Why is my furnace or heater blowing cold air?

Wondering what's going on in there?

Why a furnace blows cold air, in plain English

A furnace works like a hair dryer. A flame heats the air, and a fan blows it out. You get cold air when the fan runs but the flame doesn't.

Each number matches a step below. Cold air means the blower (4) is running while the burners (2) are off.

The thermostat asks for heat. It's the boss on your wall. When the room gets cooler than the setting, it tells the furnace to start.
The burners light. Blue flames heat a metal box inside the furnace. On older furnaces, a tiny pilot light lights them. On newer ones, an igniter glows orange or clicks.
A safety switch watches the heat. Pros call it the high limit switch. If the furnace gets too hot, it shuts off the flames but keeps the fan running to cool things down.
The blower sends the heat to your rooms. This big fan pulls room air in through the filter, past the hot metal, and out your vents.

So cold air comes from one of four things:

The fan is set to run all the time, even between heating cycles.
The flames never light: a pilot light that's out, a worn igniter, a dirty flame sensor, no gas, or a blocked pipe or drain.
The flames shut off early because the furnace overheats: a clogged filter, closed vents, or a bad safety switch.
The furnace is fine, but the warm air leaks out of the ducts before it reaches some rooms.

The questions at the top check these in the same order. This guide covers gas furnaces, which most homes have. Water around your furnace? See our guide to an AC or furnace leaking water.

Who to call

Rather not do it yourself? Call an HVAC tech.

That's a fine choice. Maybe you're busy. Maybe you're selling the house and just want it done right. An HVAC tech (a heating and cooling pro) can find out why your furnace is blowing cold air and fix it.

$75–$300Clean or replace the flame sensor

$100–$500New igniter

Most companies charge $75–$250 to come out, and many put that toward the repair. A new gas valve or control board costs more.

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What to say

Calling a pro? Here's what to say.

A few lines to read when they pick up, and the questions to ask before anyone comes out.

Before you call

Under 10 years old? Call your installer first.

Home warranty? Call them first.

4.5 stars or more, and a license number.

Say this

Hi. My furnace is running, but it's blowing cold air. When's the soonest you can get here? And what's your fee for the visit? Does it go toward the repair?

Always ask

What's your fee to come out, and does it go toward the repair?
Will you tell me the full price before you start any work?
Are you licensed and insured?

Watch out if

They push a whole new system on the first visit for a small problem.
They won't give you a price before starting.
They want to "just add refrigerant" every year without finding the leak.

Feeling pushed to decide on the spot? You can say no and hang up. You owe nothing for a phone call.

Know what's wrong before they come

Answer up to 5 quick questions. We'll give you the words a pro uses.

Answer the questions →

More of a list person?

Common symptoms, causes, tests and costs

Find the row that sounds like your furnace. The quick test is something you can check yourself in a few minutes.

What you notice	What it probably is	Quick test	Do it yourself	With a pro	Verdict
Fan set to On; air feels cool between heating cycles	The fan is set to On	Look at the Fan setting on the thermostat	Free	Not needed	FIXSee the answer: The fan is set to On
Thermostat on Cool or Off, or set below the room temperature	The thermostat is set wrong	Look at the mode and the set temperature	Free	Not needed	FIXSee the answer: The thermostat is set wrong
Cool air for a minute or two, then warm air	Nothing's wrong. It's warming up	Keep your hand at a vent for 5 minutes	Free	Not needed	FIXSee the answer: Nothing's wrong. It's warming up
Can't find or safely reach the furnace, or you rent	Let a pro (or your landlord) take it from here	None. Hand it off	Pro job	$75–$250	CALLSee the answer: Let a pro (or your landlord) take it from here
Warm air at first, then cold; filter gray and clogged	A clogged air filter	Slide out the filter and hold it up to a light	$1–$35	Not needed	FIXSee the answer: A clogged air filter
Warm air at first, then cold; several vents closed or covered	Too many vents are closed or blocked	Walk the house and look at every vent	Free	Not needed	FIXSee the answer: Too many vents are closed or blocked
No flame lights; snow, ice or leaves at the pipe ends outside	The outside vent pipes are blocked	Look at the white pipe ends on the outside wall	Free	Not needed	FIXSee the answer: The outside vent pipes are blocked
No flame lights; water pooled in or under the furnace	A clogged drain hose	Look for water under the furnace and a full or kinked hose	$0–$10	$50–$200	FIXSee the answer: A clogged drain hose
Older furnace; the small pilot flame is out	The pilot light went out	Look through the window for the tiny blue pilot flame	Free	Not needed	FIXSee the answer: The pilot light went out
Pilot is out and there's no label, or it keeps going out	The pilot light is out	Look for the tiny blue pilot flame and a relighting label	Pro job	$80–$300	CALLSee the answer: The pilot light is out
Flames light, then go out after a few seconds	The flame checker is dirty	Watch the window: flames for a few seconds, then off	Pro job	$75–$300	CALLSee the answer: The flame checker is dirty
Warm air for a few minutes, then cold while the fan keeps running	The overheat safety switch keeps tripping	Watch the window: flames stay on a few minutes, then go out	Pro job	$100–$375	CALLSee the answer: The overheat safety switch keeps tripping
No flame, and no orange glow or clicking	The part that lights the gas wore out	After the small fan starts, look for an orange glow	Pro job	$100–$500	CALLSee the answer: The part that lights the gas wore out
Orange glow or clicking but no flame, or the gas stove won't light either	Gas isn't reaching the burners	Does your gas stove still light?	Pro job	$200–$800	CALLSee the answer: Gas isn't reaching the burners
Some rooms warm, others cold; warm air leaking at duct joints	Leaky or loose ducts	Feel along any ducts you can see, with the heat on	Pro job	$200–$4,000	CALLSee the answer: Leaky or loose ducts
Fan blows cold air, but no glow, no clicking and no flame	A problem with the furnace's brain	Look through the window for a light on the board	Pro job	$250–$770	CALLSee the answer: A problem with the furnace's brain
Rotten-egg or gas smell near the furnace	You may have a gas leak	None. Leave the house	Pro job	$50–$200	CALLSee the answer: You may have a gas leak

2026 U.S. averages, rounded. Pro prices include parts and labor. Most companies charge $75–$250 to come out, and many put that toward the repair. Nights, weekends and big cities cost more. See all repair prices

Want proof it's working?

The 10-minute warm-air test

This tells you whether your furnace is really heating. You need 10 minutes and, if you have one, a kitchen thermometer.

Set the Fan to Auto and the thermostat 5 degrees above the room.
Go to the vent closest to the furnace. Cool air for the first minute or two is normal.
After that, the air should feel clearly warm, like a hair dryer on low.
Hold a kitchen thermometer in the airflow for a minute. It should read well above the room temperature.
Keep checking for 10 minutes. The air should stay warm the whole time the fan runs.

Warm the whole time: it's working. Give the house an hour or two to warm up.
Warm, then cold while the fan keeps going: the furnace is probably overheating. Check the filter and vents.
Cold from the start: the flame isn't lighting. Take the questions at the top.

Not sure it's safe to keep running?

Turn it off right away if…

You smell gas or rotten eggs. Don't touch the furnace. Leave the house and call the gas company from outside.
Your carbon monoxide alarm goes off. Get everyone outside and call 911.
You smell burning or hot plastic that doesn't fade. A dusty smell for the first few minutes of the season is normal.
It keeps going warm, then cold, over and over all day. Overheating again and again wears out parts.
You hear a loud boom or rumble when it lights.
The flames are mostly yellow, floppy or reaching out of the front. Normal flames are mostly blue.

To turn it off, set the thermostat to Off. For anything on this list except a gas smell, also flip the furnace switch off.

Got a big repair quote?

Should you repair it or replace it?

Most fixes for cold air are cheap: a setting, a filter, or a part that costs a few hundred dollars. HomeAdvisor puts the typical furnace repair at $133–$505. Ask yourself three things before a big repair:

How old is it? Most gas furnaces last about 15–20 years, and some last longer. The date is usually on a sticker inside the front panel.
Is it still under warranty? Many furnaces have parts covered for years. Check before you pay for a part.
The half rule. If the repair costs more than half the price of a new furnace, get a replacement quote too.

Example: a $200 limit switch on a 12-year-old furnace is worth doing. Sealing all the ducts for $3,000 is worth comparing with fixing only the worst sections first.

Ran into a word you don't know?

Furnace words, explained

Tap through these before you go looking. The drawings show roughly what each part looks like.

Thermostat: The control on your wall where you set the temperature. It tells the furnace when to start and stop.
Fan setting: A switch on the thermostat. Auto runs the fan only while the furnace heats. On runs it all the time.
Furnace: The big metal cabinet, usually in a basement, closet, attic or garage, that burns gas to heat air.
Furnace switch: A switch on or near the furnace that looks like a light switch, often on a red plate. It turns the furnace's power on and off.
Vent (register): The grille in a floor, wall or ceiling where warm air comes into a room.
Ducts: The metal or bendy tubes that carry air from the furnace to each vent.
Air filter: A flat screen that catches dust before air goes into the furnace. Change it every 1–3 months.
Viewing window: A small window or set of slots in the front panel. You can watch the flame through it without opening anything.
Pilot light: On older furnaces, a tiny blue flame that burns all the time and lights the burners.
Thermocouple: A small metal rod next to the pilot light. It senses the pilot flame and shuts off the gas if the flame goes out.
Igniter: On newer furnaces, a small part that glows bright orange to light the gas. Some click and spark instead.
Flame sensor: A thin metal rod that sits in the flame. If it can't feel the flame, it shuts the gas off.
High limit switch: A safety switch that shuts off the flames if the furnace gets too hot, often because of a clogged filter.
Gas valve: The gate inside the furnace that lets gas into the burners. Only a pro should touch it.
Gas shutoff: A handle on the gas pipe going into the furnace. In line with the pipe means open. Crossways means closed.
Control board: The furnace's brain. A circuit board inside the panel that runs the start-up steps. Its light blinks a code when something's wrong.
Blower: The big fan in the furnace that pushes air through your vents.
Vent pipes: On newer furnaces, two white plastic pipes that go outside. One brings in air, the other pushes out exhaust.
Condensate drain: On newer furnaces, the small trap and plastic hose that carry away water the furnace makes.
Carbon monoxide alarm: An alarm that warns you about carbon monoxide, a gas you can't see or smell. Every home with a gas furnace should have one.

Still have questions?

Questions people ask most

Why is my furnace blowing cold air when the heat is on?

The most common reason a furnace blows cold air with the heat on is the thermostat fan set to On instead of Auto. Then it blows room air between heating cycles. Next most common: a clogged filter, which makes the furnace overheat and shut off its flame while the fan keeps going.

Is a furnace blowing cold air an emergency?

A furnace blowing cold air is an emergency only if you smell gas or a carbon monoxide alarm goes off. For gas, leave the house now and call the gas company from outside. For a carbon monoxide alarm, get everyone out and call 911. Otherwise it's urgent in cold weather, so check the fan setting and filter, and call a pro today if the house is getting cold.

Is it normal for a furnace to blow cold air at first?

Yes, a furnace often blows cool air for the first minute or two. That's the air that was sitting in the ducts. If it's still cold after 5 minutes, something's wrong.

Why does my heater blow hot air, then cold?

A heater that blows hot air, then cold, is usually overheating. A safety switch shuts off the flames, but the fan keeps running to cool the furnace down. Check the filter and make sure the vents are open. If that doesn't fix it, call a pro.

Can a dirty filter make my furnace blow cold air?

Yes, a dirty filter can make a furnace blow cold air. A clogged filter chokes the airflow, the furnace overheats, and a safety switch shuts off the flame. The fan keeps blowing, so the air turns cold. A new filter often fixes it.

Can I relight my furnace's pilot light myself?

You can relight a furnace's pilot light yourself only if the furnace has a relighting label with clear steps. Follow it exactly. If there's no label, or the pilot keeps going out, call a pro. If you smell gas, don't light anything. Leave and call the gas company from outside.

Why is my heat pump blowing cold air?

A heat pump can blow cool air for a few minutes while it defrosts itself in cold weather. This guide covers gas furnaces. If a heat pump blows cool air for longer than that, call a heating pro.

Who do I call when my furnace blows cold air?

Call a heating and cooling company (an HVAC tech) for a furnace that blows cold air. Call the gas company instead if you smell gas or none of your gas appliances work. Renters should call the landlord or property manager first.

How much does it cost to fix a furnace blowing cold air?

Fixing a furnace that blows cold air often costs nothing, if it's the fan setting, the filter or closed vents. A flame sensor repair runs about $75–$300. A limit switch runs about $100–$375. A control board runs about $250–$770.

How we built this

The questions follow the order a technician uses on a "blowing cold air" call: the thermostat and fan setting, when the air turns cold, airflow (filter and vents), the flame, then the pipes, drain, pilot and gas supply. Each answer makes some causes more likely and others less likely.

Prices are 2026 national averages from published repair-cost guides, rounded. Your local prices will differ.

How we make money: some links go to parts stores that pay us a small commission, and some pro listings pay us when you call them. Verdicts come from the diagnosis, not from who pays more.

Why is my furnace blowing cold air?

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