A gas furnace that roars, clicks or blows cold air on the first cold night usually has one of a handful of problems, and most of them are repairs. Here’s what each symptom points to, which ones mean shut it off now, and what a technician should show you before you pay for a fix or a new furnace.
Why is my furnace blowing cold air?
The most common reason a gas furnace blows cold air is the simplest: the thermostat fan is set to ON instead of AUTO, so the blower runs between heating cycles. After that, the usual causes are a clogged filter that overheats the furnace until a safety switch shuts the burners off, a dirty flame sensor that shuts the burners down seconds after they light, and an ignition problem that never lets them light at all. In each case the blower can keep running, so the vents blow cold air with no flame behind it. A little cool air right at startup is normal, since most furnaces wait 30 to 60 seconds after the burners light before the blower comes on.
| What you notice | Likely cause | What to do |
|---|---|---|
| Blower runs nonstop, air never warms | Fan set to ON, or burners not lighting | Set the fan to AUTO; call if still cold |
| Burners light, then shut off within seconds, again and again | Dirty or failing flame sensor | Call for a repair |
| Runs a few minutes, burners stop, blower keeps going | Overheating from a clogged filter or blocked vents | Replace the filter and open vents |
| Small fan hums or whines, burners never light | Inducer motor, pressure switch or ignitor | Call for a repair |
| Loud boom when the burners light | Delayed ignition | Shut it off and call |
What does a normal furnace startup sound like?
A healthy gas furnace starts in the same order every time. A small fan called the inducer motor hums to clear the vent pipe, the ignitor glows or clicks, the gas valve opens with a soft whoosh of flame, and half a minute to a minute later the main blower comes on. Telling the technician where yours stops shortens the diagnosis, and a furnace that’s suddenly loud at startup is usually pointing at a specific part:
- A loud boom or bang as the burners light. That’s delayed ignition: gas builds up for a moment before it catches. Shut the furnace off at the thermostat, don’t restart it, and call.
- A whine, rattle or grinding from the small fan before the burners light. That points to the inducer motor, a repair to schedule before it stops the furnace from starting.
- Squealing or grinding while the main blower runs. Worn blower motor bearings or a loose part are the usual causes.
- A hum or buzz with no fan turning. The blower motor may be seized, or its capacitor has failed. Our flat-rate capacitor replacement is $140 to $285.
Why does my furnace click but never light?
If you hear the inducer run and the ignitor click but the burners never light, the furnace is failing its ignition sequence. The usual suspects are a worn ignitor, the pressure switch that confirms the vent is clear, the gas valve or the control board, all technician repairs. First rule out fuel. If your gas stove or water heater won’t light either, call your gas company. If you heat with propane, check the tank gauge, because an empty tank looks exactly like a dead furnace. On a high-efficiency furnace (the kind with white plastic vent pipes), a clogged condensate drain can also keep it from running.
If you can see a blinking light through the furnace’s viewing window without removing a panel, write down the pattern. It’s a diagnostic code.
Why does my furnace light, then shut off?
A furnace that lights, runs for a few seconds and shuts off is usually failing to sense its own flame. The flame sensor is a thin metal rod that sits in the burner flame and tells the control board the burners are lit. When it’s coated with residue, the board can’t read it, cuts the gas as a safety step and tries again. That on-and-off pattern is short cycling, and a winter of it wears out parts. Cleaning or replacing the sensor is a quick repair, and keeping it clean is one reason a fall heating tune-up opens the burner compartment.
A furnace that runs several minutes, then shuts its burners off while the blower keeps going is overheating, and the high-limit switch is doing its job. Check the filter and open any closed vents. If that doesn’t stop it, call us, because repeated overheating is hard on the heat exchanger.
When should you turn the furnace off and leave the house?
Leave first and make calls second if you smell gas (a rotten-egg odor) or your carbon monoxide alarm goes off. For a gas smell, don’t switch lights or appliances on or off. Get everyone out, then call from outside. If Middle Tennessee Natural Gas is your gas company, its 24/7 emergency line is 1-833-438-6864, and 911 works too. For a CO alarm, get everyone outside to fresh air and call 911. Headaches, dizziness or nausea that ease once you’re outside are a warning sign even without an alarm. Don’t run the furnace again until it’s been checked.
Shut the furnace off at the thermostat and call, without leaving, for a boom at startup, soot around the furnace or vent pipe, or a burning smell that doesn’t fade after the first run of the season.
What should a furnace repair visit show you?
Before you approve a furnace repair or replacement, the technician should be able to tell you which part failed, how they know, and what the fix costs. That goes for any company, including ours. A no-heat diagnosis normally means watching the furnace go through its startup sequence to see where it stops, then testing the part at that step. Ask for specifics: the part, the reading or photo that shows the problem, the repair price, and, if replacing the furnace is the recommendation, why that one failure justifies it. One Crossville customer wrote after a furnace check: “He took pictures of the problems and thoroughly explained what should be done to correct these issues. Not pushy at all!”
If you’re told the heat exchanger is cracked
A cracked heat exchanger is the one diagnosis that can turn a no-heat call into a replacement on the spot, so it deserves the most proof. Carrier warns that a crack can let combustion gases, including carbon monoxide, into the home’s air, and recommends a professional inspection when one is suspected. If you’re told yours is cracked, ask to see the crack in a photo or on camera, ask whether a combustion or CO test backs it up, and ask whether the heat exchanger is still under the manufacturer’s warranty. A second opinion is reasonable before you replace a furnace for a crack you haven’t seen.
Should you repair or replace a furnace that isn’t heating?
Most of the failures in this guide are repairs: a flame sensor, ignitor, pressure switch, limit switch, capacitor or inducer motor. Replacement makes sense when the repair would cost close to half the price of a new system, when the furnace is past about 15 years old, or when the same fault keeps coming back after it’s fixed. Our flat-rate repair prices are on our pricing page, and our furnace replacement cost guide covers what a new one runs here, starting around $4,500 installed for a furnace-only swap. Get the diagnosis first and the replacement quote second. They’re two separate decisions.
What can you check before you call?
These take a few minutes and clear up a real share of no-heat calls:
- Set the thermostat to HEAT, a few degrees above room temperature, fan on AUTO. Replace the batteries if the screen is blank.
- Replace the filter if it’s gray and packed with dust. One-inch filters generally need changing every one to three months.
- Make sure the furnace power switch (it looks like a light switch, on or near the furnace) is on. Reset a tripped breaker once, and call if it trips again.
- Make sure the furnace’s blower door is fully closed. Many furnaces won’t run with it loose.
If the furnace booms, smells like burning or keeps short cycling, skip the restarts and call.
Frequently Asked Questions
Can I clean a furnace flame sensor myself?
We recommend leaving it to a technician. The sensor sits in the burner compartment next to the burners and ignitor, behind panels the owner’s manual may tell you not to open, and a technician can also check whether the sensor is the whole problem.
Why does my heat pump blow cold air in winter?
A heat pump blowing cool air for a few minutes is often a normal defrost cycle, not a failure. Our guide to why a heat pump blows cold air explains how to tell defrost from a real problem.
Does Crossville Heating & Cooling repair all furnace brands?
Yes. Crossville Heating & Cooling services every make and age of furnace, heat pump and boiler. We primarily install Tempstar and Carrier, and we can install any brand you want.
Furnace Still Not Heating? Call Crossville Heating & Cooling
Crossville Heating & Cooling repairs gas furnaces across Crossville, Fairfield Glade, Lake Tansi, Crab Orchard and Pleasant Hill, with 24/7 emergency service on the same number as the office. Call (931) 484-0004 or book a heating repair visit, and tell us what you heard and where the startup stops.
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