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Oh no! The temperature has dropped and your furnace is suddenly blowing cold air. There are several common causes for this, and some you can fix yourself. Before placing a service call, check out these troubleshooting tips.
When the thermostat is set to ON, the fan blows continuously, whether or not the furnace is heating the air. To prevent this, switch the thermostat to AUTO and the fan will only blow when the furnace is producing heat.
A clogged or dirty air filter will eventually cause the furnace to overheat. A furnace safety feature, the limit switch, turns the off the furnace burners so no more heat is being generated but the fan continues to operate until the unit has cooled to a safe level. Overheating is potentially a serious problem. If the furnace repeatedly overheats, the heat exchanger will be damaged and this is very expensive to fix. The easiest and least expensive way to correct or prevent this problem is to change the air filter every 30-60 days.
The pilot light may have gone out due to a loss of power or other simple reason. In this case, it just needs to be relit. If the flame won’t ignite, however, the problem could be the flow of gas to the furnace. Make sure the gas valve switch is in the ON position and that the pilot light is clean.
If the pilot light still won’t go on or stay on, the problem could be the thermocouple, the metal rod that detects the flame and signals the gas supply to flow (or to turn off if no flame is detected). If the thermocouple is damaged or faulty, it may be unnecessarily stopping the gas flow to the the pilot light. This repair will most likely require professional service.
Cold air in your attic or crawl space can get into your ductwork through holes and leaks, and ultimately into your living spaces. This can give the impression that the furnace is not working. These leaks should be repaired to improve your comfort and also to keep your furnace from working extra hard to compensate.
An inadequate gas supply can cause your furnace to shut down for safety reasons.
Dirt and grime collecting on the burner can prevent ignition.