Why Does My Heat Pump Auxiliary Heat Stay On in St. Petersburg, FL?

What 'Aux Heat' actually means on your Florida thermostat

In St. Petersburg and across Pinellas County, the overwhelming majority of homes rely on electric heat pumps rather than natural gas furnaces. Heat pumps are exceptionally efficient at cooling and heating by moving heat instead of generating it. However, when outdoor temperatures drop below 50°F on cold winter mornings, your system engages its secondary stage: Auxiliary Heat (Aux Heat)—high-draw electric resistance coils located inside the air handler.

Auxiliary heat is designed to run only temporarily to bridge the gap until the compressor reaches your thermostat setpoint. But if you see 'AUX HEAT' or 'EM HEAT' illuminated on your thermostat for hours during the day, your heating elements are drawing 10,000 to 15,000 watts of continuous electricity—causing Duke Energy power bills to skyrocket.

Top causes of stuck auxiliary heat in St. Pete homes

1. Stuck or welded heat sequencer: The heating sequencer is an electromechanical relay that stages resistance coils on and off. Due to Florida salt air and electrical wear, sequencers frequently weld shut, keeping the heat strips energized even when the thermostat calls for off.

2. Stuck reversing valve on the outdoor unit: In heating mode, the heat pump's four-way reversing valve must shift refrigerant flow so the indoor coil becomes hot. If the reversing valve solenoid fails or the slider sticks, the outdoor unit cannot produce heat, forcing the system into permanent emergency backup heat.

3. Severely clogged air filter or restricted return: Restricted airflow prevents heat strips from dissipating warmth into the living space. The system trips its high-limit safety thermostat and struggles to warm the house, keeping auxiliary heat active in a perpetual loop.

4. Outdoor condenser defrost board failure: During cold, humid winter nights in Tampa Bay, frost forms on the outdoor coils. If the defrost board or sensor fails, the unit never initiates defrost, icing over completely and locking the thermostat into emergency heat.

How to safely test and stop electric bill spikes

Check your thermostat display first. If it is set to EM HEAT (Emergency Heat), switch it back to Heat. Emergency heat manually bypasses the efficient compressor and runs purely on expensive heat strips.

Next, verify that warm air is blowing from your supply grilles and step outside to see if the outdoor compressor is running and the fan is turning. If the outdoor unit is silent while the indoor blower runs continuously with lukewarm air, your compressor or reversing valve has locked out.

Do not let auxiliary heat run continuously for days—this can overheat wiring, melt limit switches, and run up hundreds of dollars in electrical charges. Cool Breeze provides 24/7 emergency St. Petersburg heater and furnace repair with upfront pricing and zero overtime fees. We test sequencer amp draw, diagnose reversing valves, and help you evaluate high-efficiency heat pump upgrades with utility rebates. Call 727-954-3605 or book online in St. Pete.