Auxiliary Heat Won’t Turn Off? Start With the Outdoor Temperature

First, Check the Outdoor Temperature

Auxiliary heat running is not automatically a fault. It is designed to run, and there are three conditions where it is supposed to. Trane lists them: the outdoor temperature is below about 40 °F or your set trigger point, the heat pump is in a defrost cycle, or you have raised the thermostat by more than three or four degrees at once.

So the diagnostic question is not “is aux heat on” but “is aux heat on when it has no business being on.” Trane puts it plainly: it is only a bad sign when it comes on and stays on above 40 °F.

Go and look at the outdoor temperature right now. Below 40 °F on a cold snap, aux heat running is the system doing its job and the bill is the price of the weather. Above 40 °F with the aux indicator lit for more than an hour or two, something is wrong.

Make Sure It Is Aux, Not Emergency Heat

These are different things and people mix them up constantly, which matters because one of them is a switch you may have flipped yourself.

• Auxiliary heat is automatic. The thermostat brings in the electric strips alongside the heat pump when the heat pump alone cannot keep up. You do not turn it on.
• Emergency heat is a mode you select. It shuts the heat pump off entirely and runs on the strips alone. Trane describes it as being for a true emergency — a failed compressor, say — and nothing else.

If your thermostat is showing EM HEAT rather than AUX, this is very likely the whole answer: someone set it and left it. It will heat the house, at roughly the cost of running a very large electric heater continuously, and it will never turn itself off because it is not a fault — it is a setting.

Switch it back to Heat and see whether the problem disappears. Our guide to emergency heat versus auxiliary heat covers what each actually costs.

Why It Sticks On: Six Causes, Cheapest First

1. A dirty filter or blocked return. Restricted airflow means the heat pump cannot move heat into the house fast enough, so the thermostat keeps calling for more and the strips keep answering. This is the cheapest thing on the list and a genuinely common cause.
2. You raised the setpoint too far at once. Most thermostats bring in aux heat when asked for a jump of more than three or four degrees, because that is a recovery the heat pump cannot make quickly. Nudging the temperature up two degrees at a time avoids it entirely, and a schedule that ramps gradually avoids it every morning.
3. No aux lockout temperature set. Most smart thermostats let you set an outdoor temperature above which aux heat is simply not allowed. Left unset, the thermostat will use the strips whenever they are convenient. This is the single most effective setting on the list.
4. The thermostat is configured for the wrong system type. A thermostat that thinks it is driving a conventional furnace will not stage a heat pump properly. If aux heat behaviour changed right after a new thermostat went in, check the system type and the wire configuration first.
5. The outdoor unit is iced up or failing to defrost. A heat pump that cannot shed frost loses capacity, and the strips fill the gap. Go and look: normal defrost versus a real icing problem.
6. Low refrigerant charge, or a failing compressor or reversing valve. The heat pump is producing less heat than it should, and the strips are silently covering for it. This is the expensive one, and it needs a technician — but it is also the one where the strips hide the fault until the bill arrives.

The order matters. Filters and setpoints cost nothing; a service call is the last resort, not the first.

What It Costs While You Work Through That

Auxiliary heat is electric resistance heat. Every unit of electricity produces one unit of heat — no more. A heat pump in reasonable conditions moves substantially more heat than the electricity it consumes, which is the entire reason to own one.

So running on strips is not a small inefficiency; it is switching off the mechanism you paid for. That is why an aux-heat fault shows up as a bill that has jumped rather than as a comfort complaint — the house stays warm, which is exactly why it can run for weeks before anyone notices.

If your bill is what brought you here, why aux heat spikes a heat pump bill puts numbers around it, and a circuit-level energy monitor will show you the strips drawing power in real time, which settles the question quickly.

The Settings to Change Tonight

• Set an aux lockout temperature. Somewhere around 35–40 °F is a common starting point; below your heat pump’s balance point it will be needed regardless. If your thermostat exposes this, it is the highest-value change available.
• Turn off large scheduled setbacks. A deep overnight setback saves nothing on a heat pump if recovering from it triggers the strips every morning. Shallow setbacks, or none, usually cost less.
• Raise the temperature in small steps. Two degrees at a time, not five.
• Confirm the system type in the app matches what you actually have, including whether you have a dual-fuel setup.
• Change the filter before concluding anything else.

Frequently Asked Questions

Q: Why won’t my auxiliary heat turn off?
A: First check whether it should be on. Trane lists three legitimate triggers: outdoor temperature below about 40 °F or your set trigger point, an active defrost cycle, or a setpoint raised by more than three or four degrees. Aux heat that stays on above 40 °F for more than an hour or two is the version worth investigating.

Q: What is the difference between AUX and EM HEAT on my thermostat?
A: Auxiliary heat runs automatically alongside the heat pump when it cannot keep up. Emergency heat is a mode you select that shuts the heat pump off entirely and runs on electric strips alone. If your thermostat shows EM HEAT, someone chose that — it will not turn itself off, because it is a setting rather than a fault.

Q: Is it normal for aux heat to run every morning?
A: It is common, and it is usually caused by your schedule rather than a fault. Recovering from a deep overnight setback asks for a temperature jump the heat pump cannot make quickly, so the thermostat brings in the strips. Shallower setbacks generally cost less on a heat pump than deep ones.

Q: How do I stop aux heat from coming on?
A: Set an aux lockout temperature if your thermostat offers one, avoid large setpoint jumps, reduce or remove deep scheduled setbacks, and change the filter. If it still runs above 40 °F after that, the heat pump itself is likely underperforming and needs a technician.

Q: Can a dirty filter cause auxiliary heat to run?
A: Yes. Restricted airflow stops the heat pump moving heat into the house at the rate the thermostat is asking for, so the strips make up the difference. It is the cheapest cause to rule out and worth doing before anything else.

Q: Does aux heat running mean my heat pump is broken?
A: Not necessarily, but it can be hiding a fault. Low refrigerant charge, a failing compressor or a reversing valve problem all reduce the heat pump’s output, and the strips quietly cover the shortfall. The house stays warm, so the only symptom is the bill — which is why these faults often run for weeks.

Related Guides

• Emergency heat vs auxiliary heat — what each one actually costs
• Why aux heat spikes your bill — the numbers behind a cold-snap jump
• Heat pump iced up? — when defrost failure is driving the strips
• Nest running aux heat too much — the thermostat-specific settings