Why Is My Furnace Blowing Cold Air?
A furnace blowing cold air usually has the fan set to ON. Or a dirty filter has tripped its overheat safety. Or the burners are not lighting. Set the fan to AUTO and check the filter first. If the burners never light, the blower is pushing room air through the vents, and that needs a technician.
What makes a furnace blow cold air?
Thermostat fan set to ON
With the fan on ON, the blower runs between heating cycles and pushes unheated air out of the vents. It feels cold, and nothing is broken. Move the fan to AUTO so it runs only while the furnace is heating. It takes a minute and it is safe.
Safe to check yourselfDirty filter tripping the overheat safety
A blocked filter traps heat inside the furnace. A safety control shuts the burners off, and the blower keeps running to cool the furnace down. That is why you feel cold air part way through a cycle. Changing the filter is safe, and repeated overheating shortens the life of the furnace.
Safe to check yourselfDirty flame sensor
The flame sensor is a thin metal rod that proves to the furnace that the burners actually lit. When it gets coated, the furnace lights, cannot prove the flame, and shuts the gas off within seconds. The blower then pushes cool air. Cleaning it means opening the burner compartment.
Technician repairThe burners are not lighting
The ignitor is the part that glows red hot to light the gas. When it cracks, or the spark system fails, the burners never light and the furnace may try several times before it gives up. You may hear clicks and no flame. This is inside the gas train, so it is technician work.
Technician repairBlocked drain on a high-efficiency furnace
A high-efficiency furnace squeezes so much heat out of the exhaust that it makes water all winter, and that water drains away. When the drain clogs, a safety control blocks the burners and the blower can run with no heat. You can look for standing water or a tripped switch. Clearing the drain is a service visit.
Safe to check yourselfLeaking ducts
Ducts running through a cold attic, garage, or crawl space lose heat and pull cold air in through the gaps. The furnace is heating fine, but the rooms at the end of a long duct run get lukewarm air. We find the leaks and tell you plainly what they are costing you in comfort.
Technician repairNo gas reaching the furnace
A closed valve, an empty propane tank, or a utility problem leaves the furnace with nothing to burn. You can reasonably notice whether your other gas appliances still work. Never touch a gas valve or gas piping yourself. That belongs to your utility or a qualified professional.
Technician repairWhat can you check yourself?
- Set the thermostat to HEAT, the fan to AUTO, and the temperature a few degrees above the room.
- Look at the air filter and change it if it is dirty.
- Check that every vent is open and no return grille is blocked.
- Look near a high-efficiency furnace for standing water or a small switch that has tripped.
- Listen at the start of a cycle. Note whether you hear clicks, a whoosh as the burners light, or only the blower.
Safety note: If you ever smell gas, leave the house without touching any switch or phone inside, then call your gas utility and 911 from outside.
When should you call for repair?
Call when the fan is on AUTO, the filter is fresh, and the vents still blow cold or the air swings between warm and cold. Call soon if you hear it click over and over without lighting. Call if it stops trying altogether. Call if a high-efficiency furnace has water pooling at its base. Each of those means a safety control is doing its job, and the fix is behind the cabinet door where your work should stop.
Return ducts that are too small are the most common thing we find in the Twinbrook and College Gardens ramblers. A furnace starved for air heats up, trips its overheat safety, and shuts the burners off while the blower keeps going. You feel that at the vents as cool air. The main duct in those houses usually runs through a crawl space or a low basement, so the rooms farthest from the furnace notice it first. Fixing the airflow is what stops it coming back.
Related questions
Why does my furnace blow cold air before it warms up?
A short stretch of cool air at startup is normal on many furnaces. The blower can start before the furnace is fully hot. If the air turns warm within a minute or two and stays warm, nothing is wrong. Cold air that keeps going past a couple of minutes means the burners shut down.
Why does the furnace start warm and then turn cold mid-cycle?
Warm then cold is the signature of a safety shutdown. Either the furnace overheated, usually from a dirty filter or blocked vents, or the flame sensor lost the flame and closed the gas valve. The blower keeps running to cool the furnace, which is the cold air you feel. Change the filter first.
Can a dirty filter really make a furnace blow cold air?
Yes, and it is one of the most common causes we find. A blocked filter chokes the airflow, heat backs up inside the furnace, and the overheat safety shuts the burners down while the blower keeps running. Repeated overheating also stresses the heat exchanger, the part that keeps exhaust out of your air, so a fresh filter protects the part you least want to replace.
Is it safe to keep running a furnace that blows cold air?
The safety controls are doing their job, so it is not dangerous. It is also not something to leave alone. Repeated shutdowns and failed lighting attempts wear parts out and can lock the furnace out on a cold night. Do the thermostat and filter checks. If cold air keeps coming, shut it down and book a repair.
If your furnace is blowing cold air after these checks, call Mora Heating & Air and we will get the heat back.
Or tell us what your system is doing
Send us the details and we will call you back. If the house is very hot or very cold right now, calling is faster.