The short answer
A furnace blowing cold air usually has a problem with ignition, the flame sensor, or the gas supply, so the blower runs but the burners never light. Check the thermostat and the filter first, then ignition and gas issues need a licensed technician.
When the furnace kicks on and the air coming out of the vents is cold, the blower is running but the burners are not lighting. In a Payne County January, that goes from uncomfortable to a frozen pipe risk in a hurry. The cause is almost always in the ignition system, the flame sensor, or the gas supply, and a few of those are safe to check before you call.
Start with the thermostat and the filter, since a thermostat set to ON instead of AUTO will run the blower constantly with no heat, and a dirty filter can trip the high limit and shut the burners off. If those are fine, the problem moves into gas and ignition, which is where a licensed technician takes over.
Safety comes first with any furnace issue. If you smell gas or a rotten egg odor, do not keep reading. Leave the home, leave the door open if you safely can, and call your gas utility right away. Natural gas suppliers add an odorant so you can detect leaks, and that smell is your signal to get out and let the utility handle it before any repair work starts.
Check these first
Check the thermostat. If the fan is set to ON, the blower runs all the time and pushes cold air between heating cycles. Set it to AUTO so the fan only runs when the burners are on. This single setting fixes a surprising number of cold air calls.
Safe to check yourselfSwap the air filter. A clogged filter can overheat the furnace and trip the high limit switch, which shuts the burners off but lets the blower keep running. A clean filter is the first fix and protects the furnace from damage.
Safe to check yourselfCheck the pilot or ignition. If you have an older unit with a standing pilot and the pilot is out, the burners will not light. Relighting a pilot is a homeowner task on older units, but if it will not stay lit, the thermocouple may be bad and need a pro.
Safe to check yourselfDirty flame sensor. On modern furnaces, a dirty flame sensor shuts the burners off because it cannot confirm the flame. Cleaning or replacing the sensor is a quick job for a licensed tech and one of the most common winter repairs we make.
Needs a licensed proGas supply or valve. If the burners will not light at all, the gas valve may be off or faulty, or the gas supply is interrupted. This needs a licensed technician and, if you smell gas, your gas utility.
Needs a licensed proCracked heat exchanger or limit issues. If the furnace short cycles and blows cool, the heat exchanger or a safety limit may be the cause. These are serious and need a licensed tech right away.
Needs a licensed proIf you smell gas, do not try to fix anything. Leave the home, leave the door open if you safely can, and call your gas utility right away. Then call us to inspect and repair the furnace once it is safe. Never open the gas valve or ignition system yourself. Call (405) 338-9194 for any ignition or gas issue.
Call (405) 338-9194Going deeper
The blower and the burners are two separate systems in your furnace. The blower moves air across the heat exchanger and out to the rooms. The burners heat the exchanger. When you feel cold air, the blower is working but the burners are not, so the air never gets warm. The question is why the burners are not firing.
On older units it is often the pilot or thermocouple. On newer units it is usually a dirty flame sensor that cannot confirm the flame, a faulty igniter, or a tripped limit switch from a dirty filter. Gas supply problems are less common but more serious. A licensed technician can sort out which one it is quickly and safely, and our work carries a 1-year workmanship warranty.
In the Stillwater area, the cold snap repairs we make most often are dirty flame sensors and tripped high limit switches. A flame sensor builds up a thin layer of oxidation over a season, and once it cannot sense the flame, the furnace shuts the gas off as a safety measure. A dirty filter causes the same shutdown through the limit switch. Both are quick fixes once diagnosed, and a fall tune up catches the flame sensor before the first cold night catches you.
Watch a tech walk through it
Common questions
The blower is running but the burners are not lighting. Common causes are a thermostat fan set to ON, a dirty filter tripping the limit, a dirty flame sensor, or a gas supply issue.
AUTO. With the fan set to ON, the blower runs constantly and pushes cool air between heating cycles. AUTO runs the fan only when the burners are on.
The flame sensor confirms the burner flame is lit. If it is dirty, the furnace shuts the burners off as a safety measure, so the air goes cold. Cleaning or replacing it is a quick job for a licensed tech.
Leave the home, leave the door open if you safely can, and call your gas utility right away. Then call us to inspect and repair the furnace once it is safe.
It can be in a cold snap, since a cold house risks frozen pipes. We answer 24/7. Call (405) 338-9194 and we will get a licensed technician moving.
Call (405) 338-9194 any time, day or night. A real person answers and we get a licensed technician moving across Stillwater and the towns around it.
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