Furnace Not Blowing Hot Air — Troubleshooting Guide

It’s cold outside, you’ve cranked up the heat, and nothing but lukewarm or cold air is coming from the vents. A furnace that won’t heat is urgent — but before you call for emergency service, there are several things you can check yourself.

Safety First

If you smell gas or suspect a gas leak:

1. Don’t flip any switches or create sparks
2. Leave the house immediately
3. Call your gas company from outside
4. Don’t return until they’ve cleared it

Gas leaks are rare but serious. If everything smells normal, let’s diagnose.

Quick Checks First

1. Thermostat Settings

Obvious but overlooked:

• Is it set to HEAT (not COOL or OFF)?
• Is the temperature set above the current room temp?
• Are batteries fresh (if battery-powered)?
• Is the fan set to AUTO (not ON)? “ON” runs the fan constantly even when not heating.

2. Circuit Breakers

Your furnace has its own breaker (or two). Check that it hasn’t tripped. Also check any secondary shutoff switches — there’s often one on or near the furnace itself.

3. Furnace Switch

Many furnaces have a power switch that looks like a light switch, usually mounted on or near the unit. Make sure it’s ON.

Most Likely Causes

1. Dirty Air Filter — The #1 Culprit

A clogged filter restricts airflow, causing the furnace to overheat and shut down via the safety limit switch. This is the most common cause of heating problems.

The Fix:

1. Locate your filter (in the return vent, at the furnace, or in the ductwork)
2. Check it — if you can’t see light through it, it’s too dirty
3. Replace with a new filter of the same size
4. Reset the furnace by turning it off for 30 seconds, then back on

Time: 5 minutes. Cost: $5-15.

Going forward: Check filters monthly, replace every 1-3 months.

2. Pilot Light Out (Older Furnaces)

If you have an older furnace with a standing pilot light (constant flame), it may have gone out.

The Fix:

1. Locate the pilot light assembly (usually behind a small access panel)
2. Turn the gas valve to OFF and wait 5 minutes
3. Turn to PILOT
4. Hold down the reset button while lighting the pilot with a long match or lighter
5. Continue holding for 30-60 seconds after it lights
6. Release and turn to ON

If it won’t stay lit: The thermocouple may be faulty (tells the gas valve the pilot is on). This is a $15-50 part but involves gas, so many prefer a pro.

3. Ignition Problems (Newer Furnaces)

Newer furnaces use electronic ignition — either hot surface ignitors or spark ignitors. These can fail.

Signs:

• Furnace turns on, you hear it trying, but no heat
• Click sounds but no ignition
• Furnace cycles on and off without heating

The Fix: Ignitors are replaceable parts ($20-80), but the repair involves accessing the combustion chamber and sometimes dealing with gas connections. This is moderate DIY if you’re comfortable, or a service call.

Cost: $150-400 installed by a pro.

4. Limit Switch Tripped

The high limit switch shuts down the furnace if it overheats. This is a safety feature, usually triggered by restricted airflow (dirty filter, blocked vents).

The Fix:

1. Turn off the furnace
2. Replace the filter if dirty
3. Make sure all vents are open and unblocked
4. Wait 30 minutes for the limit switch to reset
5. Turn the furnace back on

If it trips repeatedly, there may be a bigger airflow problem or the switch itself may be faulty.

5. Blower Motor Issues

The blower motor circulates air over the heat exchanger and through your ducts. If it fails, you’ll have a running furnace but no air coming out of vents.

Signs:

• No air from vents when furnace runs
• Humming sound but no air movement
• Furnace overheats and shuts down

The Fix: Blower motor replacement is a bigger repair — typically $300-700 installed.

6. Flame Sensor Dirty

The flame sensor verifies the burner lit. If it’s dirty with carbon buildup, it can’t detect the flame and shuts down the gas.

Signs:

• Furnace ignites then shuts off within a few seconds
• This cycle repeats

The Fix:

1. Turn off power to the furnace
2. Locate the flame sensor (a small rod near the burner)
3. Gently clean with fine sandpaper or steel wool
4. Reinstall

Time: 20 minutes. Cost: Free.

7. Gas Supply Issues

No gas means no heat. The gas valve might be off, or there could be a supply problem.

Check:

• Is the gas valve to the furnace open?
• Are other gas appliances working (stove, water heater)?
• Is your account current with the gas company?

Can You Fix It Yourself?

✅ DIY-Friendly:

• Replace the air filter
• Relight the pilot light
• Check and reset breakers
• Clean the flame sensor
• Open vents and check airflow
• Check thermostat settings and batteries

🛑 Call a Pro If:

• Furnace won’t stay lit after relighting pilot
• You smell gas
• Electronic ignitor has failed
• Blower motor isn’t working
• Problem persists after basic troubleshooting
• You’re not comfortable working around gas appliances

What an HVAC Tech Will Do

Diagnostic visit ($75-150):

• Test ignition system
• Check gas pressure
• Inspect heat exchanger
• Test electrical components
• Check for carbon monoxide

Common repairs:

• Flame sensor cleaning: $80-200
• Ignitor replacement: $150-400
• Blower motor replacement: $300-700
• Gas valve replacement: $200-600
• Heat exchanger replacement: $1,000-3,000 (often not worth it)

Prevent Future Problems

• Change filters regularly — monthly during heavy use
• Annual maintenance — have a tech inspect and clean the furnace before each heating season
• Keep vents open — don’t close more than 20% of your vents
• Listen for changes — unusual sounds often precede failures
• Install CO detectors — furnaces can develop cracks in the heat exchanger that leak carbon monoxide

The Bottom Line

Most furnace problems come down to dirty filters, ignition issues, or thermostat problems. Change that filter first — it fixes more furnace problems than any other single step.

If the filter is clean and the basic checks don’t help, you’re likely looking at a component failure (ignitor, sensor, motor) that’s best handled by a technician. These repairs are usually straightforward once diagnosed.