Definition
Auxiliary heat is backup heat that a heat pump's thermostat turns on automatically when the heat pump alone cannot keep up, such as below the balance point, during a defrost cycle or after a large thermostat setback. In all-electric systems it usually comes from resistance heat strips in the air handler; in dual-fuel systems, from a gas furnace.
Also called: Aux heat, Backup heat
Why it matters to you
Electric resistance strips deliver one unit of heat per unit of electricity, while a heat pump in mild weather delivers several, so aux heat running for hours costs noticeably more to operate. Seeing AUX during a hard freeze or east-wind event is expected; seeing it on an ordinary 40-degree day is worth a check.
Practical guidance
Avoid deep overnight setbacks in heating season, since recovering several degrees at once is a common aux-heat trigger. If AUX runs often in mild weather, have a licensed technician check the heat pump's charge, defrost controls and thermostat staging.
About this definition
Written by the Home Energy Group team — a family-owned HVAC company celebrating 25 years of comfort & care across SW Washington and the Portland metro, licensed in Washington (WA LIC# HOMEEGI003J7) and Oregon (OR CCB# 184255). Definitions inform; an on-site diagnosis decides.
