The first cold snap of the season is when we hear it most: “The heat is on, but the air is cold.” Here are the most common causes for a gas furnace, what you can check yourself, and how to reset it safely.
First, do you have a gas furnace or a heat pump? A gas furnace has a gas line and a metal or white plastic vent pipe. A heat pump has an outdoor unit that runs in winter. If you have a heat pump, see our guide to a heat pump not heating instead.
1. The fan is set to On
With the thermostat fan on On, the blower runs all the time, even when the burners are off. Between heating cycles you feel room temperature air, which feels cold. Set the fan to Auto.
2. The furnace is still warming up
A furnace lights its burners first and waits for the heat exchanger to warm before the blower starts. If the blower starts a little early, the first 30 seconds or so of air can feel cool. That is normal. Cold air for several minutes is not.
3. The air filter is clogged
A dirty filter makes the furnace run hot. A safety switch called the high limit then shuts the burners off, but the blower keeps running to cool things down. The result is cold air and a furnace that cycles on and off. Replace the filter and see if the problem goes away.
4. The burners light, then go out
If you hear the burners light and then shut off after a few seconds, the flame sensor may be dirty. It is a small metal rod that proves to the furnace that there is a flame. When it cannot sense the flame, the furnace shuts the gas off as a safety step. After several failed tries, most furnaces lock out for a while. Cleaning the sensor is a quick repair for a technician.
5. The ignitor has failed
Most furnaces built in the last 25 years use a hot surface ignitor that glows orange to light the gas. They crack and wear out. If you hear the inducer fan start, but the burners never light, the ignitor is a common suspect.
6. The gas supply is off
Check that the gas valve near the furnace is open (the handle runs in line with the pipe). If your water heater or stove also uses gas and is not working, the problem may be the gas supply to the house. Call your gas utility.
7. A clogged condensate drain (high-efficiency furnaces)
High-efficiency furnaces, the ones with white plastic vent pipes, make water. If the drain line or trap clogs, a pressure switch stops the furnace from lighting. You may see water near the furnace.
8. Leaky ducts
If the furnace is making heat but the air at the vents is only lukewarm, warm air may be leaking into the attic or crawlspace before it reaches your rooms. See signs your ductwork needs repair.
Should I turn my furnace off if it is blowing cold air?
If you smell gas, a rotten egg smell, do not touch the thermostat or any switches. Leave the house and call your gas utility or 911 from outside.
If there is no gas smell, it is safe to turn the heat off and try the steps above. If the furnace keeps failing to light, leave it off and call. Repeated restarts can hide a problem that needs a technician.
How to reset a furnace
- Set the thermostat to Off.
- Find the furnace power switch. It looks like a light switch on or near the furnace. Turn it off (or turn off the furnace breaker).
- Wait about 1 minute.
- Turn the power back on and set the thermostat to Heat, a few degrees above room temperature.
- Give it 5 minutes. You should hear the inducer fan, then the burners, then the blower.
Some furnaces have a reset button on the blower motor. Press it once at most. If the furnace locks out again, call us.
Signs of a dying furnace
- It is 15 to 20 years old or more
- Repairs are needed every season
- Gas bills keep climbing
- Some rooms are always cold
- The burner flame is yellow or flickering instead of steady blue
- New noises (see furnace noises explained)
- Rust, cracks or soot around the furnace
If several of these sound familiar, our guide to the $5,000 rule for repair or replace can help you decide.
Call North Alabama Heating & A/C at (256) 534-2941. We have served Huntsville, Madison and North Alabama since 1977, Monday to Saturday, 7 AM to 7 PM. See our furnace repair service.
Quick answers
Why is my furnace blowing cold air instead of heat?
Common causes are the thermostat fan set to On, a clogged filter tripping the high limit switch, a dirty flame sensor, a failed ignitor, the gas supply being off, or a clogged condensate drain on a high-efficiency furnace.
Should I turn my furnace off if it’s blowing cold air?
If you smell gas, leave the house and call your gas utility or 911 from outside. If not, it is fine to turn the heat off, replace the filter and try one reset. If the furnace keeps failing, leave it off and call a technician.
How do I reset my furnace?
Set the thermostat to Off, turn off the furnace power switch or breaker for about a minute, turn it back on, then set the thermostat to Heat a few degrees above room temperature. Allow about 5 minutes for it to start.
