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There’s nothing worse than cranking up the heat on a cold January morning in the Upstate and getting a blast of cold air from the vents. Last winter, our technician Andrew responded to a Duncan homeowner whose thermostat read 58°F and dropping — the system was running, the blower was pushing air, but there was zero heat coming through. Turned out the thermostat had lost communication with the system after the homeowner tried to switch from cool to heat mode. A thermostat replacement and system restart had the house back to 72°F within the hour.
That’s a quick fix. Other causes of cold air from the furnace are more involved. Here’s how our HVAC team works through them.
Thermostat Fan Setting
Check your thermostat fan setting first. If it’s set to ON instead of AUTO, the blower runs continuously — even when the furnace isn’t actively heating. During the off-cycle, you’ll feel cool air from the vents. Switch it to AUTO so the blower only runs when the furnace is producing heat.
Dirty Air Filter
A severely clogged filter restricts airflow so much that the furnace overheats. When that happens, the high-limit safety switch shuts down the burners to prevent damage. The blower continues running to cool the heat exchanger, pushing unheated air through your ducts.
This is one of the most common furnace calls we get — and the most preventable. Check your filter monthly during heating season and replace it when it’s visibly dirty. A 1″ filter typically lasts 30-90 days depending on your home (pets, dust levels, construction nearby).
Pilot Light or Igniter Failure
Standing pilot (older furnaces): If the pilot light has gone out, the burners can’t ignite. You can relight it following the instructions on the furnace label. If it won’t stay lit, the thermocouple — the safety sensor that detects the pilot flame — likely needs replacement.
Electronic ignition (modern furnaces): Most furnaces manufactured after 2000 use a hot surface igniter or spark ignition. Hot surface igniters are made of silicon carbide or silicon nitride and are fragile. They crack over time and eventually fail to glow hot enough to ignite the gas. This is one of the most common furnace repairs we perform — typically $200-$400 including parts and labor.
If you hear your furnace cycle on, run the blower, but never hear the burners fire, the igniter is the likely culprit.
Flame Sensor Issue
The flame sensor is a small metal rod that sits in the burner flame. Its job is to confirm that gas is actually burning. If the sensor is dirty (coated with carbon buildup), it can’t detect the flame and shuts the gas valve as a safety measure — usually within 3-5 seconds of the burners lighting.
The symptom: burners light briefly, then shut off, and the blower pushes cold air. A flame sensor cleaning is a quick, inexpensive fix that’s part of our annual maintenance tune-up.
Gas Supply Issue
If the gas valve to your furnace is partially closed or the gas supply has been interrupted, the burners can’t fire. Check that the gas valve on the supply pipe to the furnace is in the open position (handle parallel to the pipe). If you have other gas appliances (water heater, stove), check if they’re working — that tells you whether the gas supply issue is furnace-specific or house-wide.
If you smell gas near the furnace, do not attempt any troubleshooting. Leave the house immediately and call your gas utility’s emergency line.
Ductwork Problems
If the furnace is producing heat (the air at the supply register closest to the furnace feels warm) but distant rooms get cold air, the issue may be duct leaks in unconditioned spaces. Disconnected or leaking ducts in the crawlspace or attic allow heated air to escape before reaching the living space.
Cracked Heat Exchanger
This is the most serious possibility. The heat exchanger separates combustion gases from the air circulating through your home. If it’s cracked, the furnace safety systems may shut down the burners. A cracked heat exchanger is a carbon monoxide risk and should be addressed immediately.
Heat exchanger failure typically occurs in furnaces over 15-20 years old. Due to the cost of replacement (often $1,500-$3,000+), a furnace of that age usually makes more financial sense to replace entirely.
When to Call a Technician
DIY checks: thermostat setting, filter replacement, gas valve position, relighting a pilot light.
Call us: igniter failure, flame sensor issues, burners that won’t stay lit, suspected heat exchanger problem, any gas smell.
YOUTUBE EMBED: Got High Water Pressure? This One Valve Fixes It FAST — @YallCallWally
Call Waldrop Plumbing Air Electric at (864) 536-0887. We offer 24/7 emergency heating service.
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