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Heating Troubleshooting

Furnace Blowing Cool Air

Learn why a furnace may blow cool or room-temperature air, including fan settings, ignition failure, limit trips, heat-exchanger warm-up and control problems.

What It Can Mean

Start with the symptom, then narrow the cause

A short burst of cool air at the beginning or end of a normal furnace cycle can be expected because the ductwork and heat exchanger need time to warm or cool.

Continuous cool air during a call for heat is different. The blower may be operating while the burners are not producing heat, or the furnace may have shut the burners down on a safety while leaving the blower running.

Thermostat fan settings can also make the blower run between heating cycles.

Possible Causes

Common conditions associated with this symptom

Fan set to ON

The blower may circulate room-temperature air when no heat call is active.

Ignition failure

The blower may operate even though the burners never establish flame.

High-limit trip

If the furnace overheats, burners may shut off while the blower continues to cool the heat exchanger.

Flame-sensing problem

Burners can light briefly and then shut down.

Control or sequencing issue

A board, relay or wiring problem can affect when the blower and burners operate.

Heat-pump system behavior

If the home uses a heat pump rather than a furnace, normal supply air may feel less hot than gas-furnace air.

Important

Do not diagnose by symptom alone

Several different HVAC faults can produce the same homeowner symptom. Proper diagnosis may require electrical readings, refrigerant measurements, airflow testing, static-pressure measurements, combustion testing or control-sequence checks.

Do not bypass safeties, open sealed refrigerant circuits, work on gas piping or perform live electrical troubleshooting unless you are qualified to do so.

Call for Service

Professional evaluation is appropriate when

  • The furnace never establishes flame.
  • Burners shut off while the blower continues for long periods.
  • You smell gas or combustion products.
  • The furnace repeatedly trips or displays a fault code.
  • The home temperature continues to fall.
Related Troubleshooting

Similar symptoms and system topics

Need Help?

Describe what the system is doing

When requesting service, include the equipment type, approximate age, when the problem started, whether the system still operates and any unusual noise, water, odor or thermostat behavior you have noticed.

Request HVAC Service Back to Troubleshooting Center
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