Aux Heat Check
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Understand your
backup heat.

Read the thermostat message. Check the context. Know what to do next.

Check my situation Estimate strip-heat cost
Generated winter-home illustration showing an outdoor air-source heat pump and a small indoor thermostat.
Generated illustration, not a measured system or a verified installation.

Aux heat

Automatic backup when your system needs it.

Emergency Heat

A separate mode that uses configured backup heat.

No heat or a possible electrical hazard? Get appropriate professional help.

Start with what you see.

A message is a clue, not a diagnosis.

Your thermostat message

Units

Context only. Outdoor temperature does not select a universal lockout.

Runs in your browser. No answers are sent or saved.

A reason to check, not a diagnosis.

Start with the exact message and what you can safely observe. Unknown answers stay unknown.

  1. A possible hazard comes first.
  2. A home that keeps getting colder deserves professional attention.
  3. Model-specific sources help you ask a better question; they do not select a universal setting.
When to stop and get help

Same backup source.
Different control modes.

Aux heat on a heat-pump thermostat can be an ordinary automatic response. Emergency Heat is a separately selected backup mode in the manufacturer configurations described here. Read the full mode before assuming either a failure or normal operation.

Google’s Heat Pump Balance guidance describes cold conditions and defrost as reasons AUX can appear. The Honeywell Home T10/T10+ guide explains that Emergency Heat locks out the heat pump. Configured fuel and equipment still matter.

If the home keeps getting colder, ice persists or a warning continues, do not use possible defrost as reassurance. The checklist offers a next step and keeps unknown observations unknown.

Understand the two modes ↗
Generated illustration of two fictional thermostat labels: Heat with Aux, and Emergency Heat.
Generated labels, not manufacturer screenshots. The cited configuration determines what each mode does.

Heat mode

Heat pump permitted

Configured Aux may supplement as needed.

Emergency Heat

Heat pump locked out

Configured backup heat operates instead.

Authored comparison of the cited Honeywell Home heat-pump/backup configurations. Source: T10/T10+ Advanced Features. Not every thermostat label or installed fuel strategy is identical.

Cool air from the vents? Read the context and help guide.

Know the cost of the electric strips.

Your kW. Your rate. Your running time.

10 kWExample strip power$0.16/kWhExample energy rate1 hourAt that power$1.60Strip-energy cost
Worked example, not your bill. kW × USD/kWh × hours = USD. Compressor, blower, fixed fees and other use are excluded.

Use your own figures

Worked example

$1.60

10 kW × USD 0.16/kWh × 1 hour

Illustrative strip-energy cost only. Enter your own figures; this is not a calculated result for your home.

Calculation and limits
Generated closed air-handler exterior with a fictional plate reading Example only, Electric heat kit, 10 kW.
Generated illustration, not a real equipment label. Never use this fictional 10 kW value as your equipment input.

Use a relevant actual rating, with its voltage and staging conditions. If it is not safely accessible in the documentation, ask your installer. This fictional label is not a recommended power or a real nameplate.

For electric strips only. Fixed fees, compressor, blower, other use and changing tariffs are excluded. A thermostat’s Aux runtime does not establish full-power strip operation. See the full calculation limits.

Find the guide for your thermostat.

Match the exact model and the source document. A setting name is a way to ask a better question, not a universal value to copy.

ecobee EnhancedEnhanced guide, Thresholds and reading a recurring Aux alert.Google NestHeat Pump Balance paths by generation and app.Honeywell HomeT6 Pro Smart, T6 Pro II, T10/T10+ and T9 scope kept separate.Daikin One+Unitary-equipment guide, two lockouts and their different purposes.
Aux vs Emergency HeatCold air and when to ask for helpSources and method

Source: https://aux-heat-check.pages.dev/