By the team at Homegrown Heating and Air Conditioning.
If the furnace is running and the air at the register is cold, check the fan switch on your thermostat first. Set to ON instead of AUTO, the blower runs all the time, including the long stretches between heating cycles, and room-temperature air feels cold coming out of a vent. That one setting explains a large share of these calls.
If the fan is already on AUTO, the furnace is trying to heat and something is stopping it. Below is the order we work through, cheapest and safest first, and where the right move is to stop.
Stop and Call 911 If You Smell Gas or a CO Alarm Sounds
Nothing below applies if either of these is happening.
If you smell gas anywhere in the house, leave the building, take everyone with you, and call 911 from somewhere else. Do not flip switches and do not go looking for the source.
If a carbon monoxide alarm is sounding, do the same. The Consumer Product Safety Commission carbon monoxide safety center is direct: know the symptoms, get outside to fresh air immediately, and then call 911. CO is colorless and odorless, and malfunctioning fuel-burning appliances including furnaces produce it. The same guidance says not to service fuel-burning appliances without the knowledge, skill and tools, which is the line every item below is measured against.
Never bypass, jump or disable a safety control. Those switches exist to shut the gas off, and a furnace that keeps tripping one is telling you something true.
Furnace Blowing Cold Air: Work in This Order
1. Thermostat fan set to ON instead of AUTO. Move the switch to AUTO and give the system a full cycle. Safe for anyone, and it costs nothing. It is also the usual cause of a cold draft complaint during a chinook, when outdoor temperature swings 25 to 35 degrees F in a few hours and a fan left on circulates that swing through the house.
2. A dirty filter. Pull the filter and hold it up to a light. If you cannot see light through it, replace it. A loaded filter starves the blower, heat builds faster than air carries it away, and the high limit opens. Do not run the system without a filter.
3. A tripped high limit. Burners light, run a few minutes, then shut off while the blower keeps running and the air turns cold. Replace the filter, open any closed registers, and check that returns are not blocked by furniture. If it trips again after that, stop. Repeated limit trips point at airflow or an overfired burner, and both need measurement.
4. Flame sensor fouling. Burners light, hold for roughly three to seven seconds, then the gas shuts off and the blower purges cold air. This is the single most common cause of a furnace that lights and then blows cold. Nothing here is safe for a homeowner, because reaching the sensor means opening the burner compartment. Note how many seconds the flame holds and call.
5. A failed igniter. No flame at all. The inducer runs, the igniter tries, and the board locks out. Look through the sight glass to see whether anything glows, but do not touch it. Hot surface igniters are brittle and carry line voltage.
6. A blocked flue or intake. If your furnace vents through white PVC out a side wall, walk outside and look at both pipes. Clear snow, leaves or a nest away from the openings by hand. Never push a tool into the pipe. If they are on a roof, call instead of climbing.
7. A gas supply problem. Check whether other gas appliances still work. If the range and water heater are also out, the issue is upstream of the furnace. Do not adjust a gas valve or manifold pressure. That is a metered adjustment.
8. A cracked heat exchanger. Signs include a flame that shifts the moment the blower starts, soot around the burner compartment, repeated limit trips, or a CO alarm. No homeowner check exists for this. Shut the system off and call.
The Flame Sensor Is a Safety Device Doing Its Job
Most people hear “flame sensor” and picture a broken part. It is closer to the opposite. The sensor proves a flame actually exists before the furnace is allowed to keep sending gas to the burners.
It works on flame rectification. The control board puts a small alternating voltage on a metal rod sitting in the burner flame. A flame conducts electricity in one direction far better than the other, so the flame acts as a one-way valve and the board sees a small direct current coming back. That current is measured in microamps, and manufacturers publish a minimum threshold for each model.
When the rod picks up a film of oxide or silica, the current drops. Once the reading falls below the manufacturer’s threshold, the board treats it as “no flame proved” and closes the gas valve within seconds. That is why the burners light and then quit, and why the blower keeps pushing cold air. The furnace is not failing to see a problem. It is refusing to run blind.
Sensors foul faster in Northern Colorado than at sea level. A gas appliance above 2,000 feet has to have its input adjusted for elevation, and when that adjustment was never made or was made the wrong way, the burner runs rich and deposits residue on the rod. Combustion air does the same thing: a furnace in a tight mechanical room with undersized combustion air openings will foul a sensor on a schedule.
Why Cleaning Is a Repair, Not a Part Swap
A technician measures the flame signal in microamps with a meter in series and compares it to the model’s specification. We also read the board’s fault history, check static pressure across the filter and coil, check temperature rise against the range stamped on the unit, and verify input rate against the nameplate.
If the signal comes back to spec after the rod is cleaned and the combustion numbers are right, the repair is done and no part was needed. If it recovers and then collapses again within weeks, the sensor was never the problem. We would rather find the cause once.
That work sits inside furnace repair. If the heat is out now, our emergency HVAC repair team takes same-day calls seven days a week, 7 a.m. to 7 p.m. If the furnace is limping rather than dead, heating maintenance is the cheaper path, and our HVAC work in Fort Collins covers the rest.
A Diagnosis and a Repair in One Visit
A furnace that lights and then blows cold is the call we take most often once the first hard cold arrives. The work is a diagnosis first: find which safety opened and why, then fix the cause and price it before anything gets replaced. Jesse Myers called us on a furnace and got the whole sequence in one visit.
Jesse Myers wrote: “Personable service, speedy response, and a successful diagnosis and repair to our furnace for a very reasonable price!”
Speed only counts if the diagnosis holds. We quote the repair before we start, show you the readings we took, and work licensed and insured throughout Northern Colorado. Read more customer reviews from Northern Colorado homeowners.
Book a Furnace Diagnostic
Tell us what the furnace does and how long it runs before the air turns cold. Contact Homegrown Heating and Air Conditioning to book a diagnostic anywhere in Northern Colorado.
Furnace Blowing Cold Air Questions From Northern Colorado Homeowners
Why does my furnace only blow cold air on the coldest mornings?
Because that is when it runs longest. The 99 percent winter design temperature for Larimer County is around 1 degree, so a furnace here runs long cycles instead of short ones on a cold morning. A long cycle with restricted airflow reaches the high limit trip point that a short cycle never gets to.
Can I clean the flame sensor myself?
We do not recommend it. Reaching the rod means opening the burner compartment of a live gas appliance, and the CPSC guidance is explicit that fuel-burning appliances should not be serviced without the proper knowledge, skill and tools. There is also no point cleaning a sensor without measuring the signal before and after, because the number is the diagnosis.
My flame sensor was cleaned last winter and it is doing it again. Why?
A sensor that fouls on a schedule is a symptom of combustion, not a defective part. The usual causes here are an input rate that was never adjusted for elevation, or was lowered by dropping manifold pressure instead of changing orifice size, and combustion air starvation in a tight mechanical room. We measure both before quoting anything.
The furnace quit the morning after a snowstorm. Is that the same problem?
Probably not. A condensing furnace vents through PVC out a side wall, often close to grade, and drifted snow or a refreeze over the termination will open the pressure switch. The furnace shuts down as designed. Clear the snow away from the openings by hand and call if it does not restart.
How fast can someone get to my house if the heat is out?
We schedule same-day service seven days a week, 7 a.m. to 7 p.m., across Fort Collins, Loveland, Greeley, Windsor, Timnath, Johnstown, Wellington, Berthoud, Laporte and Estes Park. Tell us what the furnace is doing and how long it holds before it quits, because that detail usually narrows the fault before we arrive.
Does a furnace blowing cold air mean I need a new furnace?
Usually not. Most of these calls end in a cleaning, a filter, a sensor or an igniter. The exception is a confirmed cracked heat exchanger, which is a carbon monoxide risk, and in that case we shut the appliance down, document what we found, and give you the options in writing.
