If your furnace is blowing cold air, start with the thermostat.
A fan set to ON instead of AUTO will push room-temperature air through the vents even when the burners are off.
If that is not it, the usual causes are a clogged filter, a pilot light or igniter that will not light, a tripped safety switch, or a problem with the gas supply.
A few of these you can check in five minutes.
The rest need a tech from our furnace service and repair team.
Here is how to work through it, in the order that makes sense.
Is it actually blowing cold, or just not hot enough?
Hold your hand at a vent for a full minute after the furnace starts. A gas furnace runs the blower a short time before the burners light, so the first few seconds of air are always cool. That is normal.
What is not normal:
Air stays cool the whole time the furnace runs.
Air starts warm, then turns cold after a few minutes.
Air is lukewarm and the house never reaches the set temperature.
Each of those points to a different cause, so note which one you have before you call. It helps the tech get to the fix faster.
Check the thermostat first
Most of the time this is the five-minute fix.
Make sure it says HEAT, not COOL or OFF. Oklahoma falls bounce between both, and it is easy to leave it on the wrong one.
Set the fan to AUTO. On ON, the blower runs nonstop and pushes unheated air between cycles.
Set the temperature at least three degrees above the room so the furnace has a reason to fire.
Swap the batteries if the screen is dim or blank. A weak battery can make a thermostat lose its settings or stop calling for heat.
If the fan was on ON, switch it and give the system ten minutes. If the air turns warm, you are done.
A dirty filter can shut the burners off
A clogged filter blocks airflow across the heat exchanger. The furnace overheats, a safety switch called the high limit trips, and the burners shut off while the blower keeps running. You feel cold air. A few minutes later it cools down, relights, and does it again.
Pull the filter and hold it up to a light. If you cannot see light through it, replace it. Our guide on how often to change your furnace filter covers the right schedule for your filter type. If the furnace keeps turning on and off even with a clean filter, that is short cycling, and our post on furnace short cycling causes and fixes walks through the rest.
The burners are not lighting
If the blower runs but there is no flame, the furnace cannot make heat. Common reasons:
Pilot light out (older furnaces): Some furnaces built before the mid-1990s still use a standing pilot. If it is out, the thermocouple keeps the gas valve closed. Follow the relight steps printed on the furnace door. If it will not stay lit, the thermocouple is likely worn out.
Igniter failure (newer furnaces): Most modern furnaces use a hot surface igniter that glows orange before the burners light. Igniters crack and wear out over time.
Dirty flame sensor: The burners light, then shut off within seconds because the sensor cannot confirm a flame. A coating builds up on the sensor rod over time and needs cleaning.
Gas supply: Check that the gas valve next to the furnace is open (handle in line with the pipe) and that your other gas appliances work.
Relighting a pilot is fine for a homeowner. Replacing an igniter or cleaning a flame sensor means working inside the burner compartment with the power and gas involved. That is a job for a tech.
Condensate drain or flue problems (high-efficiency furnaces)
High-efficiency furnaces make water as they run and drain it through a PVC line, often to a small pump. If that line clogs or freezes, a float switch shuts the burners down to protect the furnace. You will often see water near the furnace or hear the pump running without stopping.
The same furnaces vent through PVC pipes on the side of the house or the roof. If leaves, a wasp nest, or ice blocks those pipes, a pressure switch keeps the furnace from lighting. Check that the pipe ends outside are clear. Do not try to take the venting apart.
Ductwork leaks
If the furnace is making heat but some rooms still get cool air, look at the ducts. In many Tulsa Area homes the ductwork runs through the attic. A disconnected or torn duct in a 30-degree attic will deliver cold air to whatever room is on the other end. If one or two rooms are cold and the rest are fine, this is a likely suspect.
If you have a heat pump
A heat pump moving heat in winter will feel cooler at the vent than a gas furnace, often in the 90s rather than the 120s. That is normal. It can also run a short defrost cycle on cold, damp days and blow cool air for a few minutes while it does. If it blows cold for longer than that, or the outdoor unit is iced over, it needs a look from our heat pump repair crew.
When you have checked the thermostat, the filter, and the gas valve and it is still blowing cold, that is the point to stop. Anything past this means opening the cabinet. Schedule service and we will find the cause and tell you straight what it needs and what it does not.
When to call right away
Call now, not tomorrow, if:
You smell gas. Leave the house first, then call the gas company from outside.
The furnace keeps trying to light and fails, and you hear clicking or a whoosh.
The breaker for the furnace trips again after you reset it.
Indoor temperatures are dropping fast in freezing weather, especially with kids or older family members at home.
For no-heat nights, our emergency heating repair page covers how to reach us. You can see everything we do on the heating side there too.
Frequently asked questions
Why is my furnace running but blowing cold air?
The most likely causes are the thermostat fan set to ON, a clogged filter tripping the high limit switch, or burners that are not lighting because of a bad igniter, dirty flame sensor, or pilot problem.
Why does my heater blow cold air for a few seconds and then warm up?
That is normal. Many furnaces start the blower before the burners are fully up to temperature, so the first air out of the vents is cool.
Can a dirty filter make my furnace blow cold air?
Yes. A clogged filter makes the furnace overheat, which trips a safety switch and shuts the burners off while the blower keeps running.
Is it normal for a heat pump to blow cooler air than a furnace?
Yes. Heat pump air usually feels cooler at the vent than gas heat, and it can blow cool air briefly during a defrost cycle.
When should I call for furnace repair?
Call if the furnace still blows cold after you check the thermostat, filter, and gas valve, or right away if you smell gas or the furnace will not light.
A furnace that blows cold air is telling you something, and most of the causes are a quick fix once someone finds them. If you have worked through the easy checks and it is still cold, schedule your furnace repair or call Pilgrim at 918-366-3040.