Air Conditioner Not Heating — thermostat

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

1. Check 1

   Read the equipment labels and thermostat menu from a normal user position, and note whether the control offers Heat, Aux, or Emergency Heat, whether a separate furnace or boiler exists, and whether the outdoor unit is described as a heat pump; do not open equipment covers or inspect internal terminals.

   Expected result: The home has a cooling-only air conditioner with a separate heater, a heat pump with backup heat, or the system type remains unknown.

   Next step: Use the exact thermostat and equipment manual for the identified system. If the type is unknown, give the model information to an HVAC professional rather than changing setup options.

   Stop if: Stop if identification would require removing a panel, touching wiring, entering a restricted equipment area, or handling a gas appliance.

   Why this matters

   Why: A cooling-only unit is not expected to heat. A heat pump uses the thermostat to coordinate heating mode and may have a backup source, while an unknown configuration should not be guessed from the outdoor cabinet alone.

2. Check 2

   At the thermostat, confirm the system mode is Heat rather than Cool or Fan, set the target above the current room reading, choose Auto fan unless the manual says otherwise, and cancel an active schedule, away state, or delay only through the documented user controls.

   Expected result: The thermostat was in Cool or Fan, the target was not calling for heat, a schedule or delay was active, or the display shows a heat call but no equipment response.

   Next step: Set one normal Heat request according to the manual and allow the system's documented response period. If the system still does not respond, stop changing settings and arrange HVAC service.

   Stop if: Stop for exposed or frayed thermostat wiring, a wet or cracked control, a burning smell, smoke, or an error that returns after the manual's reset.

   Why this matters

   Why: A mode, target, schedule, fan, or normal equipment delay can look like a heating failure. A valid heat call with no response shifts attention to power, communication, safety controls, or the HVAC equipment.

3. Check 3

   Observe whether the thermostat screen is bright, blank, flickering, or showing a low-battery or communication alert; if the model uses user-replaceable batteries, replace them only as the manual specifies and keep the thermostat mounted without disturbing its wiring.

   Expected result: The screen is blank or unstable, a battery alert is present, the screen is normal but the indoor unit does not respond, or a recurring communication or system alert appears.

   Next step: Use the exact manual for the alert and perform no more than one documented restart if it is allowed. If the screen remains blank, the alert returns, or the unit remains unresponsive, arrange HVAC service; do not remove the wall control or bridge terminals.

   Stop if: Stop for exposed wiring, a loose wall plate, a hot or damaged control, smoke, burning odor, or any instruction that requires touching terminals.

   Why this matters

   Why: A battery or control-power issue can prevent a heat call, while a normal screen with no equipment response can indicate a system power, control-board, safety, or communication fault. An alert is model-specific.

4. Check 4

   With the system off through its normal user control, inspect the accessible air filter and supply or return grilles; replace a visibly loaded filter with the exact type and orientation specified by the equipment manual, and make sure furniture or curtains are not blocking airflow.

   Expected result: The filter is heavily loaded, airflow is weak at several grilles, airflow is normal but the air is not warming, or the indoor fan does not run.

   Next step: After a safe filter change, make one normal Heat request. If airflow remains weak, the fan does not run, or the system still cannot warm the room, leave it for HVAC diagnosis rather than opening the air handler.

   Stop if: Stop for water around the indoor unit, a frozen coil, burning odor, smoke, unusual noise, a hot electrical connection, or any need to reach the blower or wiring.

   Why this matters

   Why: Restricted airflow can make a heat pump deliver weak or lukewarm air and can trigger protective shutdowns. Normal airflow with no heat points more toward mode, outdoor operation, refrigerant, reversing, backup heat, or control faults.

5. Check 5

   From a safe distance, observe the outdoor heat-pump unit during one normal Heat request and note whether it runs, is covered by light frost that clears, has thick ice, is blocked by leaves or snow, or remains silent while the thermostat calls for heat; do not remove guards, scrape ice, or touch refrigerant lines.

   Expected result: The outdoor unit runs and later warms the home, briefly enters a defrost-like cycle, has thick persistent ice, is obstructed, or does not run.

   Next step: Clear only loose debris that is plainly outside the unit and safe to reach, without touching the cabinet or coil. For thick ice, no operation, or recurring cold air, stop the heating request and arrange qualified HVAC service.

   Stop if: Stop for thick ice, water near electrical equipment, smoke, burning odor, loud mechanical noise, repeated breaker operation, or any need to remove a cover or touch a coil or line.

   Why this matters

   Why: Brief defrost behavior can be normal for a heat pump. Persistent ice, no outdoor operation, or continued cold air can indicate airflow, power, refrigerant, reversing, defrost, or control trouble that a homeowner cannot assign to the thermostat alone.

6. Check 6

   If the thermostat explicitly offers Emergency Heat or Aux Heat and the exact system manual identifies it as applicable, select the documented backup mode only when the primary heat pump is unavailable; note whether backup heat starts and return to normal Heat when qualified service directs it.

   Expected result: The backup mode is unavailable, starts heating, starts but cannot maintain temperature, or produces an alert, unusual smell, repeated cycling, or no response.

   Next step: Use backup heat only as a short-term, manual-approved measure and arrange HVAC service for the primary heat-pump fault. If backup heat behaves abnormally, turn the system off and stop troubleshooting.

   Stop if: Stop for gas odor, smoke, burning odor, abnormal heat, electrical noise, repeated cycling, a tripped protective device, or any need to alter thermostat terminals or HVAC controls.

   Why this matters

   Why: Backup heat is a system-specific secondary source, not a universal thermostat fix. It may provide temporary comfort when a heat pump is unavailable, but a failed or abnormal backup call can involve electrical, gas, control, or safety equipment.

7. Check 7

   Prepare a service note with the thermostat model, equipment type, display state, Heat/Aux/Emergency Heat options, setpoint and room reading, schedule or alert, battery result, filter and airflow observation, outdoor-unit state, and whether cooling still works; leave wiring, refrigerant, reversing, gas, and high-voltage diagnosis to a qualified HVAC technician.

   Expected result: Safe thermostat and airflow checks do not restore heat, only backup heat works, cooling works but Heat does not, or the thermostat calls while the equipment is idle or repeatedly cycling.

   Next step: Keep the system in the safe state identified by the manual and request HVAC service. Provide the exact sequence and avoid replacing a thermostat or changing its wiring until compatibility and the control fault are confirmed.

   Stop if: Stop all user diagnosis for exposed wiring, refrigerant work, gas equipment, frozen coils, water near electrics, smoke, burning odor, or repeated protective-device operation.

   Why this matters

   Why: The remaining possibilities include an incorrect configuration, sensor or thermostat failure, control communication, reversing valve, defrost, refrigerant, power, auxiliary-heat, or equipment fault.