Glossary · Heating, cooling & ventilation
Auxiliary / backup heat
Also called: auxiliary heat · aux heat · supplementary heat · supplemental heat · emergency heat · em heat
A second heat source — electric elements or a furnace — that helps a heat pump in very cold weather or during defrost.
What it means
When it's colder than the heat pump's balance point, or while the outdoor unit defrosts, auxiliary heat fills the gap. It can be electric resistance elements in the air handler or baseboards, which can run alongside the heat pump, or a furnace or boiler, which runs instead of it (see dual-fuel). 'Emergency heat' is a thermostat setting that runs only the backup.
Why it matters in an audit
Electric backup heat costs much more to run than the heat pump, so good sizing, controls and thermostat settings that avoid triggering it are key to real savings.