Heating explainer
What Is Emergency Heat (and Why Is My Bill So High)?
If you have a heat pump in Gallatin, those EM and AUX indicators on your thermostat control the most expensive heat you own. Here is how to use them right.
(615) 412-2495- Aux vs. emergency heat, plainly explained
- The thermostat mistakes that spike bills
- When backup heat signals a real problem
The short answer
Backup heat is expensive by design
Every all-electric heat pump system in Gallatin has a set of electric resistance heating elements tucked inside the indoor air handler. They go by two names, auxiliary heat and emergency heat, and they exist for one reason: to keep you warm when the heat pump cannot do it alone. The catch is that they heat your home the expensive way, like a giant space heater, at roughly two to three times the operating cost of the heat pump itself.
Used correctly, they are a safety net you barely notice. Used incorrectly, or running because of an underlying problem, they are the reason winter electric bills double. Understanding how heat pumps use backup heat in cold weather makes the whole picture click, but here is the practical version.
The difference
Auxiliary heat vs. emergency heat
| Auxiliary (AUX) heat | Emergency (EM) heat | |
|---|---|---|
| What it is | Backup elements that assist the heat pump automatically | The same backup elements, run manually without the heat pump |
| How it starts | System calls for it on its own during defrost or extreme cold | You select EM on the thermostat yourself |
| Outdoor unit | Keeps running alongside it | Stays off entirely |
| When to use it | Let the system decide | Only if the heat pump itself has failed |
The key insight: auxiliary heat is automatic and normal in small doses. Your heat pump briefly reverses itself to melt frost off the outdoor coil, and during those defrost cycles the backup elements keep warm air coming from your vents. On the coldest Gallatin mornings, aux heat also helps the system meet the thermostat setpoint. Both are by design.
Emergency heat is the manual override. Switching your thermostat to EM tells the system to stop using the heat pump altogether and heat only with the resistance elements. That setting exists for one scenario: the outdoor unit has failed and you need heat until a technician arrives. Running it as your normal heat setting is one of the most expensive mistakes a homeowner can make.
The expensive mistakes
Thermostat confusion that doubles bills
We see three versions of this every winter across Sumner County. First, someone bumps the thermostat to EM heat during a cold snap and forgets to switch it back, sometimes for weeks. Second, a new thermostat gets installed or a system gets serviced, and the thermostat is configured for a conventional system instead of a heat pump, so it treats the backup heat as the primary heat source. Third, a failing outdoor component means the heat pump cannot carry the load, so aux heat runs for hours trying to compensate.
All three look the same on your bill: a shocking jump. And all three are fixable. Check your thermostat right now if your winter bill looks wrong. If it says EM, switch it back to HEAT. If it says AUX constantly rather than occasionally, something deserves a look. Homeowners in neighborhoods like Kennesaw Farms and Clear Lake Meadows, where all-electric heat pump systems are common, run into this more than most.
Wiring and configuration issues are squarely in thermostat service territory, and they are quick for a tech to verify. Do not let anyone sell you a new system because "the heat pump can't keep up" until someone has confirmed the controls are actually telling it to run properly.
When to worry
When backup heat means a real problem
Some aux heat in winter is normal. Constant aux heat is not. Call for heating repair in Gallatin if you notice any of these:
- The thermostat shows AUX heat for hours at a time in moderate cold (40s and up).
- The outdoor unit never seems to run, or runs but the air from the vents is lukewarm.
- Your winter electric bill jumped far beyond last year's with similar weather.
- The system was recently serviced or got a new thermostat, and the problem started after.
Common underlying causes include low refrigerant, a failed defrost control, a bad outdoor sensor, or that thermostat misconfiguration. A proper diagnostic finds which one it is instead of guessing. Gallatin HVAC Repair Pro approaches these calls the same way we approach everything: diagnose first, explain in plain English, and show you the options in writing.
If your system is older and aux heat struggles are becoming a pattern, it may also be worth a broader conversation about your heating setup. Our heating installation page covers heat pump sizing and dual-fuel options that reduce how often backup heat is needed in the first place.
Common questions
Emergency and auxiliary heat FAQs
Auxiliary and emergency heat are electric resistance heating elements that back up a heat pump. They produce heat at two to three times the operating cost of the heat pump itself, so when they run a lot, whether by design in extreme cold or by mistake, the electric bill jumps.
Brief AUX heat during defrost cycles or very cold mornings is normal. If it shows constantly, the heat pump may be struggling with low refrigerant, a failed outdoor component, or a thermostat misconfigured for the system, and it deserves a diagnostic visit.
Check the thermostat display for an EM or Emergency indicator, and notice whether the outdoor unit ever runs. If the thermostat is set to EM heat, only the indoor backup elements are heating, and the outdoor unit sits idle. Switch back to HEAT and watch whether normal operation resumes.
Yes, in moderation. Heat pumps routinely use short bursts of auxiliary heat during defrost cycles and on the coldest mornings. It becomes a problem when it runs for hours at a time or dominates your heating, which usually signals an underlying issue.
AUX heat running nonstop?
We will find out why and get your heat pump back to efficient operation.
(615) 412-2495 Call Gallatin HVAC Repair Pro