How to Turn Off Auxiliary Heat on a Honeywell Thermostat (Without Killing Backup Heat)

You usually cannot permanently “turn off” auxiliary heat on a Honeywell thermostat the way people mean it — there is no homeowner toggle that disables backup heat forever while leaving a heat pump otherwise normal. What you can do is stop unnecessary Aux use: set outdoor Aux lockouts and compressor balance points in installer setup (when your model and outdoor temperature source support them), avoid big setpoint jumps that force strip heat on mild days, and leave Emergency Heat alone unless the heat pump itself has failed. Disconnecting the Aux/E wire to “save money” also kills your backup when you actually need it — that is a bad trade on almost every house.

This guide was substantially rewritten and re-fact-checked in August 2026 against Honeywell Home / Resideo manuals and U.S. energy-efficiency guidance on heat-pump strip heat.

Key takeaways

• Auxiliary heat supplements the heat-pump compressor automatically in Heat mode. Emergency (Em) heat is a manual mode that locks out the compressor and runs only backup heat — Honeywell Home documents both clearly.
• On modern Honeywell Home / Resideo heat-pump thermostats, the real controls are installer settings: compressor lockout / balance point and Aux heat outdoor lockout (for example ISU 355 and 356 on T5+/T6 Pro lines), which need an outdoor temperature source.
• Aux running briefly in deep cold, after a big setpoint recovery, or during defrost is often normal. Aux running for hours on a mild day is usually a control, wiring, or equipment problem — not a reason to yank the wire.
• Do not treat Em Heat as a “colder weather” setting. It is for a failed or iced-out heat pump until a tech can fix it.
• If your display shows Aux Heat On constantly, start with filters, outdoor unit ice, and a thermostat restart — then call an HVAC tech before you rewire anything. Related: how to reset a Honeywell thermostat and Honeywell thermostat not cooling (same “equipment vs thermostat” logic applies in heat).

What auxiliary heat is (and why Honeywell shows it)

On a heat-pump system, the compressor is stage 1 heat: it moves heat from outdoor air into the house. When that capacity is not enough — or when the thermostat stages up for other control reasons — the system calls auxiliary heat: usually electric resistance strips in the air handler, or (on dual-fuel setups) a gas/oil furnace. That backup is what Honeywell labels Aux / Aux Heat On.

Per Honeywell Home’s Aux Ht vs Em Ht support article (updated September 2025): Aux Ht is backup heat that turns on to help the compressor maintain your temperature setting; both Aux and the compressor may run at the same time; you see Aux Heat On on the display when it is running. Emergency Heat only runs when you move the system switch (or mode) to Em Ht — and in that position the compressor does not run, only emergency/backup heat does. Honeywell also notes it is common for Em and Aux to be the same physical heat source (strips or fossil fuel).

That last point matters for anyone tempted to “turn Aux off” by cutting a wire: on many systems Aux and Em share the same strips. Kill the circuit and you remove both everyday staging help and the emergency path.

Aux heat vs emergency heat

Mode	How it starts	Compressor	What you want it for
Heat + Aux	Automatic when the thermostat stages backup heat	May run with Aux (electric backup)	Cold days, recovery after setback, times the heat pump alone cannot hold setpoint
Em Heat	You select Em Ht / Emergency Heat	Locked out — does not run	Heat pump failed, faulted, or cannot defrost; temporary heat until repair

Resideo’s ElitePRO documentation frames the same split: Aux runs as backup while the thermostat is in Heat (when load conditions call for it); Em Heat is a manual mode that locks out the heat pump. See Honeywell Home ElitePRO advanced features — Em Heat and Auxiliary Heat.

When Aux should run — and when it should not

Every house has a balance point: the outdoor temperature where the heat pump’s heating capacity roughly matches the home’s heat loss. Below that point, you need supplemental heat to stay comfortable. U.S. Department of Energy / FEMP acquisition guidance puts it in practical terms: cold climates need heat pumps that can operate colder before switching to resistance heating, and because electric resistance elements are inefficient, their use should be kept to a minimum — with heat-pump-aware thermostats that ramp setpoints slowly so strips do not slam on during recovery (DOE FEMP — purchasing energy-efficient residential air-source heat pumps, tips section, December 2024 update).

Bonneville Power Administration’s PTCS notes on strip-heat control are blunt for the Pacific Northwest climate they serve: with a proper install, backup heat hours should be relatively few, and Aux should be locked out on relatively mild outdoor temperatures — BPA’s program language uses 35°F as that “relatively mild” outdoor lockout reference (BPA — Notes on Auxiliary Heat Controls and Thermostats). Your climate and equipment will differ, but the principle is the same: strip heat on a 50°F day is usually a control mistake, not a feature.

Normal Aux (usually):

• Outdoor temps near or below your system’s real capacity limit
• Large upward setpoint changes (many controls stage Aux aggressively on big recoveries)
• Short periods around defrost cycles on the outdoor unit
• Dual-fuel switchover when the installer set a fossil-fuel changeover temperature

Not normal Aux (investigate):

• Aux Heat On for long stretches while it is mild outdoors
• Em Heat left selected by accident after a guest or previous season
• Aux that never shuts off even after the house is at setpoint for hours
• Ice-locked outdoor coil, blocked airflow, or a heat pump that is no longer moving heat

Can you turn Aux heat off on a Honeywell thermostat?

Short answer: not as a permanent “off” switch for most homeowners — and you should not want a permanent off if strips (or a furnace) are your only backup. What Honeywell / Resideo actually give you is staging and lockout control in installer setup, plus the Em Heat mode you should almost never leave engaged.

Older advice floating around the web (including the old version of this page) leaned on disconnecting the W2 / Aux wire behind the thermostat. That does stop Aux calls from that thermostat — and it also removes Emergency Heat when Aux and Em share the same heat source, which Honeywell says is common. BPA’s PTCS notes separately stress that Aux and Em circuits should be verified and, where required, controlled separately so emergency heat still works when needed. Disabling backup to chase a bill is not a manufacturer-supported “setting.”

How to reduce unnecessary Aux use on Honeywell / Resideo models

These steps are the honest path. Exact menu names vary by model; always match your printed Resideo guide for your SKU.

1. Confirm you are not in Em Heat

Open System / Mode. If it says Em Heat, Emergency, or similar, switch back to Heat (or Auto). Per Honeywell Home, Em Heat locks out the compressor — so every Btu comes from expensive backup. That is the opposite of “saving” on Aux.

2. Use outdoor Aux lockout and compressor lockout (installer setup)

On the T5+ Smart Thermostat, Resideo documents:

• ISU 130 Outdoor Temp — outdoor temperature source (Internet when registered to the Resideo app, or none). Outdoor temp is required for lockout ISUs.
• ISU 355 Balance Point (Compressor Lockout) — Off, or 5°F–60°F in 5°F steps. Below this outdoor temperature, the thermostat locks out the heat pump and runs Aux only. Shown when the system is a heat pump with Aux/E stages configured and outdoor temp is available.
• ISU 356 Aux Heat Lock Out — Off, or 5°F–65°F in 5°F steps. Prevents the more expensive Aux source from running above the outdoor limit you choose — explicitly framed as an energy-bill optimization setting.

Source: Resideo T5+ Smart Thermostat guide, 33-00474.

The T6 Pro Series installer setup uses the same idea with matching numbers: ISU 355 Compressor Lockout / Balance Point and ISU 356 Outdoor Lockout Backup Heat, plus options for how Aux and E terminals are driven together or independently on eligible models (Resideo T6 Pro Series, 33-00181EFS). Non-Wi‑Fi / TH6320U–TH6220U Pros need a wired outdoor sensor for ISU 355; Wi‑Fi models can use Internet outdoor temp when ISU 130 is configured.

On older TH7000-series heat-pump setups with an optional outdoor sensor, Resideo documents Function 0350 (compressor lockout) and Function 0360 (auxiliary lockout). With electric backup, when outdoor temp is above the Aux lockout, only the compressor runs; between the two lockouts, both can run; below the compressor lockout, only Aux runs (Resideo TH7000 Series, 69-2668ES).

Installer caution: These menus are usually behind an installer password or hold-to-enter setup sequence. Wrong lockout values can leave you underheated on design days or strand a dual-fuel system on the wrong fuel. If you are not sure what outdoor temperature your equipment can actually carry, have the installing contractor set 355/356 (or 0350/0360) from the equipment’s performance data — do not copy a number from a forum post for a different climate.

3. Stop forcing Aux with setpoint shocks

FEMP’s heat-pump tips specifically call out programmable / heat-pump thermostats that raise temperature gradually so electric resistance does not activate on recovery. Practical version: raise the setpoint a degree or two at a time and let the compressor catch up; avoid 8–10°F jumps from deep setbacks if Aux is lighting up every morning on 45°F days.

4. Fix the house and the equipment first

Lockouts cannot fix a dirty filter, a iced outdoor coil, low refrigerant, or leaky ducts. Those problems make the heat pump look “weak,” so the thermostat stages Aux early. Annual service between heavy seasons beats any thermostat trick. BPA’s strip-heat notes also remind installers that poor airflow and control mistakes drive Aux hours that should not exist.

5. What about space heaters and “just lower the setpoint”?

Lowering the setpoint a little overnight can reduce total runtime — that is basic load reduction, not a Honeywell Aux switch. Portable heaters can warm one room, but they do not replace a correctly staged heat pump, and they are not a substitute for outdoor lockouts or a working Em Heat path. If you do use one, follow CPSC portable-heater safety guidance (wall outlet only, clearance, never unattended).

When you should NOT turn Aux (or Em) off

• True emergency heat need. Compressor dead, outdoor unit iced solid and not clearing, or a heat-pump fault that needs Em Heat until a tech arrives — leave backup available.
• Design-day cold. Below your real balance point, Aux is the plan. Locking Aux out too aggressively (or disconnecting it) can leave rooms cold and push people toward unsafe space-heater misuse.
• Dual-fuel changeover. On gas/oil backup systems, “Aux” may be the furnace. The compressor lockout / changeover temperature is a fuel and comfort decision, not a money-saving kill switch. Dual-fuel staging is also meant so the heat pump and furnace do not run together (coil damage risk) — see Honeywell Home’s ElitePRO advanced features.
• Rental / unfamiliar installs. If you did not commission the system, do not cut thermostat wires. Document what you see and call the landlord’s HVAC contractor.

If Aux Heat On will not go away

1. Check Mode — confirm you are in Heat, not Em Heat.
2. Note outdoor temperature and how long Aux has been on. Mild day + hours of Aux → problem. Single-digit outdoor temp + Aux → often expected.
3. Replace a dirty filter; clear snow/debris from the outdoor unit; look for heavy ice that is not clearing after a defrost cycle.
4. Try a simple restart (breaker or soft reboot) if the display is stuck — see the model steps in How to Reset a Honeywell Thermostat. A reset will not repair a failed compressor.
5. If Aux still runs continuously while the house is at setpoint, or heat output is weak, stop DIY wiring experiments and get a licensed HVAC tech. Continuous Aux on strip heat can run a painful electric bill and still leave you with an undiagnosed heat-pump fault.

When to call a pro

Call a licensed HVAC technician (not just a thermostat swap) if:

• Aux or Em is the only heat that warms the house
• The outdoor unit will not run, short-cycles, or stays iced over
• You need lockouts set and you do not have the installer menu / outdoor sensor / app outdoor-temp path configured
• Someone previously disconnected Aux/E and you want backup restored safely
• Breakers trip when Aux stages on (strip circuit or wiring issue)

I am a homeowner researching manufacturer docs — not a licensed HVAC tech. Heat-pump controls, reversing valves, and strip circuits are easy to miswire; wrong Aux/E landing can run heat continuously or leave you without emergency heat. When the house is cold and the display says Aux Heat On for the wrong reasons, paying for diagnosis beats a viral “cut the brown wire” tip.

How this was researched

Claims about Aux vs Em wording come from Honeywell Home’s current support article. Lockout / balance-point ranges and ISU numbers were checked in Resideo’s T5+ (33-00474), T6 Pro (33-00181EFS), and TH7000 (69-2668ES) documentation. Efficiency and “minimize resistance heat / ramp setpoints” guidance comes from DOE FEMP’s residential air-source heat pump page. Outdoor Aux lockout practice and the mild-weather strip-heat concern are reinforced by BPA’s PTCS auxiliary-heat control notes. I did not bench-test every Honeywell SKU; menu paths differ, so treat your model’s Resideo PDF as the final word for button presses.