Florida cold front · heat pump thermostat

AUX Heat vs Emergency Heat: What Your Florida Heat Pump Thermostat Is Telling You

The first cool morning you switch to heat, your thermostat may show AUX or EM HEAT. They are not the same. Lennox says aux heat typically works alongside your heat pump to give it a boost, while emergency heat completely shuts off the heat pump and runs only off the backup source. Brief AUX on a cold morning is usually normal. EM HEAT is a manual setting for when the heat pump has failed, and Orlando's utility, OUC, tells heat pump customers to keep it in normal mode during cold snaps. Answer the questions below for your safe next step. Educational only. Not a service page.

Safety first: your controls are the thermostat, the filter at the return grille, and, when it is safe, the breaker at the panel face. Trane notes the electric heat strips sit inside the indoor air handler. Never open the air handler, any unit cover, the electrical panel cover, or the outdoor disconnect. Never touch the heat strips or wiring, and don't meter or test anything. Burning smell or smoke: thermostat off, breaker off, call a licensed HVAC tech. Flames: get everyone out and call 911.

What is my thermostat telling me?

Pick what you can see on the thermostat and from outside, without opening anything. The finder checks emergencies first, then hard stops, then the normal cases.

What does the thermostat show? Outside temperature How long has AUX been showing? Did you just raise the heat setting more than 3–4 degrees? Outdoor unit (look only, from a distance) Is the house warming up? Any smell or smoke? Heat/AC breaker (look only, cover stays on)

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Pick your answers and tap the button.

AUX heat

Automatic. The thermostat adds backup heat to help the heat pump catch up. Trane lists three triggers: below 40 degrees outside (or a set trigger temperature), defrost mode, or raising the setting by more than 3–4 degrees. It should end once the house reaches the setting.

EM HEAT

Manual. It shuts the heat pump off and runs only the backup heat (Lennox, Trane). Trane says use it only in a true emergency, when the heat pump has failed. OUC says emergency heat uses significantly more electricity and adds strain to the grid.

Homeowner checklist

Leave it in HEAT, not EM HEAT.If the thermostat is set to Emergency Heat or EM Heat, Lennox says to switch it back to Heat (or Auto) and see if the system returns to normal. The exception: a tech told you to run EM HEAT while you wait for a repair.
Give it a few minutes, and don't keep cycling it.Lennox says the system sometimes needs a moment to restart the heat pump, and warns: “Don't keep switching the system on and off.”
Raise the setting a degree or two at a time.Trane says a jump of more than 3–4 degrees can bring on AUX, and advises bumping the temperature only a degree or two at a time. OUC and Trane both point to a setting around 68 degrees.
Time the AUX light.Trane says if it's above 40 degrees and AUX HEAT shows for more than an hour or two, it's worth looking into, and AUX that stays on above 40 degrees can point to a heat pump or thermostat problem. That's a call, not a DIY fix.
Check the filter at the indoor return grille.Turn the thermostat off first. Change only the filter at the wall or ceiling return grille, never one inside a unit cabinet. Lennox says dirty filters restrict airflow and can push the system toward emergency heat.
Clear debris around the outdoor unit, from the outside only.Lennox says to remove leaves, grass clippings, and debris from around the heat pump. Never open the unit, never chip ice, and never use tools on it.
Breaker tripped? One reset only, and only if safe.Turn the thermostat off first. If nothing is wet, burning, or buzzing, reset the tripped breaker once at the panel face with the cover closed (firmly Off, then On). If it trips again, leave it off and call a licensed HVAC tech.

Hard stops — do not DIY: AUX stays on for more than an hour or two when it's above 40 degrees; the outdoor unit won't run while heat is on; the house won't warm; EM HEAT won't go back to normal; a breaker trips again after one reset. For any of these, shut the heat/AC off at the breaker and call a licensed HVAC tech. Never open the air handler, panel, or outdoor disconnect, never touch the heat strips or wiring, and never meter or test anything. Burning smell or smoke: thermostat off, breaker off, call. Flames: get everyone out and call 911.

Why Florida homeowners hit this every first cold front

Many Florida heat pumps sit idle in heat mode for most of the year, so the first cold morning is often the first time anyone sees AUX on the screen. A DIY StackExchange answer to a confused homeowner put it plainly: “If you have a modern heat pump in a relatively warm area, it is quite possible you have never actually needed the emergency heat.” That's also why EM HEAT gets switched on by mistake. During January's record cold in Central Florida, OUC told customers: “If your home has a heat pump, keep it in normal mode. Emergency heat uses significantly more electricity and adds strain to the grid.”

Heat smells like burning the first time it runs? See heat smells like burning in Florida. Steam coming off the outdoor unit while AUX is on? See heat pump steaming in defrost.

This is a homeowner check, not a repair diagnosis or quote. Thermostats and systems vary, so follow your own equipment's manual. We don't list prices, products, or shops. No form, no shop phone.

Common questions

What is the difference between AUX heat and EM heat on my thermostat?

Lennox says aux heat typically works alongside your heat pump to give it a boost, while emergency heat completely shuts off the heat pump and runs only off the backup source. Trane says the thermostat controls aux heat automatically, while emergency heat is a setting you turn on manually.

Is it normal to see AUX heat on a cold Florida morning?

Usually, yes. Trane says the thermostat triggers aux heat when it is below 40 degrees outside (or a set trigger temperature), when the heat pump is in defrost mode, or when you raise the heating setting by more than 3-4 degrees. Once the house is up to temperature, aux mode should end.

Should I switch to emergency heat during a Florida cold front?

No, not for normal cold. OUC, the Orlando utility, tells heat pump customers to keep it in normal mode because emergency heat uses significantly more electricity and adds strain to the grid. Trane says to use emergency heat only in a true emergency, when the heat pump has failed. Lennox says to use it if a technician tells you to while you wait for a repair.

My thermostat is on EM HEAT. What should I do?

Lennox says to make sure the mode is set to Heat or Auto, not Emergency Heat, then see if the system returns to normal. Give it a few minutes and don't keep switching it on and off. If it doesn't return to normal, the outdoor unit won't run, or the house won't warm, shut the heat/AC off at the breaker and call a licensed HVAC tech.

When is AUX heat a problem?

Trane says it is only a bad thing if aux heat comes on and stays on when it is above 40 degrees outside, and that AUX HEAT showing for more than an hour or two above 40 degrees is worth looking into. If that happens, or the outdoor unit won't run, shut the heat/AC off at the breaker and call a licensed HVAC tech. Never open the air handler or touch the heat strips.

Also here

Heat smells like burning · Heat pump steaming in defrost · Air filter size finder · Maintenance checklist · Tools hub.

Sources (retrieved Oct 23, 2026)

OUC (Orlando Utilities Commission), “OUC Encourages Energy Conservation During Peak Morning Hours of Record Cold” (Jan 30, 2026): heat pump homes should keep it in normal mode; emergency heat uses significantly more electricity and adds strain to the grid; OUC recommends setting thermostats around 68 degrees. ouc.com/about/news/ouc-encourages-energy-conservation/
Lennox (manufacturer), “What is Emergency Heat and When Should You Use It?” (Jan 30, 2026): aux heat works alongside the heat pump, emergency heat shuts it off and runs only the backup; use it if a technician instructs you to while waiting for a repair; if the thermostat is on emergency heat, switch it back and see if the system returns to normal; give it a few minutes; don't keep switching the system on and off; dirty filters can trigger emergency heat; clear debris around the outdoor unit. lennox.com/residential/lennox-life/consumer/what-is-emergency-heat
Trane (manufacturer), “What is Auxiliary Heat and How Does it Affect HVAC Performance?” by Anne Fonda (Sep 23, 2024): aux triggers are below 40 degrees, defrost mode, or raising the setting more than 3–4 degrees; aux that stays on above 40 degrees can indicate a problem; AUX HEAT for more than an hour or two above 40 degrees is worth investigating; heat strips sit in the indoor air handler; emergency heat is a manual setting for a true emergency; keep the temperature around 68 degrees; bump up the temperature only a degree or two at a time; if the heat pump won't turn on at all, call for service. trane.com/residential/en/resources/blog/heat-pump-auxiliary-heat/
DIY StackExchange, “Heat pump failed to work in aux/emergency mode?” (asked Apr 1, 2024): homeowner question; answer notes a modern heat pump in a relatively warm area may never have actually needed the emergency heat. diy.stackexchange.com/questions/297330

AUX stuck on, EM HEAT won't clear, or no heat? Shut the heat/AC off at the breaker and call a licensed HVAC tech.