If your gas furnace is blowing cold air then you know how unpleasant it can be! It’s a cold problem that no one should have to deal with during winter in the Athens and Lancaster Ohio areas!

When I was a little girl I used to wear a flannel nightgown with little blue flowers on it in the winter. I would stand over our registers in the morning and wonderful warm air would poof it out and create a bubble of heat.  Nothing is more satisfying than a warm bubble when it’s freezing cold out.  

The last thing you want to have happen when chilly weather arrives is for your furnace to not be working!  Or, even worse, you discover that it’s actually blowing cold air! Brrr!  When you want warm air, cold air is worse than no air!  How frustrating to have to deal with a broken furnace right when you need it the most.

But don’t panic.

It’s quite possible the solution is a simple fix and you won’t need to call us at all. There are several possible causes for a gas furnace blowing cold air. So it’s hard to give you good advice without more information.

But here are 5 common causes you can fix yourself. See if it’s possibly one of these causes and hopefully you’ll have warm heat blowing out of your registers warm in no time.  

Your Thermostat is Set to “On”

The first thing to do is to check your thermostat.  If there’s a constant flow of cool air coming from your furnace the chances are your thermostat’s fan setting is set to “on,” in which case your furnace blower will constantly run even when your furnace isn’t heating the air.

You may ask yourself why anyone would want cold air blowing all the time?  What crazy person set this fan to on?  The main reason is to have better air quality in your home.  During allergy season, I often have my fan setting to “on.”  When your fan is constantly running, air is being cycled through your filter day and night. Also, this setting allows air to be more evenly distributed through all the rooms in your home, which will improve your comfort and make it so some rooms aren’t cold while other are hot.

That being said, if the air blowing from your registers is blowing right on you as you watch tv, you will be uncomfortable no matter how even the temperature is throughout your home.  So your best bet is to turn the fan setting on your thermostat to “auto.”  This will make it so it only blows air when the gas furnace is running and the air will be warm.

Cold Air is Pushed Out By Warm Air

If you turn on the furnace in the morning and it start by blowing cold air but then in a few minutes starts blowing warm air, then there isn’t anything to worry about.

Think of when you first start your car in the winter, it always blows cold air first and then it heats up.  Or a hot water faucet does the same thing.  That water can be very cold when you first turn it on.  The air actually inside your ductwork is not warm so that cold air has to be pushed out by the warm air coming from your gas furnace.  In addition, your furnace itself will not be at full capacity instantly.  Never fear, however, once it gets going there should be toasty air coming from those registers.

Obviously, if warm air doesn’t follow after a few minutes, something else might be going on and that’s time to schedule a service call.

Your Air Filter is Clogged

But what if your furnace blows hot air first, then cold air second, but then stops blowing any air at all after a while. What’s the possible problem then?

It’s possible your furnace overheated. If so, the limit switch, a furnace safety device, turned the furnace burners off.  The fan limit switch reads the temperature inside a furnace and once the temperature becomes warm enough, it turns the blower motor on sending warm air into a home.  The blower keeps blowing and keeps the furnace cooled to a safe level.  If the furnace gets overheated, then the limit switch fulfills its “limit” and shuts off the burners to be safe.

What would cause a gas furnace to overheat?

Restricted airflow. The number one cause of restricted airflow is a dirty filter.  Because air isn’t going through your system efficiently your furnace has to run longer to heat your home and it ultimately will overheat. When your furnace overheats, you will only get cold air.

The solution is very simple.  Replace your dirty air filter with a nice, clean one. Once airflow to the furnace is no longer blocked, your furnace will be less likely to overheat and should function normally.

Do NOT ignore this one.  You can ignore it, once your furnace cools down enough, it will blow warm air again so you may be tempted to procrastinate.  However, repeated overheating will damage your furnace’s heat exchanger and then instead of a relatively cheap fix (a new filter) you’ll be replacing the gas furnace heat exchanger.  This is one of the most expensive repairs on your whole furnace and often requires replacing the entire furnace!  

Many of our customers are signed up with our Planned Maintenance program where we change the filters for them.  Others buy multiple filters and keep at least one on hand to change out when needed.

The Pilot Light is Out

If your gas furnace is only blowing cold air, then the pilot light may be out, especially if your furnace is older.  You can’t have warm air from a gas furnace if there is no fire.  

Here are three possible solutions:

  1. Relight the pilot light.  If you feel comfortable doing this yourself, it’s a good place to start. Does the flame easily light and then stay on?  Great!
  2. Make sure there is gas. If the pilot refuses to light, ensure that gas is flowing into the furnace. Make sure the furnace’s gas valve switch is in the “on” position.  “On” means the switch on the valve is parallel with the gas supply pipe.  
  3. Thermocouple. If the pilot lights, but won’t stay lit, then there’s a problem with the thermocouple.  From time to time we sell home handymen their own thermocouples.  Usually, this is a fix for your local Heating and Cooling technician to handle.

The Flame Sensor is Dirty

Many modern furnaces are pilotless ignition furnaces.  These use a flame sensor to keep the furnace burning once it starts. If this flame sensor is dirty, your furnace will turn on and begin heating but then turn cold relatively quickly.

Like the thermocouple, this one is probably best left to the HVAC technicians.  Furnace components aren’t as simple as they were years ago and getting a professional in when you are dealing with such a key part of your home is a good idea.

Gas Furnace Still Not Working?

Still having problems? If your troubleshooting efforts don’t resolve the issue, something more serious may have gone awry.

The cold air coming from your furnace could be due to a malfunctioning circuit board, damaged ductwork, a faulty igniter, or any number of more serious problems. These are issues your HVAC tech can easily address.

Your Dave White technician will work through a series of tests to pinpoint the exact source of trouble, and get your heating system up and running again as soon as possible.

David White Services experienced technicians can inspect your whole system and make sure that warm air is coming out of those registers.  Give us the details about your problem, and we’ll get back to you ASAP!

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