Why Is My Furnace Blowing Cold Air? Fast Fixes Before You Call

A furnace blows cold air when the fan runs but the burners do not light or stay lit. The most common causes are a thermostat fan set to On, a dirty filter that tripped the high limit switch, a dirty flame sensor, a failed igniter, a closed gas valve or a clogged condensate line on a high efficiency furnace. With a heat pump, defrost mode or a failed reversing valve can also cause cold air.

In Las Vegas, this problem shows up most on the first cold night of the season. The furnace sat for months collecting dust, and the flame sensor or igniter picks that moment to fail.

What Should You Check First?

Start with the thermostat and the filter.

Thermostat fan setting

If the fan is set to On, it runs all the time, even between heating cycles. That air feels cool. Set the fan to Auto.

Thermostat mode and batteries

Make sure it is set to Heat and the set point is a few degrees above room temperature. Replace weak batteries.

Dirty filter

A clogged filter makes the furnace overheat. The high limit switch shuts off the burners to protect it, but the fan keeps running and blows cool air. Replace the filter and wait a few minutes.

Is the Furnace Getting Gas and Power?

Check these before calling.

Gas supply

Make sure the gas valve near the furnace is open, with the handle in line with the pipe. If other gas appliances are not working, contact your gas utility. If you smell gas, leave the home and call the gas company or 911 from outside.

Furnace power switch

Many furnaces have a light switch style power switch nearby. Make sure it is on. Check the breaker too.

Furnace door

The burner door has a safety switch. If the door is not seated, the furnace will not run.

Why Does the Furnace Light and Then Shut Off?

A dirty flame sensor is the most common reason. The furnace lights, the sensor fails to confirm the flame, and the control shuts the gas off for safety. The fan may keep blowing cold air.

Flame sensor

The sensor is a thin metal rod in front of the burner. A light cleaning by a technician often fixes it. This is one of the most common first cold night calls in the valley.

Igniter

A hot surface igniter glows bright orange before the burners light. If it is cracked or does not glow, the furnace cannot light. Typical igniter or flame sensor repairs in the valley run about $150 to $400.

Error codes

Most furnaces have a small light that blinks a code. The code chart is often on the inside of the door. Note the pattern before you call.

What If You Have a Heat Pump?

Many Las Vegas homes heat with a heat pump, not a gas furnace. Cool air from a heat pump can have different causes.

Defrost mode

On cold, damp mornings, the heat pump briefly runs a defrost cycle and the air can feel cool for a few minutes. That is normal.

Heat pump air feels cooler than gas heat

Heat pump air is warm, often around 85 to 100 degrees at the vent, not hot like a gas furnace. It can feel cool on your hand even when it is working.

Reversing valve or auxiliary heat problem

If the air stays cold for long periods, the reversing valve, defrost control or auxiliary heat may have failed. See what is a heat pump for how these parts work.

Could It Be the Ducts?

Yes, especially in attic installs. A disconnected or crushed duct in a cold attic can blow cold air into one room. If only one room gets cold air, ducts are a likely cause.

When Should You Call a Technician?

How Can You Prevent Cold Air Next Winter?

1. Change the filter before heating season.
2. Book a fall heating check to clean the flame sensor and test the igniter.
3. Test carbon monoxide alarms. The U.S. Consumer Product Safety Commission recommends alarms on each level and near sleeping areas.
4. Run the heat for a few minutes on a mild fall day to find problems early.

Our heating repair in Las Vegas page lists what a fall check includes. If your furnace is old, read how long furnaces last.

A Typical Las Vegas Call: The First Cold Night

A furnace lit, ran for a few seconds and shut off, while the fan kept blowing cool air. The flame sensor was coated after sitting unused since spring.

Cleaning the sensor fixed it in minutes. This is one of the most common calls on the first cold night of the season, and a fall check prevents most of them.

Need Heat Restored in Las Vegas?

We connect callers across the valley with independent providers who repair gas furnaces and heat pumps. Ask for the price before any work. See heating repair in Las Vegas. If you rent and the heat is out, read how long a Nevada landlord has to fix AC and heat. First cold night calls come in fast from Centennial Hills and Summerlin. For cooling problems, see AC repair in Las Vegas.

Frequently Asked Questions

Why is my furnace running but blowing cold air?

The fan is running but the burners are not lit or not staying lit. Common causes are a fan set to On, a dirty filter, a dirty flame sensor, a failed igniter or a gas supply problem.

How do I reset my furnace?

Turn the thermostat off, switch off the furnace power switch for about 30 seconds, then turn it back on and set the thermostat to heat. If it fails again, call a technician.

Can a dirty filter make a furnace blow cold air?

Yes. A clogged filter can overheat the furnace, which trips the high limit switch and shuts off the burners while the fan keeps running.

What does a dirty flame sensor do?

It fails to confirm that the burner is lit, so the control shuts off the gas. The furnace lights and then shuts down, often blowing cold air.

Why does my heat pump blow cold air in winter?

It may be in defrost mode, which is normal for a few minutes. If cold air lasts longer, the reversing valve, defrost control or auxiliary heat may need service.

Is it dangerous if my furnace blows cold air?

Usually not by itself. But if you smell gas or a CO alarm sounds, leave the home and call for help from outside.

How much does it cost to fix a furnace blowing cold air in Las Vegas?

Flame sensor and igniter repairs often run about $150 to $400 in the valley. Gas valve and blower motor repairs cost more.

Why is only one room getting cold air?

A disconnected, crushed or leaky duct in the attic is a common cause. A technician can inspect the duct run to that room.