EM Heat vs. Heat: What Emergency Heat Means on Your Thermostat

What "EM heat" means

EM heat (emergency heat) is a thermostat setting on heat pump systems. It turns the heat pump's outdoor unit off and heats the house with the backup heat alone. In most San Antonio homes with a heat pump, that backup is a set of electric heat strips inside the air handler. In a dual-fuel system it is a gas furnace.

Regular Heat mode uses the heat pump first, because it moves heat from outdoor air instead of making it, and brings the backup in only when it needs help.

EM heat vs. heat vs. AUX heat

• Heat: the heat pump does the work. The cheapest way to heat a heat pump home.
• AUX (auxiliary) heat: the thermostat adds backup heat automatically while the heat pump keeps running, for example during defrost or when you raise the temperature a lot at once. Seeing AUX come on for short periods is normal.
• EM heat: you choose it by hand. The heat pump stays off and only the backup heat runs, for as long as the setting is on.

When to use emergency heat

Use EM heat only when the heat pump itself can't run, so the house stays warm until it is repaired. Typical reasons:

• The outdoor unit is damaged, iced over in a solid block that will not clear, or making grinding or banging noises.
• The outdoor unit will not start at all while the indoor fan runs.
• A technician told you to use it until a part arrives.

San Antonio winters are mild enough that a working heat pump should almost never need EM heat. Short freezes like the ones Texas sees every few years are the exception, and even then normal Heat mode with AUX is the usual setting.

Why EM heat costs more to run

Electric heat strips turn electricity straight into heat. A heat pump moves existing heat into the house, so it typically delivers several units of heat for each unit of electricity it uses. Running on strips alone can raise a winter electric bill noticeably if it goes on for days. Switch back to Heat as soon as the heat pump is repaired.

If the EM or AUX light stays on

If your thermostat shows EM heat and you did not turn it on, or AUX stays lit for hours while the house struggles to warm, the heat pump is likely not doing its share. Common causes are low refrigerant, a failed outdoor fan motor or capacitor, a defrost-control problem, or a thermostat wiring or setup issue. These need a licensed technician to measure, not a guess.

Cowboy's checks heat pumps 24/7. See heat pump repair or call (210) 495-7771.

What not to do

• Do not chip ice off the outdoor coil with tools; you can puncture it. Light frost that comes and goes is normal defrost.
• Do not leave EM heat on all winter "just in case." It costs more and hides a problem that is cheaper to fix early.
• Do not keep resetting a breaker that trips. One reset is the limit; if it trips again, leave it off and call.

Frequently asked questions

What is the difference between EM heat and heat?

Heat mode runs the heat pump and adds backup heat only when needed. EM heat shuts the heat pump off and runs only the backup heat, usually electric heat strips, so it costs more to run.

Is it bad to run emergency heat?

It will not damage the system, but it uses more electricity than normal heat pump heating. Use it only until a broken heat pump is repaired.

Why is my thermostat on AUX heat?

AUX heat comes on automatically to help the heat pump during defrost or a large temperature change. Short periods are normal. If AUX stays on for hours, have the heat pump checked.

Should I use EM heat when it freezes in San Antonio?

Usually no. A working heat pump with AUX backup handles San Antonio freezes. Use EM heat only if the outdoor unit has stopped working.

Do gas furnaces have emergency heat?

No. EM heat is a heat pump setting. A dual-fuel system uses the gas furnace as its backup, so EM heat there means furnace only.

Related service: heat pump repair in San Antonio · heating repair in San Antonio · thermostat service. Cowboy's AC answers 24/7 at (210) 495-7771.