Aux Heat Locks Out When It Should Engage Balance Point Decision Tree
Why this matters
A heat pump that will not bring on auxiliary strip or backup heat when the house is falling behind on a cold day leaves the customer cold while the compressor labors alone past its useful range. This is an aux-heat lockout that engages too high or never lifts when it should. The usual cause is the balance-point or outdoor-temperature lockout logic: an aux-heat lockout set too low, a defective outdoor sensor reading warm, a thermostat misconfigured for the equipment, or a wiring fault that never energizes the strips. Get this wrong and either the customer freezes (aux locked out when needed) or, if you over-correct, pays for strip heat the heat pump could have avoided. This tree isolates the lockout-logic and wiring causes so aux engages exactly when the load and balance point require it, and not before.
Symptom presentation
On a cold day the space drops several degrees below setpoint and stays there. The compressor runs continuously, supply air is lukewarm, and the auxiliary heat never energizes (no aux indicator, no rise in supply temperature, strips cold to a clamp meter). The house cannot recover because the only heat source is a heat pump below its balance point with no backup assist. In a milder variant, aux engages eventually but far later than the load warranted, after the space has drifted well below setpoint, because the lockout or staging logic delays it too long.
Quick checks before diagnosing
Confirm the strips themselves work. Energize aux directly (force emergency-heat mode) and confirm the elements draw current and raise supply temperature. If the strips are dead, this is an element, sequencer, contactor, or fuse problem, not a lockout-logic problem; inspect the strip heat package.
Read the aux-heat lockout temperature and any outdoor-temperature lockout for compressor or aux. Note whether the thermostat is configured for a heat pump with the correct stage count and whether an outdoor sensor is installed and assigned.
Verify the outdoor sensor reading against actual ambient. A sensor reading warmer than reality (open or shorted reading high, or poor placement near a heat source) holds aux locked out because the control thinks it is mild outside.
Isolation tree
Branch 1: aux-heat lockout set too low.
- Thermostat aux lockout set below the outdoor temperature where the heat pump runs out of capacity (the balance point): aux is forbidden in exactly the conditions it is needed. Reset the lockout above the measured balance point, or remove it if the balance-point math does not support one. Determine the balance point from the capacity curve and the Manual J load, then set the lockout accordingly.
Branch 2: outdoor sensor fault.
- Outdoor sensor open, shorted, miswired, or in a warm location: the control reads a false high ambient and locks out aux (and may also misbehave on compressor lockout and defrost economy). Measure sensor resistance against the OEM curve, move it to shaded outdoor air away from condenser discharge and building heat, and replace if out of tolerance.
Branch 3: thermostat configuration wrong for the equipment.
- Thermostat set as a conventional system or with the wrong heat-pump stage count, so the aux stage is never properly assigned or is mapped to a stage the control never reaches: aux cannot be called. Reconfigure the thermostat for a heat pump with the correct compressor and aux stage assignment, and confirm the reversing-valve orientation (O versus B) so the rest of the logic is coherent.
- Droop or staging differential for aux set so wide that the space must fall implausibly far before aux is permitted: tighten the aux staging differential to a sensible value so aux assists before the house is uncomfortable.
Branch 4: wiring fault to the aux stage.
- W2/E/aux conductor open, on the wrong terminal, or never reaching the strip-heat contactor: the thermostat commands aux but nothing energizes. Trace the aux call from the thermostat terminal to the air-handler strip controls; confirm continuity, correct termination, and that the call energizes the sequencer or contactor.
Branch 5: balance-point logic is actually correct.
- Aux is locked out at a temperature where the heat pump genuinely still meets the load, and the real complaint is a heat-pump capacity or charge problem masquerading as an aux-lockout issue. Before raising the lockout and paying for unnecessary strip heat, verify the heat pump is delivering rated capacity: airflow, charge by weight against the heat-mode table, defrost behavior, and supply rise. A correctly performing heat pump above its balance point should not need aux; if it is short on capacity, fix the heat pump rather than masking it with premature strips.
Confirming the diagnosis
After correcting the lockout setting, sensor, configuration, or wiring, recreate a load that should call aux (cold morning recovery or a forced low setpoint at cold ambient) and confirm aux energizes at the intended point: aux indicator on, strip current drawn, supply temperature rising into the comfortable range, and the space recovering to setpoint. Confirm aux does not run when the heat pump alone is meeting the load above the balance point. Verify across the relevant ambient band that aux engages above the balance point and stays off below it where the heat pump suffices.
Remediation summary
| Finding | Root cause | Action |
|---|---|---|
| Aux forbidden below balance point | Lockout set too low | Set lockout above measured balance point |
| Control thinks it is mild out | Outdoor sensor fault | Verify curve, reposition, replace sensor |
| Aux never assigned | Thermostat config wrong | Reconfigure for heat pump, correct stages |
| Aux called, nothing energizes | Wiring or contactor fault | Trace aux circuit to strip controls |
| Heat pump short on capacity | HP charge or airflow fault | Fix heat pump, do not mask with strips |
Do not defeat an aux-heat lockout by removing all lockouts and letting strips run freely. That can leave resistance heat running far above the balance point and double the customer's bill. Set the lockout from the measured balance point and a correct load calculation, not by guesswork.
References
- ACCA Manual J - Residential Load Calculation (heating design load).
- ACCA Manual S - Residential Equipment Selection (balance-point determination).
- AHRI Standard 210/240 - Performance Rating of Air-Source Heat Pump Equipment (low-temperature capacity).
- Honeywell/Resideo T-Series Heat Pump Installation Guide - aux lockout, outdoor sensor, and stage configuration.
- Ecobee Installation and Settings Guide - compressor and aux lockout temperatures and reversing-valve orientation.