A furnace that runs but blows cool or room-temperature air is a common and frustrating problem, and the cause is often something simple. Here is how to work through it safely before you call for service.
Check the thermostat fan setting first
This is the most common and easiest cause. If your thermostat fan is set to ON instead of AUTO, the blower runs continuously, including between heating cycles, so it pushes unheated room-temperature air through the vents. Switch the fan to AUTO and it will run only when the furnace is actually producing heat.
A dirty filter or airflow problem
A clogged filter or blocked airflow can cause a furnace to overheat. When that happens, a safety device shuts off the burner to protect the equipment, but the fan keeps running, so the furnace blows cool air. Check and replace a dirty filter and make sure return and supply vents are open.
The furnace is in a safety lockout
Modern furnaces protect themselves. After repeated faults, a furnace can enter a lockout where it stops firing but the fan may still cycle. Many units have a status light that flashes a code. A lockout is a sign the furnace needs attention rather than just a reset.
Ignition or gas supply issues
If the burner is not igniting, there is no heat to deliver. Causes range from an ignition or flame-sensor fault to a gas supply problem. Confirm other gas appliances are working, but ignition components and gas issues are best left to a qualified technician.
When to call a professional
If the simple checks do not solve it, the furnace keeps locking out, or you ever smell gas, stop troubleshooting and call for service. If you smell gas, leave the home first and call your gas utility or emergency services from outside. Persistent cold air usually points to a problem a technician should diagnose.
Frequently asked questions
Why is my furnace blowing cold air?
The most common cause is the thermostat fan set to ON instead of AUTO, which blows unheated air between cycles. Other causes include a dirty filter, a safety lockout, or an ignition problem.
Should the thermostat fan be on AUTO or ON?
AUTO is the usual setting. It runs the blower only when the furnace is heating, while ON runs the fan constantly and pushes room-temperature air through the vents between cycles.
Why does my furnace blow cold then shut off?
A dirty filter or restricted airflow can overheat the furnace, which trips a safety limit that cuts the burner while the fan keeps running. Replacing the filter and clearing vents often helps.
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