 20 kW.

Here’s a list of heat strip sizes by kiloWatt and BTUs (British Thermal Units):

Heat Strip Sizes (kW): BTUs

  • 3 kW: 10,000 – 15,000

  • 5 kW: 15,000 – 20,000

  • 7.5 kW: 20,000 – 30,000

  • 10 kW: 30,000 – 40,000

  • 15 kW: 50,000 – 55,000

  • 20 kW: 65,000 – 70,000

  • 25 kW: 85,000 – 90,000

The strips won’t come on all at once; that electrical draw might dim your lights. The system will bring them on one at a time.

Are Heat Strips as Efficient as a Heat Pump?

Electric heat is 100% efficient. You put a dollar in, you get a dollar worth of heat out. With a heat pump, when you put a dollar in, you get three, maybe three and a half dollars out of it. So when the heat pump can’t create enough heat anymore, it will still keep working, even if the outside temperature gets down to zero.

You’re still getting BTUs out of it, and it’s probably cheaper than the electric resistance heat. 

Though heat strips are technically efficient, they consume a lot of electricity. A 1500 kW electric heat package for your air handler is about the same as 15 electric heaters. That would be like an electric heater in every room. The electricity bill will soar.

Some installers or other HVAC contractors might tell you that when it gets cold, you should turn the heat pump off. That’s false. In fact, the supplemental heat provided by heat strips isn’t sized to heat your entire home. You still need the heat pump to provide some heat.

The nice thing is if something is wrong with the heat pump, you can turn it off and still get heat. And the great thing about heat strips is that they get red hot. When the blower is circulating that heat, you’ll know instantly.

Gas Furnaces as Emergency Heat

You may be operating an HVAC system that we call “dual fuel.” It means that you use an electric heat pump with a gas (or propane or oil) furnace. You are using two heating sources that have two different fuel sources.

In a traditional gas system, you’re using only the furnace for heating, and you’re using only the AC for cooling. Dual fuel means you’re using your heat pump for heat first, with your gas furnace as back-up. You’re going to let the heat pump do its heating until it gets too cold, then the gas furnace will turn on as a secondary heat source.

Whether this is a good idea largely comes down to what the going rate for electricity and natural gas are. If gas is cheap, then relying on your gas furnace will make the most financial sense, even if it’s more inefficient. (A standard-efficiency gas furnace must operate at 80% Annual Fuel Utilization Efficiency or AFUE, or higher. That means if you put a dollar’s worth of energy into it, it will give you 80 cents of heat, and the other 20 cents is waste.)

It also depends upon the efficiency of your gas furnace. A unit in need of repair or that’s 20 years old might be operating at a rate of 60% AFUE or lower. It can heat your home, but it won’t be efficient.

On the other hand, some gas furnaces have an AFUE rate of up to 98%.

What's an AFUE Rating? - Buying a New Furnace

How Does Your System Know When to Switch to Emergency Heat?

The two most common emergency heat uses are when the heat pump has a mechanical failure or freezes up, and when the outside weather is too cold for the heat pump to adequately warm your home.

Your heat pump will still run even if it can’t keep your home warm. They used to have an outdoor temperature gauge, an outdoor thermostat built into them. After you configured that, it would determine when the auxiliary heat source would come on. 

One problem that could arise in that situation is that the outdoor sensor could be fooled if the sun were on it. It would heat up even though the air temperature was low.

Now, it’s all run by the indoor thermostat. The thermostat signals to the heat pump and electric furnace to switch over when it senses the house temperature has dropped to a certain degree.

When you manually switch to emergency heat, you’ll probably get some sort of indicator light, which will stay on until you stop using it. On a call for heat, no signal will be sent to the heat pump.

The supplemental heat is also referred to as second-stage heating. The first stage is the heat pump working solo. The second stage is when the heat strips kick on.

Different systems and thermostats have different ways of determining when the second-stage heat comes on to assist the heat pump, but it is always automatic. The two stages will work together in the colder months, and it is not necessary to switch your thermostat to emergency heat.

Obviously, if the emergency light comes on by itself and stays on, your heat pump has suffered a serious malfunction, and you should contact an HVAC professional ASAP.

Staying Warm in Columbus, Ohio

Cold weather is a fact of life in Columbus and Central Ohio. 

If your HVAC system has been maintained as it should, you shouldn’t have any problem with your home. (Pity your heat pump can’t fix the snow-covered streets.)

You can do small things to ensure everything is running properly, including:

  • Change your furnace filter when it’s dirty

  • Check your furnace to see if any water is leaking from it

  • Schedule routine maintenance

  • Call an HVAC expert if you start hearing new noises

Peace of mind can be found on our maintenance plan page, where we offer three levels of service. Enter your zip code in the map below to see if you’re in our service area. We look forward to hearing from you.

Read more:

HVAC Buyer’s Guide

How to Stay Warm When Your Furnace Won’t Keep Up With Extreme Cold

What’s the Difference Between a Furnace and a Heat Pump?

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