Furnace Blowing Cold Air? Here's What's Going On

You hold your hand over the register and the air moving past it is the same temperature as the room. The furnace is clearly doing something — you can hear the blower running in the basement — but nothing warm is showing up where you live.

That one symptom covers about six different problems. They range from a switch on your thermostat that costs nothing to flip to a part that needs replacing tonight. I'll take them in the order I actually find them, cheapest first. Most of the cold-air calls we take turn out to be one of the first two on this list, and the first one costs nothing to fix.

Check the Fan Switch Before Anything Else

Go look at your thermostat. Somewhere on it is a fan setting with two positions: AUTO and ON.

If it's on ON, that's very likely your whole problem. ON means the blower runs continuously, all day, whether or not the burners are lit. Your furnace doesn't make heat continuously — it fires, warms the house a few degrees, satisfies the thermostat, and shuts the burners off. On AUTO, the blower shuts off with them. On ON, it keeps right on pushing air through the ducts for the twenty or forty minutes until the next call for heat, and that air is room temperature or a little cooler after a trip through a cold basement.

So you walk past a register between cycles, feel cool air, and conclude the furnace is broken. It isn't. Switch the fan to AUTO and see what happens over the next hour.

Some people run the fan on ON deliberately, for even temperatures upstairs and down or because they've got good filtration and want the air moving through it. That's a legitimate choice and I'm not going to talk you out of it. Just know that it comes with cool air between burns, and that's normal for the setting, not a fault. While you're at the thermostat, make sure it's set to HEAT and not on a leftover cooling or fan-only mode, and that the batteries aren't dead. Our post on a thermostat that isn't responding covers the rest of that list.

The Cold Air at Startup Is Supposed to Be There

The other non-problem. When the furnace gets a call for heat, the burners light first and the blower waits — usually thirty to sixty seconds — for the heat exchanger to come up to temperature before it starts moving air. If it started the fan at the same instant as the burners, you'd get a shot of cold air across the house on every single cycle.

That fan-on delay is built into the control board on purpose. But if a supply register sits right where you stand at the kitchen counter, you feel the first few seconds of it, and the timing is easy to misread.

Stand at a register through one full cycle. If the air goes from cool to warm inside of a minute and then stays warm until the furnace shuts down, everything is working. If it starts cool and stays cool for five minutes, keep reading.

A Loaded Filter Shuts the Burners Off

This is the one I find most in real houses, and September is when it surfaces, because the filter has been in there catching everything the air conditioner pulled through it since May.

A furnace is built around a specific volume of air moving across the heat exchanger, and that moving air is what carries heat away from the metal. Choke the airflow with a matted filter and the heat exchanger runs hotter than it was built to run. A safety called the high limit switch is watching that temperature, and when it goes past its setpoint the limit opens and cuts the burners off.

The blower does not stop. It keeps running to pull heat out of that metal and bring it back down, which is exactly what you want it doing. What you feel at the register is a furnace that ran for a few minutes, quit heating, and is now blowing cold.

Then the metal cools, the limit closes, the burners light again, and the whole cycle repeats. Short bursts of heat, long stretches of cold air, a house that never quite gets to temperature.

Pull the filter and hold it up to a light. If you can't see light through it, you found it. Put a fresh one in, let the furnace run through two or three full cycles, and see if the heat holds. If you're not sure how often that should be happening, we wrote a whole piece on filter changes and what actually drives the interval.

One more airflow item while you're down there: closed or blocked registers. People shut vents in unused rooms to save money, and past a certain number of closed vents you've created the same restriction as a dirty filter. Furniture over a return grille does it too.

The Burners Never Lit at All

If the filter is clean and the fan is on AUTO and you're still getting cold air, the next question is whether the furnace is lighting.

Go stand in front of it during a call for heat. You should hear a sequence: a small motor spinning up for a few seconds, a click, and then the burners catching with a soft whoosh. If you get the motor and the click and then nothing, ignition is failing, and there are a handful of usual reasons.

• The flame sensor is dirty. A thin metal rod sitting in the flame that confirms to the control board that there's actually fire. It carbons over with a gray film, stops conducting properly, and the board shuts the gas off a few seconds after lighting as a safety. You'll see the burners light and then die, over and over. This is the cheapest real repair in the trade — it's a cleaning, not a part.
• The hot surface ignitor is cracked. The glowing element that lights the gas on most furnaces built in the last thirty years. They're brittle, they have a service life, and they usually fail at the start of the season when they're asked to glow for the first time since spring.
• A pilot is out, on an older unit with a standing pilot. Relighting instructions are printed on a label inside the door.
• The gas is off. Worth a look. A valve gets bumped during a water heater job or a basement project and nobody notices until October.

Most furnaces will try three or four times and then stop trying, which the trade calls lockout. There's an LED behind the little window on the furnace door that flashes a code while it's locked out, and the legend for that code is printed on the inside of the door panel. If you're going to call us, reading us that flash pattern over the phone is genuinely helpful.

I was in a Hudsonville basement on this call a couple of falls ago. The homeowner had gotten two quotes for a new furnace and was days from writing a check. The flame sensor was carboned over. I cleaned it with a piece of emery cloth, watched it run through three cycles, and charged him the diagnostic. He had a fifteen-year-old furnace that was going to need replacing eventually, and I told him so, but eventually wasn't that week. That's the difference between furnace repair and a sales call, and it's the whole reason we do the first one.

High-Efficiency Furnaces and a Plugged Drain

If your furnace has white PVC pipe running out of it instead of metal flue pipe, it's a condensing furnace, and it makes water when it runs — a couple of gallons a day in cold weather. That water leaves through a drain line and usually a small trap.

Those lines plug. Sludge, mineral scale, sometimes a spider. When the drain backs up, water fills the condensate trap and the inducer, and a pressure switch senses that the venting isn't drawing right and refuses to let the burners light. The furnace goes through its startup sequence, fails the pressure switch check, and quits. Same symptom at the register: air, no heat.

Wet floor around the furnace, water in the drain pan, or a gurgling noise from the plastic tubing is your tell. It's a routine repair and not an expensive one, but it needs somebody who knows which parts of that assembly can be cleared and which will crack if you lean on them.

Stop Resetting It

The habit I'd most like people to break: the furnace shuts itself down, somebody flips the breaker or the door switch, it runs again for a while, and everybody moves on. Repeat nightly.

Every one of those resets is another cycle of whatever the furnace was protecting itself from. If the limit switch is opening because the thing is overheating, resetting it doesn't fix the airflow, it just gives the heat exchanger another overheating cycle. Metal that gets taken past its design temperature over and over develops cracks, and a cracked heat exchanger is not a repair. It's a furnace, and it's the failure that can put combustion gases into the air you breathe.

What You Check, and Where You Stop

Yours to do, in this order: fan switch to AUTO, thermostat on HEAT with good batteries, fresh filter, registers and return grilles open and clear, gas valve on, and a look at the flash code behind the furnace door.

Ours from there. Anything involving the gas valve, the burners, the ignitor, the pressure switch, the wiring, or the heat exchanger is a licensed job, and half of it is only diagnosable with a manometer and a combustion analyzer in your hand. If you've done the free checks and you're still getting cold air, you've done the part that was worth doing yourself.

If the house is dropping into the fifties, there's a baby or somebody elderly in it, or the temperature outside is heading for the teens, don't wait on it — our emergency line is answered day and night. And most of what's on this page is what a fall tune-up is for: the sensor gets cleaned, the ignitor gets tested before it fails, the drain gets cleared, and the temperature rise across the heat exchanger gets measured while it's still 60 degrees outside. Cheaper in September than in January, and easier to get on the schedule.