Why Is My Heat Pump's Auxiliary Heat Not Working?

A heat pump is efficient because it moves heat rather than creating it. But on the coldest days, moving heat is not enough — and that is when the system switches on its auxiliary, or backup, heat. If your heat pump runs but blows cool air when the temperature drops, the auxiliary heating coils are a prime suspect.

How Auxiliary Heat Works

Auxiliary heat comes from electric resistance heating coils built into the air handler or fan coil unit. When the thermostat decides the heat pump alone cannot keep up — or when you switch to emergency heat — current flows through these coils and converts directly into warmth. Like any resistance element, they wear out over time — much the same way kiln elements do, as covered in Signs Your Kiln Elements Are Failing.

Common Causes When Auxiliary Heat Fails

A few culprits account for most failures:

A burned-out heating coil. Years of heat cycling eventually cause the resistance wire to thin, sag, and break. A broken coil produces no heat at all.

A failed sequencer or relay. These components stage the coils on and off. If one fails, the coil never receives power even though it is otherwise fine.

A blown fuse or tripped breaker. Heat strips draw heavy current and are protected by their own fuses. A blown fuse cuts the circuit.

A tripped high-limit switch. A safety switch shuts the coil down if it overheats — often a sign of restricted airflow from a dirty filter or blocked coil.

How to Tell If the Coil Is the Problem

With the power safely off, a technician can check the heating element for continuity. A healthy coil shows a continuous, measurable resistance; a failed one reads open. Visible breaks, heavy sagging, or burned spots on the coil confirm it. If the fuses, sequencer, and switches all test good but the coil reads open, the element itself needs replacing.

Replacing the Element

Heat pump heating coils are resistance wire wound to a specific wattage and voltage — see Understanding Kiln Voltage and Amps for why the voltage match matters so much. A replacement must hit those values exactly so the system heats as designed. Reliable Heating Coil winds custom replacement coils for heat pump auxiliary and backup heat, built to your equipment's original specifications and shipped fast. Request a quote — because losing your backup heat in cold weather is not something that can wait.