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Likely cause: the gas is off or the tank is empty
A furnace can't make heat without fuel. A gas handle turned across the pipe, or an empty propane or oil tank, leaves the blower pushing cold air.
Why this happens
Carrier, Lennox and Trane each list the gas or oil supply among the causes of a furnace blowing cold air. HVAC.com explains that when the gas handle is perpendicular to the pipe, the gas is off. Lennox notes that most furnaces made since 2010 don't use a standing pilot light, while older ones may need theirs relit once fuel is restored.
How to fix it
- Propane or oil: call your fuel supplier to refill the tank. Ask them or an HVAC technician to restart the furnace if it doesn't come back on.
- Natural gas: if you know why the handle was turned off and there is no gas smell, turn it so it lines up with the pipe.
- If you don't know why it was off, leave it off and call an HVAC technician to check before the gas goes back on.
- Set the thermostat to Heat and give the furnace a few minutes. If it still won't heat, call a technician.
- If you smell gas at any point, leave the house and call 911 or your gas utility from outside.
When to stop. If your furnace has a standing pilot light that went out, don't try to light it. Call an HVAC technician.
If this does not fix it, tell the HVAC technician
- Whether the gas handle was off or the tank was empty, and for how long.
- Whether your furnace has a standing pilot light.
- Whether the furnace restarted once fuel was back.
Build the full list: what to tell the HVAC technician.
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