Furnace Blowing Cold Air? What Spokane Homeowners Should Check

You hear the furnace kick on, you hold a hand over the register, and the air coming out is cool. A furnace blowing cold air is one of the most common calls we get in the last week of October, and it is less alarming than it feels. Sometimes nothing is wrong at all. Sometimes the furnace is doing exactly what it was designed to do to protect itself. Here is how to tell which, and what to check before you call.

Why a Furnace Blows Air With No Heat Behind It

Most people picture a furnace as one machine that either makes hot air or doesn’t. It is really two jobs that run separately. The burners heat a metal heat exchanger. The blower pushes house air across the outside of that heat exchanger and back out through the ducts. The air never touches the flame.

Because those are separate, the blower can run while the burners are off. When that happens, the blower is just moving room air around the house. Air at 66 degrees feels cold against skin at 98, so it reads as “cold air” even though it is the same temperature as the room.

So the real question is not why the furnace is making cold air. It is why the blower is running while the burners are not.

Furnace Blowing Cold Air: The Common Causes

1. The thermostat fan is set to On

Check this first. Most thermostats have a fan setting of Auto or On. On Auto, the blower runs only while the furnace is heating. On On, it runs all the time, including the long stretches between heating cycles, and that air is room temperature. Switch it to Auto. If the cold air stops, you are done.

2. It is the end of a heating cycle

When the burners shut off, the blower keeps running for a minute or two to pull the leftover heat out of the heat exchanger. The air starts warm and gets cooler until the blower stops. That is normal. If the cool air lasts more than about three minutes, or comes in the middle of a cycle, keep reading.

3. A dirty filter tripped the high limit switch

This is the cause we find most often, and it explains a pattern people find strange: the air is warm for a few minutes, then turns cold while the blower keeps running.

A clogged filter starves the furnace of airflow. With too little air moving across it, the heat exchanger overheats. A sensor called the high limit switch sees that and shuts the burners off, but it leaves the blower running to cool the heat exchanger down. That is the cold air. Once it cools, the burners light again, and the cycle repeats.

In Spokane, a filter that went in before wildfire smoke season can be loaded by late October even if it looks fine at a glance. Hold it up to a light. If you cannot see through it, a clean filter is the fix.

4. The burners light and then drop out

If the flame sensor cannot confirm a flame, the gas valve closes within seconds as a safety measure. When that happens partway through a cycle, the blower finishes its run with nothing heating the air. A dirty flame sensor is a common cause. So is a propane tank running low, which is worth checking first in places like Rathdrum and Athol where many homes run on propane.

5. The condensate drain is blocked

High-efficiency furnaces make water and drain it through a small plastic trap and hose. If the line clogs, a safety switch shuts the burners down. Depending on the furnace, the blower may keep going. Look for water around the base of the furnace or a hose full of sludge.

6. Only some rooms get cold air

If most registers blow warm and one or two blow cool, you do not really have a furnace blowing cold air. You have a duct problem. A duct that has come apart in a crawlspace or attic dumps warm air there before it reaches the room. That is a job for our air duct services team, not a furnace repair.

Do You Have a Heat Pump?

If what seems like a furnace blowing cold air is really a heat pump, air coming out at 90 to 95 degrees is normal, and it will feel cooler than furnace air at 120. A heat pump can also blow cool air for a few minutes during a defrost cycle. That is a different set of checks. Start with our heat pump repair page.

What We Do Not Recommend

Do not turn the thermostat up to 80 to force more heat. A furnace makes air at one temperature. A higher setpoint only makes it run longer, and if the high limit is tripping, it will just trip more often.

Do not ignore a furnace that keeps tripping its high limit. Each overheat stresses the heat exchanger, and years of it is how heat exchangers crack. A cracked heat exchanger can let carbon monoxide into the air the blower sends through your house. Keep working carbon monoxide detectors on every floor. The EPA’s carbon monoxide guidance explains the symptoms to watch for.

And do not let anyone quote you a replacement for a furnace blowing cold air before they have checked the filter, the airflow, and the temperature rise across the heat exchanger. Most of the time, the furnace is not the problem.

When to Call a Technician

Call us if the furnace is still blowing cold air after you have set the fan to Auto and put in a clean filter, if the burners keep lighting and going out, if there is water around the furnace, or if the air turns cold every cycle. We will measure the temperature rise and static pressure, test the flame sensor and high limit, and check the heat exchanger. Our furnace repair team covers Spokane, Spokane Valley, Liberty Lake, Coeur d’Alene, Post Falls, and Hayden.

If your furnace will not start at all, see our guide to a furnace not turning on. A yearly furnace maintenance visit catches dirty flame sensors, blocked drains, and airflow problems before the first cold week. You can also request service directly.

Frequently Asked Questions