Auxiliary Heat vs. Emergency Heat: What Each One Costs You

If your thermostat is showing AUX HEAT, your heat pump asked for backup and got it automatically. You did not do anything wrong. If it is showing EM HEAT, someone switched the system into emergency mode by hand, and the heat pump is now shut off entirely.

Both settings run the same electric heat strips, and both cost roughly three times what your heat pump costs to run on its own.

Here is the part most articles skip: in North Alabama, aux heat coming on is a weaker excuse than it is up north. Our average January low in Cullman is 34°F. A properly working heat pump should handle almost all of that on its own. If yours is leaning on the backup strips regularly, that is usually worth a look rather than something to shrug at.

The Short Answer

The single most useful thing to understand: aux heat is your system helping itself. Emergency heat is your system giving up. They use the same hardware, so they cost about the same to run, but they mean completely different things about the health of your system.

How a Heat Pump Actually Heats Your House

A heat pump does not create heat. It moves it. Even on a cold morning there is heat energy in the outside air, and the refrigerant cycle pulls that energy inside. This is why heat pumps are so efficient — moving heat takes far less electricity than making it.

The catch is that the colder it gets outside, the less heat there is to move, and the harder the system works to move it. At some outdoor temperature, your heat pump can no longer keep up with how fast your house is losing heat. That temperature is called the balance point, and for most systems in our area it lands somewhere between 30°F and 40°F depending on how well the house is insulated.

Below the balance point, the thermostat turns on electric resistance heat strips in the air handler to make up the difference. Those strips are essentially large toasters. They work every time, they never struggle in the cold, and they are wildly expensive compared to the heat pump.

Aux heat also kicks on during defrost cycles. When frost builds on the outdoor coil, the system briefly reverses to melt it off, which means it is temporarily cooling your house — so the strips fire to cover the gap. That is normal and lasts a few minutes.

What Auxiliary Heat Costs You Per Hour

This is the number nobody publishes, so here it is.

Cullman Electric Cooperative's residential rate runs roughly 13.3 cents per kilowatt-hour. Most homes in our area have either 10 kW or 15 kW of heat strips. Running the math:

So the strips themselves add roughly $1.33 to $2.00 for every hour they run.

Put that on a calendar. If your 10 kW strips run four hours a day for a month — which is very achievable if the balance point is set wrong or the system is struggling — that is about $160 added to a single power bill. Run them most of the day because someone left the thermostat on EM HEAT, and you are looking at considerably worse.

These are estimates based on the co-op's published average rate and typical residential strip sizes. Your exact numbers depend on your rate schedule, your strip capacity, and how long the system actually calls for them. But the ratio holds everywhere: backup heat costs about three to five times what the heat pump costs, and that ratio is why a January power bill can double without anything visibly breaking.

Why Aux Heat Running Is a Bigger Deal in Cullman Than in Cleveland

Almost every article you will find on this topic is written for the national average, and the national average includes a lot of places where winter means single digits. In Minneapolis, aux heat running daily through January is just how heat pumps work.

Cullman is a different climate. Our January averages a 34°F low and a 51°F high. That means the coldest part of a typical winter day sits right around most systems' balance point, and the rest of the day sits comfortably above it. A correctly sized, correctly charged heat pump in a reasonably tight house should carry the load by itself for the large majority of winter hours here.

That changes what the AUX indicator means for you. Where a homeowner in Ohio can reasonably ignore it, a homeowner in Cullman County has a genuine diagnostic signal.

When Aux Heat Is Perfectly Normal Here

• During a real cold snap, when we drop into the low 20s or teens
• For a few minutes during a defrost cycle, which you may notice as a brief whoosh and a puff of steam off the outdoor unit
• After you have bumped the thermostat up several degrees at once — the strips help the system catch up faster
• The first cold morning of the season, when the house has cooled off overnight and has a lot of ground to make up

When It Means Something Is Wrong

• AUX is on and it is above 40°F outside
• AUX runs for hours at a stretch, or shows up every single day
• Your power bill jumped sharply and nothing else in the house changed
• The vents are blowing air that feels lukewarm even with AUX showing
• The outdoor unit runs constantly and never seems to satisfy the thermostat
• You are seeing defrost cycles that feel frequent or unusually long

Any of these is worth a phone call rather than a wait-and-see. The cost of finding out is one diagnostic visit. The cost of not finding out is an entire winter of strip heat.

Four Things That Make Aux Heat Run When It Shouldn't

1. Low refrigerant charge. This is the most common one we find. A slow leak reduces the system's capacity, so it cannot keep up in weather it used to handle fine, and the strips step in to cover the shortfall. The house stays warm, the bill goes up, and nothing looks broken. This is exactly why the problem so often goes unnoticed until the power bill arrives.

2. A thermostat set up wrong. Heat pump thermostats have a balance point or aux lockout setting that controls when the strips are allowed to run. Set too high — or left at a default meant for a colder climate — and the strips fire at temperatures where the heat pump was doing fine on its own. We see this constantly on thermostats that were swapped out by someone who did not configure the heat pump settings.

3. A dirty coil or a restricted filter. Anything that chokes airflow reduces how much heat the system can actually deliver into the house. The thermostat sees the room falling behind and calls for backup. A filter that has not been changed since summer is a genuinely common cause of a high December power bill.

4. Defrost controls that are stuck or misbehaving. If the defrost board is cycling too often or running too long, you are paying for strip heat every time — and each cycle is also briefly cooling the house.

There is a fifth possibility worth naming honestly: the system may simply be undersized or at the end of its life. If your heat pump is fifteen-plus years old and leans on aux heat every cold morning, no repair is going to change the fundamentals. In that case we will tell you so, walk you through replacing the system, and point you at financing if it helps. We are not going to sell you a repair that does not fix anything.

When You Should Actually Use the EM HEAT Setting

Rarely. That is the honest answer.

Emergency heat is not a "it is really cold out" setting. It shuts the heat pump off entirely and runs your house on the most expensive heat source you own. Switching to EM HEAT because it is 25°F outside makes your bill worse, not your house warmer.

Use it when:

• The outdoor unit is visibly broken — a fan that will not turn, a coil buried in a solid block of ice, loud grinding or screeching
• The heat pump is running but producing no heat at all, and you need to keep the house warm until a technician gets there
• A technician told you to while you wait on a part

If you do switch to EM HEAT, treat it as a countdown, not a solution. Get the system looked at, and switch back as soon as it is repaired. And if your system has been quietly sitting on EM HEAT since someone flipped it last winter, that alone can explain a power bill that has never made sense.

What You Can Check Yourself Before Calling Anyone

Some of this you can genuinely sort out in ten minutes, and we would rather you did.

1. Confirm which setting you are actually on. Look at whether the thermostat says AUX (automatic, heat pump still running) or EM HEAT (manual, heat pump off). If it says EM HEAT and nothing is broken, switch it back to normal heat.
2. Pull the filter and look at it. If light does not pass through it easily, replace it. This is free, takes two minutes, and solves a surprising share of the calls we get.
3. Walk out and look at the outdoor unit. The fan should be turning. The coil should not be packed with leaves, grass clippings, or a solid shell of ice. Clear any debris from around it.
4. Check the outside temperature against the AUX indicator. If it is comfortably above 40°F and AUX is lit, note that — it is the most useful single detail you can hand a technician.
5. Stop raising the thermostat in big jumps. Going up five degrees at once guarantees the strips fire. One or two degrees at a time lets the heat pump do the work.

What is not a DIY job: checking refrigerant charge, configuring balance point and lockout settings, testing the defrost board, or anything involving the strips themselves, which run on high-voltage circuits. Those need gauges, meters, and a license.

The Cheapest Fix Is the One You Do in October

Most of the aux heat problems we find in January were visible in October. A charge that has drifted low, a coil that needs cleaning, a defrost board starting to act up — all of it shows up on a fall heating tune-up before the cold weather arrives to make it expensive.

If you are reading this in the fall and your thermostat has not shown AUX yet this year, that is the right time to get ahead of it. If you are reading this in January with a power bill in your other hand, call us anyway. We will figure out what your system is actually doing.

Veterans Cooling and Heating LLC — veteran-owned, licensed AL#24210, serving Cullman County and the surrounding communities. Call (256) 915-2826, Monday through Friday, 7:00 AM to 6:00 PM.