Why is the Furnace Cold?
From dirty flame sensors to failed igniters, we break down why your gas furnace is failing to provide warmth when it's needed most.
Primary Causes & Explanations
Dirty Flame Sensor
50% Prob.The most common culprit for 'short-firing'. The furnace lights but shuts off after 3-5 seconds because it can't sense the flame.
Failed Hot Surface Igniter
25% Prob.These silicone carbide elements have a limited lifespan. If they don't glow, the gas will never ignite. Look for small cracks.
Tripped High Limit Switch
15% Prob.Caused by overheating, usually due to restricted airflow (dirty filter). The system shuts down as a safety precaution.
Pressure Switch Fault
10% Prob.If the inducer fan isn't pulling enough vacuum (clogged chimney or failed motor), the gas valve will never open.
Standard Troubleshooting Protocol
1
Check that the thermostat is calling for heat and the gas valve is in the 'On' position.
2
Observe the ignition sequence: Does the inducer start? Does the igniter glow? Does the flame light?
3
Inspect the flame sensor for carbon buildup and clean with a non-abrasive pad.
4
Check for any LED blink codes on the furnace control board.
5
Ensure all intake and exhaust vents are clear of snow, ice, or bird nests.
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