No Heat But Electric Bill High: Aux Strip Lock-On Cross System Decision Tree

Why this matters

"My heat pump is not really heating and my electric bill doubled" is the classic auxiliary-strip lock-on complaint. The compressor side has quit or is being bypassed, the electric resistance strips are carrying the whole load, and the strips draw several times the power of the compressor for the same heat. The strips keep the house lukewarm, so the customer feels under-heated while the meter spins. The cause can be a failed compressor circuit, a defrost or reversing-valve fault, a thermostat/control wiring error forcing emergency heat, or an outdoor unit that simply is not running. This tree isolates whether the heat pump is making heat at all and why the strips are locked on, so you restore the efficient compressor heat and stop the runaway bill.

Symptom presentation

Heat pump system delivers weak/lukewarm heat and the electric usage is far above normal for the weather. The aux/emergency heat indicator may be on. Supply air is warm but not hot, or the outdoor unit is not running while the indoor strips are.

Variants:

• Outdoor unit not running at all, strips carrying everything: compressor/contactor/control or a stat in emergency heat.
• Outdoor unit runs but blows cold/neutral, strips covering it: reversing valve, low charge, or defrost stuck.
• Strips energize on every call even with the compressor running fine: thermostat staging/wiring forcing aux, or a balance-point/lockout misconfiguration.
• Strips run continuously, never cycling with demand: a stuck sequencer/relay or a control calling W constantly.

Quick checks before the meter panel

Read the thermostat mode and staging. Confirm it is not set to Emergency Heat (which locks out the compressor and runs strips only). Confirm the heat-pump configuration, aux/emergency wiring, and any outdoor-temperature lockout or balance point are set correctly. A stat miswired so W (aux) energizes with or instead of Y (compressor) is a frequent cause.

Confirm the outdoor unit is actually running on a heat call. If the compressor and outdoor fan are off while the house calls for heat, the strips are the only heat source and the bill climbs. Determine why the outdoor unit is not running before anything else.

Check defrost state and supply temperature. A heat pump stuck in defrost, or with a reversing valve in the wrong position, blows cool air and the strips mask it. Read supply-air temperature on a steady compressor heat call (should be meaningfully above return).

Isolation tree

Branch 1: outdoor unit not running.

• No Y to the contactor or contactor not pulling in: trace the heat call through the stat, board, and any pressure/defrost controls. Correct miswire, replace a failed contactor, or address a tripped safety.
• Compressor electrical fault (open winding, failed capacitor, hard-start): diagnose and repair so the compressor carries the load again.
• Outdoor lockout active (low-ambient or fault lockout): confirm whether it is a legitimate balance-point lockout or a misconfiguration forcing strips too early.

Branch 2: reversing valve / wrong mode.

• Outdoor unit runs but the system is not actually in heat (reversing valve stuck or O/B control logic wrong), so supply is cool and strips fill in: verify reversing-valve operation and the thermostat O/B configuration. Correct the valve or the control logic so the system makes compressor heat.

Branch 3: defrost stuck or excessive.

• Unit stuck in defrost or defrosting far too often, running on strips most of the time: check the defrost control/sensor, coil condition, and airflow. A defrost board misreading or an iced coil keeps the system in defrost with strips carrying the load. Repair the defrost control or the airflow/charge cause of excessive frosting.

Branch 4: low charge / poor compressor heat.

• Compressor runs but makes little heat (low charge, restricted metering, airflow): the strips compensate and the bill rises. Verify charge by the proper method and airflow; correct so the compressor delivers full heat and the strips drop to backup duty.

Branch 5: aux staging / control forcing strips.

• Strips energize on normal calls even with good compressor heat: thermostat staging brings in aux too aggressively, a droop/differential is set wrong, or W is being called continuously. Reconfigure aux staging and balance point, and confirm aux only supplements below the balance point or on a real recovery demand.
• A stuck sequencer/relay or a shorted W line keeping strips energized: repair the stuck control or the wiring short so strips de-energize when not demanded.

Confirming the diagnosis

After the fix, run a heat call and confirm the compressor carries the primary load: outdoor unit running, reversing valve in heat, supply-air temperature well above return on compressor-only heat, and the aux strips energizing only when intended (below balance point or on recovery), not continuously. Clamp the strip circuit to confirm it de-energizes when not demanded. Verify defrost initiates and terminates correctly.

Document thermostat mode and staging, outdoor-unit operation, reversing-valve verification, supply/return temperatures, charge and airflow readings, balance-point/lockout settings, and a measurement showing the strips no longer lock on. Explain to the customer that restoring compressor heat is what brings the bill back down.

Remediation summary

References

• NFPA 70 (National Electrical Code), Article 424 - fixed electric space-heating equipment, conductor and overcurrent requirements for resistance strips.
• ACCA Manual S / Manual J - heat-pump sizing and balance-point determination governing when auxiliary heat should engage.
• ACCA Standard 5 (HVAC Quality Installation Specification) - heat-pump and auxiliary-heat configuration and verification.
• AHRI Standard 210/240-2023 - heat-pump performance rating context, including defrost and low-temperature operation.
• Manufacturer installation and control instructions - reversing-valve (O/B) logic, defrost control, aux/emergency staging, and balance-point/lockout settings for the specific equipment.