Emergency heat is only the name of a setting on your thermostat. It isn’t a service, and switching it on doesn’t call anyone. Aux heat is the backup that steps in to help your heat pump on its own. Emergency heat is a setting that replaces the heat pump, and it’s meant for a heat pump that has broken down. So if you’re wondering when to use emergency heat on heat pump systems, the short answer is simple. Leave the thermostat on Heat and let aux do its job.
Three worries come with that light. It’s one you’ve never seen before. Your bill could climb if the wrong setting runs for weeks. And the house could stay cold if the heat pump has failed. Each worry has a different answer, and none of them calls for guesswork at the thermostat.
What Is the Difference Between Aux Heat and Emergency Heat?
Aux heat and emergency heat lean on the same backup, but they don’t run the same way. According to one A/C maker’s consumer guide, aux heat typically works alongside the heat pump to give it a boost during very cold days. Emergency heat, that guide says, completely shuts off the heat pump and runs only off the backup source.
That backup is a source such as electric-resistance coils. The guide says electric resistance makes warmth directly, instead of moving heat in from outside the way a heat pump does. It calls that less efficient than your heat pump. The guide also says that, depending on the thermostat, emergency heat may come on by itself, though it can be turned on by hand too.
Carry one line with you to the thermostat. AUX is the backup that helps the heat pump. EM is the setting that replaces it. That single difference drives what you should do about each.
Should You Ever Switch the Thermostat to Emergency Heat?
Yes, but the window is narrow. Another A/C maker’s article describes emergency heat as a heat pump thermostat setting to use only when the heat pump has failed. Switching to it shuts the heat pump down, so the backup becomes your main source of heat. Think of it as a stopgap.
That first guide lists the cases for turning it on. A technician specifically instructs you to while a repair appointment or parts are pending. The heat pump has completely failed and you need temporary heat until a professional can fix or replace it. The outdoor unit is frozen solid or damaged and can’t operate safely until someone inspects it. Short of something like that, the setting stays put.
The same guide is plain about the cost side. Electric-resistance heat is less efficient than the heat pump, so it says your energy bill can climb quickly if emergency heat runs for days or weeks. It also says occasional use during a repair or an extreme cold snap won’t harm your system. But it treats regular reliance on the setting as a sign that something needs attention.
That second article adds one step. If you’ve engaged the emergency setting, it says to call for service. The switch itself is the signal. Something made you reach for it, and that something deserves a certified technician’s eyes.
Does a Lit Aux Light Mean Something Is Wrong?
Not by itself. That second article calls aux heat supplementary heat, there to help the heat pump reach the temperature you set, and it says the thermostat controls that automatically. The same article says aux can come on during a defrost cycle, or after a large jump in the heating setpoint. A lit light alone isn’t proof of a fault.
Picture a cool morning after a cold front moves through. The thermostat was set back overnight, then raised by several degrees once the house woke up. That’s a setup that can bring aux on, and it’s the thermostat doing its job. A certified technician can tell you what’s normal for your system. The pattern on its own isn’t cause for alarm.
There’s another side to it. That second article says aux heat that comes on and stays on when the weather isn’t cold can point to a problem with the heat pump or the thermostat. That first guide goes further. It names problems behind a system that keeps leaning on backup heat. A refrigerant leak is one. A faulty sensor or thermostat is another. A blocked or frozen outdoor coil is a third.
If you aren’t sure the system is a heat pump at all, settle that first. A separate Honest Air post on whether a Florida home needs a heat pump or a cooling-only system covers how to choose between them and the clue your thermostat gives you.
When a Long-Running Aux Light Needs a Technician
Your side of this is simple, and all of it happens at the thermostat. Check that the mode isn’t set to Emergency Heat or EM heat. That first guide says to check the setting, and that it should be on Heat or Auto. Set it back if it’s wrong. Read what the display says, including any error code or warning light. Give the system a few minutes. And don’t keep switching it off and on.
If the light keeps running, or the house stays cold either way, that’s the point to hand it off. A certified technician can trace the cause. Don’t try to diagnose it yourself.
Honest Air’s heat pump repair page says every repair begins with a comprehensive inspection of your heat pump system. The examination covers coils, refrigerant levels, electrical connections, motors and airflow pathways.
Honest Air’s A/C maintenance page describes a comprehensive 32-point inspection and cleaning. It lists the thermostat and the air filter among what a visit covers.
One related problem is worth keeping separate. A system that won’t change between heating and cooling at all is a different fault, and why a heat pump won’t switch between heating and cooling covers that one.
Frequently Asked Questions About Emergency Heat and Aux Heat
When should you use emergency heat on a heat pump?
Only if the heat pump itself has broken down, or if a certified technician tells you to while a repair is pending. One A/C maker’s guide and another maker’s article point the same way. The setting is a stopgap for a failed heat pump, not a substitute for normal heating. If you aren’t sure yours has failed, ask a certified technician before changing the setting.
What does AUX mean on a thermostat?
AUX is short for auxiliary, which just means backup. It’s the automatic helper that runs alongside the heat pump when the heat pump needs a hand reaching the temperature you set. The thermostat decides when to use it. You don’t set it yourself. EM, or Emergency Heat, is the setting that shuts the heat pump off and runs the backup alone.
Will emergency heat raise my electric bill?
It can. One A/C maker’s consumer guide says electric-resistance heat is less efficient than the heat pump. So your energy bill can climb quickly if emergency heat runs for days or weeks. The guide doesn’t put a figure on it. If emergency heat keeps running, ask a certified technician to look at the heat pump.
Is emergency heat the same as an emergency service call?
No. Emergency heat is only the name of a thermostat setting, and switching it on doesn’t call anyone. The two have nothing to do with each other beyond the word. If the heat pump has stopped heating your home, that’s a separate decision. Ask a certified technician to look at it.
What should I do if the thermostat is stuck on emergency heat?
Start at the thermostat. Check that the mode isn’t set to Emergency Heat or EM heat, and set it back to Heat or Auto if it is. Note anything unusual, like a sound, a smell or a message on the display. Give the system a few minutes to respond. Don’t keep switching it off and on. If it won’t clear, call a professional and leave the equipment alone.
Leave the Setting Alone and Ask About a Light That Won’t Quit
A repair or maintenance visit with Honest Air includes its 32-point inspection, which lists a thermostat check: level, secure to the wall and accurate.
So a short note helps. Jot down what the thermostat showed and when you saw it, and a technician has somewhere to start.
If that aux light keeps running, or you’re still unsure about emergency heat on your heat pump, you can book an A/C service visit with Honest Air. Treasure Coast homeowners can start there.
