Heat Pump Not Heating: Check Mode, Airflow, Defrost, and Power Before Service

Use only checks that match the exact appliance or system and its official guide.

1. Check 1

   Check the thermostat without opening it. Confirm the system mode is Heat, the target temperature is above the room temperature, and the fan is set to Auto unless your equipment manual specifies another setting. Note whether the thermostat shows a defrost message, a heat call, a low-battery warning, or an error.

   Expected result: The thermostat may be in Cool, have the fan set to On, show an active defrost cycle, or be correctly calling for heat. The room temperature may be slowly rising, unchanged, or falling.

   Next step: Correct only the clearly wrong mode, setpoint, or fan setting, then allow one normal heating cycle. If the thermostat is correct or the symptom persists, continue to the filter and power checks.

   Stop if: Do not remove thermostat wiring or bypass controls to force a heat call.

   Why this matters

   Why: A wrong mode can make a working heat pump appear not to heat, and Fan On can circulate unheated air between cycles. A brief cool period during defrost can be normal; an unchanged or falling room temperature after defrost keeps a fault open.

2. Check 2

   Inspect the accessible indoor air filter with the system stopped by its normal controls. Replace or clean it only according to the equipment instructions if it is visibly clogged; do not operate without a required filter. Check that supply and return grilles are open and not covered by furniture.

   Expected result: The filter may be clean, loaded with dust, damp, or the wrong size. Several grilles may be blocked, or airflow may be normal at the accessible registers.

   Next step: Correct only the accessible filter or grille issue, then observe one heating cycle. If airflow remains weak or the home stays cold, proceed to power and outdoor-unit checks and prepare for service.

   Stop if: Do not remove blower or coil panels or reach into the air handler.

   Why this matters

   Why: A clogged filter or blocked grille restricts airflow and can make a heat pump deliver weak or lukewarm air or shut down. A clean filter and open grilles do not rule out hidden duct, indoor blower, coil, or outdoor-unit faults.

3. Check 3

   Look at the electrical panel and the normal equipment power switches from a dry, safe position. Identify whether the indoor and outdoor equipment have lost power or whether a breaker is in a tripped or middle position. Reset a clearly tripped breaker only once according to the panel and equipment instructions; do not remove covers or replace a breaker.

   Expected result: Both sections may have power, one may be off, or a breaker may trip again as soon as the system calls for heat. A normal breaker position does not prove the unit is receiving power.

   Next step: If power was interrupted and the breaker remains on, allow the system's normal delay and observe one cycle. If the breaker trips again, power cannot be confirmed, or there is burning odor, heat, sparking, or damaged wiring, stop and call a qualified professional.

   Stop if: Do not hold a breaker on, fit a larger breaker, remove panel covers, or touch wiring.

   Why this matters

   Why: Heat-pump indoor and outdoor sections can have separate power protection. A single interruption can explain no operation, but a repeat trip indicates an electrical or equipment fault rather than a setting problem.

4. Check 4

   Inspect the outdoor unit from outside the grille. Check whether leaves, snow, vegetation, or stored items restrict the airflow space, and whether the coil has light frost that clears or a thick ice coating that remains. Keep hands and tools away from the fan and coil; remove only loose debris that is clearly outside the cabinet.

   Expected result: The outdoor unit may be clear and running, obstructed by loose debris, lightly frosted during a defrost cycle, or covered in persistent ice. The fan may be running normally, stopped, or making an unusual sound.

   Next step: Clear only safe external obstructions and observe one normal cycle. If the coil remains heavily iced, the fan is abnormal, or heating does not recover, stop repeated operation and call HVAC service.

   Stop if: Do not chip ice, pour water on the coil, pressure-wash it, bend fins, or reach through the grille.

   Why this matters

   Why: Restricted outdoor airflow can reduce heating performance. Light frost that clears can be normal, but heavy ice, a stopped fan, or repeated refreezing can indicate low airflow, refrigerant, fan, or defrost trouble.

5. Check 5

   Compare the symptom across operating modes only if the system is safe, powered normally, and your controls support the change. Record whether it heats poorly but cools normally, fails in both modes, or has recently changed behavior after a service or thermostat change. Do not repeatedly switch modes to provoke a fault.

   Expected result: Cooling may work while heating does not, both modes may be weak, or neither mode may run. The outdoor coil may also show ice, and the thermostat may show a recurring fault.

   Next step: Give the mode comparison, filter and airflow results, power behavior, ice condition, and any codes to a qualified HVAC professional rather than opening the system.

   Stop if: Do not test refrigerant pressure, disconnect tubing, bypass controls, or open electrical compartments.

   Why this matters

   Why: Heating-only failure keeps the reversing valve or heat-side controls in the diagnostic path. Weak operation in both modes keeps airflow, power, refrigerant, and broader system faults open; the observation does not identify a replaceable part.