Auxiliary heat and emergency heat are two backup settings on a heat pump, and they are easy to confuse. Aux heat starts automatically to help the heat pump on cold nights, while emergency heat is a manual setting that replaces the heat pump with electric strips. The mix-up costs real money in Florida, where heat pumps run mostly for cooling and owners see the AUX light only a few nights a year, usually just before a bigger electric bill.
This guide explains the five things to know in plain language, shows when each setting is right, and points to West Air Conditioning for Heat Pump Heating Services when the settings are not the problem.
5 Things To Know About Auxiliary Heat Vs Emergency Heat
1. Auxiliary Heat Is Automatic Backup That Works With The Heat Pump
Auxiliary heat is a secondary heating function that the thermostat switches to on its own. It runs alongside the heat pump, usually through electric heat strips in the indoor unit, to make up for heat the heat pump cannot pull from cold outdoor air. According to HVAC.com, it can start when outdoor temperatures are near or below freezing, when the indoor temperature falls three or more degrees below the set point and during defrost mode. The usual trigger is below 40 degrees or during a rapid temperature adjustment. That is why a cool morning can light the AUX indicator with nothing wrong, and why some owners suspect a Heat Pump Blowing Cool Air when the unit is only defrosting.
2. Emergency Heat Is A Manual Setting That Replaces The Heat Pump
Emergency heat must be switched on by hand. While it is on, the heat pump is bypassed, and the electric strips do all the heating, so aux heat and emergency heat never run together. HomeServe advises turning it on only when the heat pump is not working and treating it as temporary until a repair restores normal operation. In practice, that means a frozen outdoor unit, loud noises from the unit or a system that runs nonstop without warming the home. The first time the strips run each winter, they can also give off a faint Heater-Burning Smell as dust burns off, so do not mistake that for a failure.
3. Both Settings Run On Electric Strips, Which Cost More Than The Heat Pump
Electric strips convert electricity to heat at roughly 100 percent efficiency, while a heat pump moves heat and can deliver about three times as much. Both aux heat and emergency heat use a lot of energy and raise the electric bill. That efficiency gap is why long stretches on backup heat show up on the bill so quickly, since every hour on the strips costs more than the same hour on the heat pump. The size of the jump shows in a warning from a Florida utility. In February 2026, FPL warned that auxiliary heating used more than 12 hours can add up to $15 a day, WFLX reported. FPL does not serve most of our area, so treat that number as an example. Your own utility, such as Duke Energy or OUC, can tell you your rate.
4. You Can Avoid Triggering Aux Heat Most Of The Time
The easiest way to avoid aux heat is to avoid big jumps in the thermostat setting. Avoid raising the thermostat more than two degrees at a time, and HVAC.com gives the same two-degree interval. Letting the house cool deeply overnight and then asking for a ten-degree rise in the morning is a classic trigger. Small changes in the house help too. Duke Energy says sealing air leaks around windows, doors and vents can save 10 to 20 percent in heating costs, and changing the air filter regularly keeps the system from working harder than it needs to. Both reduce how often the heat pump needs help.
5. Constant Aux Heat, Or A Need For Emergency Heat, Points To A Problem
Aux heat should be a temporary setting that turns off once the house warms. If the aux indicator stays on after outdoor temperatures reach 40 degrees or higher, there may be a malfunction. The warning signs are unusual noises from the outdoor unit, ice that does not melt, a system that runs nonstop without warming the home, refrigerant leaks and odd smells. Any of these is a reason to call for Heat Pump Repair before the bill arrives. A technician can check the refrigerant, the defrost controls, the strips and the thermostat settings, and tell you which one is making the strips work too hard.
Should You Ever Switch To Emergency Heat?
Switch to emergency heat only when the heat pump has stopped working, and you are waiting on a repair. HVAC.com advises against using it unless it is a real emergency, and says to turn it off once the home is within three degrees of the set point. If the house will not stay warm and it is 40 degrees or higher outside, the heat pump is the problem, not the weather, and the better step is to call a technician.
If the heat pump is down on a cold night, emergency heat can keep the house livable while you call for emergency service from West Air Conditioning at (407) 739-1667.
Use this table to choose:
| Situation | Aux Heat | Emergency Heat | What To Do |
| Cold morning, thermostat raised a few degrees | May start, normal | Not needed | Raise in small steps and wait |
| Steam from the outdoor unit during defrost | May run, normal | Not needed | Let the cycle finish |
| AUX stays on above 40 degrees outside | Possible fault | Not the fix | Call a professional for inspection |
| Outdoor unit frozen solid or silent | May not keep up | Use temporarily | Switch to EM HEAT and call a technician |
| Smoke, a strong burning smell or a tripped breaker | Do not rely on it | Do not use | Turn the system off and call |
A house that never catches up with the set point can also be a duct problem and not a heat pump problem, so ask for Duct Cleaning if the strips run but some rooms stay cold.
What Is Normal Aux Heat Use In A Central Florida Cold Snap?
Short periods of aux heat are normal. Duke Energy meteorologists put December and January peak lows in the 30s and mid-40s, so a few cold mornings each winter will call for backup heat. Aux heat is also normal during the defrost cycle and after a sharp thermostat change, and it should switch itself off as the house warms. What is not normal is an AUX light that stays on all day in mild weather, a house that never catches up with the set point, or a bill that climbs by far more than the weather explains.
How Can You Keep Aux Heat From Running Up The Bill?
Test the system before the cold front, not during it. A fall tune-up lets a technician check the strips, the sequencer, the defrost controls and the thermostat while the weather is mild, and ENERGY STAR recommends heating checks in the fall for that reason. Replace the filter, keep the outdoor unit clear of leaves and set the thermostat once instead of chasing it. Keeping a short log of when the AUX light comes on and what the outdoor temperature was gives the technician a head start.
Final Thoughts
If your AUX light stays on or your winter bill jumped, West Air Conditioning can test the heat pump, the strips and the thermostat and tell you plainly which one is at fault. Request a Heat Pump Estimate or call (407) 739-1667 and tell us what your thermostat shows.
Frequently asked questions
What Is The Difference Between Aux Heat And Emergency Heat?
Aux heat starts automatically to help the heat pump, while emergency heat is a manual setting that replaces the heat pump with electric strips and costs more to run.
When Should I Use Emergency Heat On My Heat Pump?
Use emergency heat only when the heat pump has stopped working, such as when the outdoor unit is frozen or damaged, and switch back to normal heat after repairs.
Is It Bad If My Heat Pump Is On Aux Heat?
Not for short periods. Aux heat is normal on cold mornings, during defrost and after big thermostat changes. If it stays on above 40 degrees outside, call a technician.
Does Aux Heat Make My Electric Bill Higher?
Yes. Electric strips use far more electricity than a heat pump, and FPL warned that backup heat running more than 12 hours can add up to $15 a day.
Why Does My Thermostat Say Aux Heat In Mild Weather?
Aux heat can start in mild weather after a big thermostat change or during defrost. If it stays on once outdoor temperatures reach 40 degrees, a fault is possible.