Gas Furnace Blowing Cold Air

A gas furnace blowing cold air is commonly caused by a clogged air filter, failed pilot light, or the thermostat being set to "on" and not auto. Before calling a professional check your filter, make sure the pilot light is on, and reset your furnace. If the problem persists call a service technician, you may have a dirty flame sensor, faulty limit switch, or cracked heat exchanger.

Severity

Moderate

What It Means

The furnace is running but only blowing cold air. This problem can usually be fixed by replacing the dirty air filter, resetting the system, or lighting the pilot light. If that doesn't work you will need the help of a professional.

Safe To Use

This problem is usually due to a clogged filter or failed pilot light and not an immediate danger. However, if you have a cracked heat exchanger it can leak carbon monoxide and should be fixed immediately.

Estimated Repair Costs (including service fees)

• New Flame Sensor: $100 to $250
• New Limit Switch:$200 to $300
• Cracked Heat Exchanger:$1,000 to $3,000 (may want to replace whole unit)

Causes

Causes include dirty air filter, thermostat turned to "on" and not auto, overheating, tripped limit switch, failed pilot light, cracked heat exchanger, and condensate line blockage.

Repair vs Replace

You should probably repair your furnace for this problem as it may just need a changed air filter or system reset. If it has a cracked heat exchanger and your furnace is older than 10 years old, you may want to replace the whole unit.

Prevention

Change air filters every 3 months, make sure your pilot light is lit, keep your thermostat on "auto", and schedule annual maintenance to clean the sensors.