1. Furnace Blowing Cold Air: The primary causes of a furnace not heating up are dirty filters, a pilot light that has blown out, or a malfunctioning thermostat. Additional factors may involve ignition problems, issues with the gas valve, the blower motor, or other internal components.
2. Heat Keeps Turning On and Off (Cycling): Frequent cycling, characterized by repetitive on-off heating, can result from a damaged fan limiter, a grimy flame sensor, or a faulty thermocouple. If you observe this pattern, it is advisable to contact a local heating and cooling expert.
3. Blower Fan Does Not Turn Off: Typically, the fan limit switch might be set to “manual override,” but the cause could also be a short-circuited wire in the thermostat or at the fan limit switch.
4. Noisy During Operation: A buzzing noise emanating from the furnace might point to an electrical issue. If a loud noise occurs during startup, there could be a condition called “delayed ignition,” a serious problem where an excessive amount of natural gas accumulates in the combustion chamber before igniting.
5. Carbon Monoxide Detector Going Off or a Sulfur Smell Around Furnace: Carbon monoxide, a byproduct of natural gas combustion in furnaces, is typically contained within the heat exchanger and then vented through the flue to the outdoors. Smelling a sulfur odor, excessive condensation on windows, soot stains, or smoke stains around the furnace might indicate a potential gas leak of carbon monoxide.