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HVAC Diagnostic Guide HIGH URGENCY

Furnace Blower Runs But No Heat

Indoor blower fan operates normally but the furnace never produces warm air.

Quick Check Before You Call a Pro

  1. Check thermostat — is FAN set to ON (which runs blower without calling for heat)?
  2. Verify thermostat mode is HEAT, not just FAN
  3. Look at the furnace through the inspection window — are burners igniting?
  4. Listen for the inducer draft fan — should run before ignition attempts

01 · Most Likely Causes (Ranked)

35%

Thermostat FAN Set to ON (Most Common)

What's happening

FAN setting ON makes the blower run continuously regardless of heat call. Thermostat may be in COOL or OFF mode while the FAN is ON, creating the illusion of furnace failure when nothing's wrong.

How to check

Look at thermostat. There should be separate MODE (HEAT/COOL/OFF/AUTO) and FAN (ON/AUTO) settings. FAN ON + MODE OFF or COOL = blower runs without heat.

How to fix

Set FAN to AUTO. Set MODE to HEAT. Set temperature setpoint 5°F above current room temp. Heating should engage within 1-2 minutes.

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20%

Failed Hot Surface Ignitor

What's happening

The ignitor that lights the gas has cracked or burned out. Without ignition, the gas valve may open briefly then close (gas valve safety), but no flame. After 3 attempts the control board locks out. Blower may continue if thermostat is still calling.

How to check

Watch the furnace at startup. The ignitor should glow bright orange-red within 15-30 seconds. No glow = failed ignitor.

How to fix

Replace ignitor with matching type (silicon nitride lasts longer than silicon carbide). $25-60 part. Handle by ceramic base only — skin oils shorten lifespan.

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15%

Failed Pressure Switch

What's happening

Pressure switch verifies the inducer fan is drawing proper draft. If it doesn't close, ignition never starts — system goes to fault mode. Blower runs because thermostat call is still active.

How to check

Inducer fan runs but ignitor never glows. LED code indicates pressure switch issue. Also check vent pipes for blockages (snow, debris, bird nests).

How to fix

Clear any vent obstructions first. If switch itself is bad, replace ($30-80 part). Match manufacturer part number for proper pressure rating.

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12%

Failed Gas Valve

What's happening

Gas valve won't open when commanded by the control board. Ignitor glows but no flame because no gas. After 3 failed attempts, control board locks out.

How to check

Multimeter test at gas valve coil — should show 24V during ignition attempt. If voltage is present but valve doesn't open, valve has failed.

How to fix

Replace gas valve. Licensed gas tech recommended. $80-200 part. Match the model number exactly.

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10%

Locked-Out Control Board

What's happening

After 3 failed ignition attempts, the control board enters lockout for safety. Blower may continue from thermostat call. Until lockout clears, no further ignition attempts will occur.

How to check

LED diagnostic code shows lockout. Furnace had been trying to ignite earlier but stopped attempting.

How to fix

Power-cycle at the breaker for 1 minute. Then troubleshoot the underlying cause (ignitor, flame sensor, gas valve) before next attempt.

8%

Failed Inducer Motor

What's happening

Inducer draft fan doesn't run, so pressure switch never closes, so ignition never starts. Main blower may run independently due to thermostat call.

How to check

Listen for the smaller draft fan during startup. Should hear it spin up before ignition. Silence = motor failed or wiring issue.

How to fix

Replace inducer motor assembly. $150-300 part. Moderate difficulty job — requires disconnecting venting.

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ThermostatsHot Surface IgnitorsPressure SwitchesGas ValvesInducer Motors

02 · Frequently Asked Questions

Why does my furnace fan run but no heat?
Two possibilities: (1) thermostat FAN is set to ON instead of AUTO (most common — easy fix), or (2) actual ignition failure where the burners aren't lighting (ignitor, gas valve, pressure switch issue). Check thermostat FAN setting first.
How do I know if my ignitor is bad?
Watch the furnace at startup through the inspection window. A healthy ignitor glows bright orange-red within 15-30 seconds. No glow or visible cracks = failed. Sometimes you hear gas valve open with no ignition (whoosh-but-no-flame).
Why does my blower run after the heat is off?
That's the delay-off cycle — most furnaces run the blower 60-120 seconds after the burners shut off to extract residual heat from the heat exchanger. Normal, improves efficiency. If it continues for many minutes, fan switch may be stuck.
Can I replace a furnace ignitor myself?
Yes — straightforward DIY. Turn off power AND gas. Most ignitors are held by 1-2 screws and have a 2-pin connector. Handle the new one ONLY by its ceramic base. Total job: 15-20 minutes.

03 ·Related Symptoms

Furnace Not IgnitingFurnace Blowing Cold AirFurnace Not Blowing Air
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
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