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UNITED STATES · SIZING & EQUIPMENT

Auxiliary heat: understand a heat pump’s winter electricity use

By SAVPWR Editorial · Published · 2 min read

For U.S. residential systems. General education, not a site diagnosis or engineering certification.

THE DIRECT ANSWER

Why does the thermostat show auxiliary heat?

Auxiliary heat supplements the heat pump when the system or controls call for it. Occasional use can be normal; persistent use or an unexplained bill increase deserves a controls and performance check.

Separate normal support from a fault claim

Supplemental heat can operate when compressor capacity is insufficient or during particular control sequences, including some defrost operation. The display alone does not prove a broken heat pump. Record outdoor conditions, setpoints and how long the indication remains present. [1]

Identify the actual backup type

Electric resistance, a gas furnace and other backup sources have different operating costs and control requirements. Ask for the installed heater size and documented settings. Do not infer backup energy consumption simply from the thermostat’s symbol or stated run time. [1][2]

Avoid changing installer settings by guesswork

Deep setbacks or recovery behavior can affect supplemental operation, but the right settings depend on the equipment. Request a qualified review of staging, lockouts and capacity if use seems excessive. Do not disable freeze protection or disconnect backup based on a generic online tip.

PUT IT TO WORK

Decision exercise

A hypothetical 10-kW electric heater operating continuously for one hour consumes 10 kWh, costing $2.40 at $0.24/kWh. A thermostat hour that cycles the heater or uses fewer stages is different. This example explains the scale; it does not estimate your actual auxiliary consumption.

Any numbers in this exercise are illustrative assumptions, not product performance, an installed quote or a personal savings estimate.

What to bring to the conversation

  • Record the backup type and rated input.
  • Keep a brief weather, setpoint and display log.
  • Request a professional controls review before changing lockouts.

One more useful distinction

Should I switch to emergency heat to save electricity?

Do not assume that. Emergency mode can change which heat sources run; use it only as directed for the actual system and situation.

Sources and verification

Technical statements draw on the linked references. Decision exercises and comparison frameworks are SAVPWR’s own explanatory examples. Current rules and program availability should be rechecked before a purchase.

  1. DOE Building Science Education / CEE — You Installed an Air Source Heat Pump. Now What?Source checked 2026-11-09 · External primary reference
  2. PNNL — Cold Climate Heat Pump Sizing and SelectionSource checked 2026-11-09 · External primary reference

AI-assisted preparation; checked against public sources. These articles have not received a licensed HVAC professional’s review. Read our editorial policy.

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