How to Stop Auxiliary Heat From Coming On

Auxiliary heat costs several times more per unit of warmth than the compressor does. Four settings decide when it engages, and most systems are handed over configured badly.

Before you change anything

Auxiliary heat is not a fault. It is the system covering a gap the compressor cannot close on its own, and there are conditions where it running is exactly right.

Aux heat is behaving correctly when:

• The outdoor temperature is below your system's balance point, the temperature at which heat pump output no longer matches the house's heat loss.
• A defrost cycle is running, where backup heat tempers the supply air so it does not blow cold.
• The house is recovering from a deep setback and the thermostat is trying to close a large gap quickly.

Suppressing aux heat in genuinely cold weather does not save money, it just makes the house cold and the system run continuously without catching up. The goal is not zero aux heat. The goal is aux heat only when the compressor genuinely cannot cope.

The signal that something is wrong is aux heat running on mild days, or running for long stretches in weather your system used to handle. If that is what you are seeing, the settings below are where to look, and aux heat that will not turn off covers the mechanical faults that produce the same symptom.

The four settings that control it

Names differ by brand, so these are described by what they do. All of them normally live in the installer or advanced menu rather than the everyday screen.

1. Auxiliary heat lockout (outdoor temperature). The most powerful of the four. It prevents backup heat engaging at all when the outdoor temperature is above a value you set. Setting it near your balance point stops strip heat running on days the compressor could have handled alone. This is the single highest-value change for most homeowners.

2. Droop, or the aux differential. This controls how far behind the setpoint the house must fall before backup engages. Set too tight, backup joins in at the first sign of a small gap. Widening it slightly gives the compressor more time to do the work at a fraction of the running cost.

3. Adaptive or smart recovery. This decides how the system approaches a scheduled temperature change. Configured for a heat pump it ramps gradually; configured for a furnace it closes the gap fast, which means strip heat. Our thermostat guide covers which models handle this well.

4. Equipment type. Worth verifying rather than assuming. A thermostat configured as a conventional furnace system rather than a heat pump with auxiliary heat will behave wrongly in ways no other setting can correct.

Stop using deep setbacks

This is the behavioural change that saves the most, and it is counterintuitive if you are used to a furnace.

With a furnace, turning the thermostat down at night and back up in the morning saves money, because a furnace closes the gap at the same efficiency regardless of how large the gap is. A heat pump does not work that way. Faced with a large gap it brings in resistance backup, which costs several times more per unit of heat than the compressor. The saving from eight hours at a lower temperature is frequently wiped out by twenty minutes of strip heat closing the gap in the morning.

Practical guidance:

• Prefer steady setpoints. Heat pumps are most efficient running long and gentle rather than in bursts.
• If you do set back, keep it shallow, a couple of degrees rather than eight, and let recovery start early.
• Do not manually jump the setpoint several degrees to warm the house faster. That is precisely the instruction that calls for backup heat.

Emergency heat is a different thing entirely

These two get confused constantly, and the confusion is expensive.

	Auxiliary heat	Emergency heat
How it engages	Automatically, alongside the compressor	Manually, by you selecting it
Compressor	Keeps running	Shut off entirely
When it is right	Below the balance point, and during defrost	Only when the heat pump is broken
Running cost	High, but offset by the compressor still working	Highest, because all heat is now resistance heat

If your thermostat has been left on emergency heat, that alone explains a very large bill, and switching it back to normal heat is the entire fix. It is worth checking before adjusting anything else.

When settings are not the answer

If the lockout and differential are configured sensibly and backup heat still runs on mild days, the cause is mechanical rather than configurational. The usual candidates:

• A stuck relay or contactor energising the strips when nothing has called for them.
• A failed outdoor temperature sensor, so the lockout you set is being evaluated against a wrong reading.
• Low refrigerant charge, which reduces heat pump capacity so the compressor genuinely cannot keep up in weather it used to handle.
• An undersized system, which leans on backup heat by design rather than by fault. Covered in oversized versus undersized problems.

A useful test: note the outdoor temperature when aux heat engages, several times. If it engages consistently well above where your lockout is set, either the lockout is not being applied or the sensor feeding it is wrong. That single observation saves a technician a lot of time.

Common questions

Can I just turn auxiliary heat off completely? You can, on most thermostats, and you should not. Below the balance point the compressor cannot meet the house's heat loss on its own, and disabling backup leaves you cold while the system runs continuously. Set a sensible lockout temperature instead of removing the function.

What temperature should the aux lockout be set to? Near your system's balance point, which depends on your house and your equipment rather than on a universal number. If you do not know it, a starting point below freezing is common, and the better approach is to ask your installer for the balance point from the design calculation.

Why does aux heat come on when I raise the temperature manually? Because a large sudden gap between room temperature and setpoint is exactly the condition that calls for backup. Raise the setpoint in small increments, or let a gentle schedule do it, and the compressor handles it alone.

Is it normal for aux heat to run during defrost? Yes, and it is desirable. During defrost the indoor coil goes cold, and backup heat tempers the supply air so your vents do not blow cold. Those are short bursts and are not what makes a bill large.

How this guide is put together

This page separates the cases where auxiliary heat is behaving correctly from the cases where it is misconfigured, because turning off aux heat that is genuinely needed leaves you cold rather than saving money. Settings are described by function, since the label differs between thermostat brands.

• Sources. Manufacturer documentation, industry pricing surveys and the references linked at the foot of this page.
• The DIY line. Anything involving refrigerant is legally restricted to an EPA Section 608 certified technician. Sealed systems, capacitors and line-voltage wiring are called out as technician work rather than left ambiguous.
• Prices. Given as ranges, because regional labour rates and local competition move them more than any other factor. Confirm against written quotes.

Sources

Primary references for the figures and procedures on this page. Where a number here is a range drawn from field experience rather than a published measurement, it is labelled as such in the text.

• ENERGY STAR: Air-Source Heat Pumps — Federal efficiency criteria and guidance on heat pump operation and controls.
• AHRI Directory of Certified Product Performance — Look up the certified low-temperature capacity of your system when judging its balance point.
• EPA Section 608 Technician Certification — Relevant where reduced capacity turns out to be a refrigerant charge problem.

Last checked September 2026. Setting names vary by thermostat brand; confirm against the installer manual for your model.