Furnace Won’t Turn On but Fan Works? Common Causes and Fixes

It’s a confusing scenario: You can hear your furnace running. You can feel air coming out of the vents. But the air is ice cold, and the house temperature is dropping. If the furnace fan is working but the heat isn’t, your system has power, but the ignition sequence is failing.

This specific problem helps narrow down the culprit immediately. Since the blower motor is operational, we can rule out a total power outage. The issue lies within the heating components—safety switches, ignitors, or fuel supply.

Why the Furnace Fan Works but Heat Doesn’t

To fix this, you need to understand how your system thinks. Most modern HVAC systems treat the blower fan and the heating element as two separate operations controlled by the same board.

How a Furnace Separates Fan and Heating Functions

When you set your thermostat to “Heat,” it sends two signals: one to the blower (fan) and one to the ignition system (gas valve/burners). If the ignition system fails—perhaps due to a safety lockout—the control board may still run the fan to clear out any potential gas or heat buildup. Understanding how HVAC blower motors work can help you distinguish between a fan issue and a heating issue.

Common Reasons Furnace Won’t Turn On but Fan Works

Thermostat Fan Set to ON Instead of AUTO

This is the #1 cause of “cold air” complaints. If your thermostat fan setting is toggled to ON, the fan runs 24/7, regardless of whether the furnace is heating or not. Between heating cycles, the fan will circulate room-temperature air, which feels cold on your skin.

Dirty or Clogged Air Filter Causing Overheating

A restricted air filter causes the heat exchanger to trap too much heat. The High-Limit Switch detects this danger and cuts the fire to the burners, but it keeps the fan running to cool down the unit. This is a classic safety lockout mode.

Faulty Ignitor or Pilot Light Problem

If the fan turns on, runs for a minute, but you never hear the “whoosh” of the burners lighting, your ignition source is likely dead. On modern furnaces, the cracked Hot Surface Ignitor is a common failure point.

Bad Flame Sensor Shutting Off the Burners

If the furnace fires up, produces heat for 5 seconds, and then shuts off immediately (while the fan keeps running), your Flame Sensor is dirty. It thinks there is no fire, so it cuts the gas for safety. This is a simple cleaning fix.

Tripped High-Limit Safety Switch

As mentioned with the filter, if the limit switch trips frequently, it may wear out and stay stuck in the “Open” position. This prevents the burners from ever igniting, even though the fan blows.

Gas Supply or Gas Valve Issues

Check other gas appliances in your home. If the stove works, the supply is fine, but the gas valve inside the furnace might be stuck closed or not receiving voltage.

Furnace Control Board or Wiring Problems

If the thermostat is calling for heat but the signal isn’t reaching the gas valve, you could have a wiring break. Read our guide on diagnosing thermostat clicks with no start to trace electrical issues.

Step-by-Step Troubleshooting Checklist

Check Thermostat Heat and Fan Settings

Ensure the mode is “Heat” and the fan is set to “Auto,” not “On.” Raise the set temperature 5 degrees above the current room temp to force a call for heat.

Replace or Clean the Furnace Air Filter

Pull the filter. If it is grey and clogged, replace it. If the furnace was overheating, it may need up to an hour to cool down and reset the limit switch automatically.

Reset Power to the Furnace

Locate the power switch on the side of the furnace (or the circuit breaker). Turn it OFF for 30 seconds, then back ON. This resets the control board and clears minor error codes.

Inspect Pilot Light or Ignitor

Remove the furnace door (tape the safety switch if needed to observe). Watch the startup sequence. If the ignitor doesn’t glow, it needs replacement. If you have an older system with a pilot light, ensure it is lit.

Check Furnace Safety Switches

If you have a multimeter, check for continuity across the limit switch and rollout switches. If they are “open,” they have failed or detected a dangerous condition.

When to Call an HVAC Technician

While changing a filter or cleaning a flame sensor is DIY-friendly, some issues require a pro.

H3: Signs the Furnace Needs Professional Repair

Gas Smell: Never troubleshoot gas leaks yourself.
Rapid Cycling: If the unit turns on/off every minute (Short Cycling).
No Ignition Glow: Replacing an ignitor involves handling fragile components.
Control Board Failure: If the diagnostic light on the board isn’t blinking at all.

Frequently Asked Questions

Why Is My Furnace Blowing Air but Not Heating?

The most common reason is that the thermostat fan setting is on “ON” instead of “AUTO,” causing the fan to run continuously even when the heat is off. Other causes include a dirty flame sensor or a tripped safety limit switch.

Can a Bad Thermostat Cause the Fan to Work but No Heat?

Yes. If the thermostat fails to send a signal specifically to the ‘W’ (Heat) wire but keeps the ‘G’ (Fan) wire energized, you will get fan-only operation. Check our guide on smart thermostat efficiency and settings to ensure your device is programmed correctly.

Is It Safe to Run the Furnace Fan Without Heat?

Yes, it is safe. Running the fan helps circulate air and filter dust, but it will not warm your home. It typically costs a few cents per hour to run just the fan.

How Much Does It Cost to Fix This Furnace Problem?

Cleaning a flame sensor or changing a filter is less than $20 (DIY). Replacing an ignitor typically costs $150-$300. A new control board or gas valve can range from $400 to $800+ if hired out.