Category: Furnaces

A furnace blowing cold air is most often caused by the thermostat fan set to ON, a furnace that has shut down for safety (overheating from a dirty filter, or a failed ignition), or heat lost through leaky ducts. Start with the thermostat and filter. If the burners aren’t lighting, the problem is in the ignition sequence. If they light and then shut off, the flame sensor or limit switch is the likely cause.

Quick answers

• Thermostat fan set to ON makes air blow between heating cycles. Set it to AUTO.
• A short delay of cool air at startup is normal.
• A clogged filter can trip the high-limit switch and stop the burners.
• Burners that light and go out point to a dirty flame sensor.
• A heat pump in defrost mode can blow cool air for a few minutes. That’s normal.

1. The fan is set to ON

With the fan on ON, the blower runs constantly, even when the burners are off. Between cycles, the air feels cool. Set the fan to AUTO so it only runs while heating.

2. The furnace is still warming up

When heating starts, the burners light and heat the heat exchanger before the blower starts. Some furnaces start the blower a little early, and the first air through the ducts can feel cool. Give it 2 to 3 minutes.

3. A dirty air filter

A clogged filter restricts airflow, the heat exchanger overheats, and the high-limit switch shuts off the burners while the blower keeps running to cool things down. That feels like cold air. Replace the filter and note how often it clogs. How Often Should You Change Your Air Filter?

4. A dirty flame sensor

If the burners light and then shut off after a few seconds, the flame sensor may not be detecting the flame. Cleaning it is a common fix. Furnace Flame Sensor: How to Clean It Safely

5. Ignition failure

If the burners never light, the furnace may still run the blower in some situations. See Furnace Not Igniting: Causes and Step-by-Step Fixes.

6. The gas supply is off

Check that the gas valve near the furnace is open (handle parallel to the pipe) and that other gas appliances work. If your gas service was interrupted, follow the utility’s instructions to restore it.

7. Clogged condensate line (high-efficiency furnaces)

90%+ efficient furnaces produce water. If the drain or trap clogs, a safety switch prevents the burners from firing. Look for water in the tubing or around the furnace and clear the line.

8. Blocked vent pipes

Ice, snow, leaves or nests in the PVC intake and exhaust pipes can trip the pressure switch. Check the pipes outside and clear them carefully.

9. Leaky or uninsulated ducts

If air is warm near the furnace but cool at distant vents, heat is lost along the way. Ducts in unheated attics, crawl spaces or garages need insulation and sealed joints. Disconnected ducts can dump warm air into the attic entirely.

10. A heat pump in defrost mode

If you have a heat pump, it periodically reverses to melt frost from the outdoor coil. During defrost, the air may feel cool unless backup heat turns on. That’s normal for a few minutes. See Heat Pump Defrost Mode: Normal or a Problem?

Diagnose with the sight glass and LED

Look through the small window on the furnace door:

• Steady blue flames while cold air comes out: duct or blower issue.
• No flames: ignition, gas or safety switch issue.
• Flames that go out quickly: flame sensor or gas pressure issue.

Then check the LED code: Furnace Blinking Light: How to Read Your Furnace’s Code.

When to call a technician

Call for service if the high-limit switch keeps tripping with a clean filter, the flame sensor problem returns quickly after cleaning, or you notice any gas smell, soot or scorching. A technician should also inspect the heat exchanger if the furnace is old and overheating frequently.

Frequently asked questions

Is it bad if my furnace blows cold air?
It’s not dangerous by itself, but it often means a safety switch is protecting the furnace from overheating or failed ignition. Find the cause.

Why does my furnace blow cold air at the end of a cycle?
The blower keeps running for a minute or two after the burners turn off to pull remaining heat from the heat exchanger. The air cools during that time.

Can a bad thermostat cause cold air?
Yes. A misconfigured thermostat (for example, set for a heat pump when you have a furnace) can run the blower without the burners.

Related guides

• How Long Do Furnaces Last?
• Thermostat Not Working
• Furnace Flame Sensor: How to Clean It Safely

A gas furnace typically lasts 15 to 25 years, and an electric furnace often 20 to 30 years because it has fewer parts that wear out. The real limit for a gas furnace is usually the heat exchanger. When it cracks or corrodes, or when repairs become frequent and expensive, replacement makes more sense than repair.

Quick answers

• Gas furnace: about 15–25 years. Electric furnace: about 20–30 years. Oil furnace: about 15–20 years.
• Yearly maintenance and clean filters add years.
• A cracked heat exchanger usually means replacement.
• Rising bills, uneven heat and frequent repairs signal decline.

Lifespan by furnace type

Furnace type	Typical lifespan	Main wear points
Gas (80% efficiency)	15–25 years	Heat exchanger, igniter, inducer motor
Gas (90%+ condensing)	15–20 years	Secondary heat exchanger, condensate system
Electric	20–30 years	Heating elements, sequencers, blower
Oil	15–20 years	Burner, nozzle, heat exchanger

What affects how long your furnace lasts

• Maintenance: yearly service catches small problems early and keeps it running efficiently.
• Filters: dirty filters cause overheating, which stresses the heat exchanger.
• Sizing: oversized furnaces short-cycle, which wears igniters and controls.
• Installation quality: proper venting, gas pressure and airflow matter.
• Climate: furnaces in Minnesota run many more hours than those in Georgia.
• Humidity and combustion air: corrosive fumes (from laundry chemicals or pool supplies near the furnace) can shorten heat exchanger life.

Warning signs your furnace is failing

1. Age over 15 years plus a major repair.
2. Frequent repairs, especially more than one per winter.
3. Rising gas bills without a change in usage or prices.
4. Uneven heating or rooms that never warm up.
5. Yellow or flickering flames instead of steady blue (combustion problem; call a technician).
6. Soot around the furnace or registers.
7. Strange noises: banging, rumbling or screeching.
8. Carbon monoxide alarm activation (leave the home and call for help).

The heat exchanger question

The heat exchanger separates combustion gases from the air you breathe. Cracks can let combustion gases into the air stream. If a technician finds a cracked heat exchanger, ask to see the evidence (photos or camera inspection). Replacing only the heat exchanger is possible under warranty on newer units, but on older furnaces, replacing the whole furnace is usually more sensible.

Repair or replace?

Use these guidelines:

• Under 10 years old: repair in almost all cases.
• 10–15 years: repair unless the cost is more than about a third of a new furnace.
• Over 15 years: consider replacement for any major repair (heat exchanger, control board plus inducer, blower motor).

A common rule: multiply age by repair cost. Over 5,000 suggests replacement. Costs: Furnace Replacement Cost in 2026.

Should you replace with a heat pump?

If your AC is also old, it may be worth considering a heat pump or a dual-fuel system instead of a new furnace and AC separately. See Heat Pump vs Furnace: Which Is Cheaper to Run? and Dual fuel and emergency heat basics.

How to find your furnace’s age

Check the rating plate inside the furnace door. Some list the manufacture date. Others encode it in the serial number. Many brands use a year and week code. Searching the brand name plus “serial number date” usually finds the format.

Frequently asked questions

Can a furnace last 30 years?
Some do, especially electric furnaces or well-maintained gas furnaces in mild climates. Very old furnaces are typically 60–70% efficient, much less than modern units.

Is it worth repairing a 20-year-old furnace?
Only for small, inexpensive repairs. Start planning a replacement.

What’s the best time to replace a furnace?
Late spring or early fall, when contractors are less busy and you’re not without heat.

Related guides

• Furnace Flame Sensor: How to Clean It Safely
• Furnace Not Igniting: Causes and Step-by-Step Fixes
• Furnace Blowing Cold Air: 10 Reasons and Solutions

When a gas furnace won’t ignite, the most common causes are a dirty flame sensor, a failed hot surface igniter, a closed gas valve, a clogged filter or condensate drain, or a safety switch that has tripped. Modern furnaces follow a fixed startup sequence, so noticing where it stops tells you where to look. Before anything else: if you smell gas, leave the house and call your gas utility or 911 from outside.

Quick answers

• Smell of gas: don’t touch switches, leave the home, call the gas company from outside.
• Check the thermostat is set to HEAT and above room temperature.
• Replace a dirty filter and make sure the furnace switch is ON.
• Flame lights then goes out after a few seconds: usually a dirty flame sensor.
• No orange glow from the igniter: likely a failed igniter.

How a gas furnace starts

Most furnaces built in the last 25 years follow this sequence:

1. The thermostat calls for heat.
2. The draft inducer motor starts to vent the combustion chamber.
3. The pressure switch confirms the venting is working.
4. The hot surface igniter glows bright orange (or a spark igniter clicks).
5. The gas valve opens and burners light.
6. The flame sensor confirms a flame within a few seconds.
7. After a short delay, the blower starts to send warm air through the house.

If any step fails, the control board stops and tries again. After several failed attempts, many furnaces go into a lockout for about an hour.

Start with these quick checks

1. Thermostat: set to HEAT and at least 3 degrees above room temperature. Replace the batteries if the screen is dim or blank.
2. Furnace power switch: looks like a light switch on or near the unit. It should be ON.
3. Breaker: check the furnace breaker in your panel.
4. Furnace door: the front panel must be fully closed to press the safety switch.
5. Air filter: a clogged filter can make the furnace overheat and shut down. Replace it.
6. Gas supply: the gas valve handle on the pipe leading to the furnace should be parallel to the pipe. Make sure other gas appliances work.
7. Condensate drain (high-efficiency furnaces): if the drain or trap is clogged, a pressure switch may stop ignition.

Where the sequence stops, and why

What you observe	Likely cause
Nothing happens at all	No power, thermostat, door switch, blown control fuse
Inducer runs, no igniter glow	Pressure switch issue, blocked vent or drain, failed igniter
Igniter glows, no flame	Gas valve closed or faulty, no gas supply
Flame lights, then shuts off in 3–10 seconds	Dirty flame sensor, poor ground
Burners run, blower never starts	Blower motor, capacitor or limit switch issue
Repeats several times then stops	Lockout after failed attempts

Fix 1: Clean the flame sensor

The flame sensor is a thin metal rod in front of the burners. Over time it gets a coating that weakens its signal, so the control board thinks there’s no flame. Cleaning it is one of the most common DIY furnace fixes. Step-by-step: Furnace Flame Sensor: How to Clean It Safely.

Fix 2: Check the igniter

Watch through the sight glass when the furnace starts. A working hot surface igniter glows bright orange within a few seconds. If it doesn’t glow, it may be cracked or burned out. Igniters are fragile ceramic parts. Replacement is a moderate DIY job for experienced homeowners but is often best left to a technician, who will also confirm the cause.

Fix 3: Clear the vent and condensate drain

High-efficiency furnaces vent through PVC pipes, often out the side of the house. Snow, ice, leaves or nests can block them. Check outside that both pipes are clear. Inside, look for water in the condensate trap and tubing. A blocked drain is common after a long summer of AC use.

Fix 4: Reset after a lockout

Turn the furnace switch off for 30 seconds and back on, or turn the thermostat off and on. This clears a lockout. If the furnace fails again, the underlying problem remains. Don’t reset repeatedly.

Read the diagnostic light

Most control boards have an LED that blinks a code. The code chart is usually printed on the inside of the furnace door. Read it before calling a technician. Guide: Furnace Blinking Light: How to Read Your Furnace’s Code.

When to call a professional

Call a licensed technician if:

• You smell gas or see scorch marks
• The igniter or gas valve needs replacement and you’re not experienced with gas appliances
• The pressure switch keeps failing (this can indicate venting problems)
• The furnace is more than 15 years old and has repeated failures

Frequently asked questions

Why does my furnace click but not turn on?
Clicking usually comes from relays on the control board or a spark igniter. If the burners don’t light, check the gas supply, igniter and pressure switch.

How long should a furnace take to ignite?
Usually 30 to 90 seconds from the thermostat call, including the inducer pre-purge.

Can I relight my furnace’s pilot light?
Only older furnaces have standing pilot lights. Follow the instructions on the furnace label. Most modern furnaces have no pilot to relight.

Related guides

• Furnace Blowing Cold Air: 10 Reasons and Solutions
• Furnace Replacement Cost in 2026
• Furnace Flame Sensor: How to Clean It Safely

A dirty flame sensor is one of the most common reasons a gas furnace lights, runs for 3 to 10 seconds, and shuts off. The sensor proves to the control board that a flame is present. When a thin layer of oxide builds up on it, the signal weakens and the furnace shuts the gas off for safety. Cleaning it takes about 15 minutes with a fine abrasive pad and a screwdriver or nut driver.

Quick answers

• Symptom: burners light, then go out within seconds, and the furnace retries several times.
• Tools: 1/4-inch nut driver or screwdriver, fine abrasive pad or very fine sandpaper, clean paper towel.
• Turn off power and gas before starting.
• Don’t touch the fragile hot surface igniter next to it.
• If cleaning doesn’t help, check the sensor’s ground connection or replace it.

What the flame sensor does

The flame sensor is a thin metal rod, usually bent, with a white ceramic base, mounted so its tip sits in the flame of one burner. The control board sends a tiny electrical current through the flame to the sensor. If the current isn’t detected within a few seconds after the gas valve opens, the board closes the valve. This prevents unburned gas from filling the furnace.

Signs of a dirty flame sensor

• Burners light, then shut off after a few seconds
• The furnace tries several times, then goes into lockout
• The diagnostic LED shows a flame sense or ignition failure code (how to read it)
• The problem gets worse over weeks

Safety first

1. Set the thermostat to OFF.
2. Turn off the furnace power switch (and the breaker if you’re unsure which switch is the furnace).
3. Turn the gas valve on the furnace supply pipe to OFF (handle perpendicular to the pipe).
4. Wait for the furnace to cool if it was just running. Burner areas get very hot.

If you smell gas at any point, stop, leave the house and call your gas utility from outside.

Step-by-step cleaning

1. Remove the furnace front panel(s). Usually the upper panel lifts off.
2. Find the burners and look for the flame sensor: a thin rod with a single wire attached, held by one screw. Don’t confuse it with the igniter, which is a flat or round gray/black ceramic element with two wires. The igniter is extremely fragile.
3. Take a photo so you remember the position.
4. Remove the screw holding the sensor bracket (often 1/4-inch hex).
5. Pull the sensor out gently. You can leave the wire connected or disconnect it if it slides off easily.
6. Clean the metal rod by rubbing lightly with a fine abrasive pad (like a non-scratch scouring pad) or very fine sandpaper until it’s bright. Don’t use coarse sandpaper or a file, which can scratch the surface and make it dirty faster.
7. Wipe it with a clean paper towel. Don’t touch the cleaned rod with your fingers.
8. Reinstall it in the exact same position, tighten the screw firmly (the screw also provides the ground), and reconnect the wire.
9. Replace the panel, turn the gas back on, restore power and set the thermostat to HEAT.
10. Watch a full cycle through the sight glass. Burners should stay lit.

If it still shuts off

Possible cause	What to check
Sensor cracked or worn	Look for cracks in the ceramic base; replace the sensor (inexpensive part)
Poor ground	Tighten the sensor screw and check the furnace ground wire
Sensor out of the flame	Make sure the tip sits in the flame path
Wrong polarity on power supply	Some boards need correct hot/neutral wiring; a technician can test
Weak flame	Low gas pressure or dirty burners; requires a technician
Control board	Less common; a technician can measure the flame signal in microamps

How often should you clean it?

During a yearly tune-up, a technician usually checks and cleans the flame sensor. If yours needs cleaning every few weeks, something else is causing contamination, such as dirty combustion air, a lot of dust near the furnace, or chemical fumes from nearby storage.

Frequently asked questions

Can I use steel wool?
It’s better to avoid it. Steel wool can leave fibers behind. Use a fine abrasive pad or very fine sandpaper.

How much does a new flame sensor cost?
The part itself is usually inexpensive (often 10 to 40 USD). A service call to replace it costs more, typically including the diagnostic fee.

Is it safe to bypass the flame sensor?
No. Never bypass a safety device. The sensor prevents gas from building up in the furnace.

Related guides

• Furnace Not Igniting: Causes and Step-by-Step Fixes
• HVAC Maintenance Checklist: Spring and Fall
• How Long Do Furnaces Last? Signs It’s Time to Replace