Gas Furnace Blowing Cold Air? Common Causes and How to Fix Them

gas furnace blowing cold air

Does your gas furnace not seem to be heating your home right lately? Is that furnace blowing cold air at times?

This is a common problem that people will sometimes experience with their gas home heating system.

Does it mean you have to get out your bigger wallet and call your HVAC service person ASAP? Although that is a possibility it’s not always the case. There are some things that you can do that might offer you a quite inexpensive solution to solving your gas furnace problems.

That’s why we have taken the time to write this article. Once you read it you will be able to do a few simple troubleshooting steps that if they work, they will get that nice warm air you love flowing again. You can also save yourself the money you would have spent on an HVAC service call.

Eliminating Simple Furnace Blowing Cold Air Problems First

Here are a few simple troubleshooting steps for when your gas furnace is only blowing cold air. Some of them may seem obvious but you would be surprised how many people overlook these things and call an HVAC service company to come out to their home. This often results in a costly and unnecessary service call bill.

Make Sure Your Furnace is Getting Gas

Your furnace blowing cold air could be caused by the fact that it has no gas to burn. A gas furnace will either be supplied by connecting to a city’s natural gas line or by being supplied by your own personal LP storage tank and gas line.

If you have your own LP gas tank you need to check to make sure it’s not empty. You just locate the tank in your yard and open up the domed cover on top of it. Under the domed cover, there will be a gas fill connection and a pressure gauge. Check the pressure gauge; if it reads less than 25 you need to call your LP gas supplier for a fill-up.

To determine if you are getting gas to your furnace if it’s supplied by a natural gas line from your city is much more difficult. There is no gauge you can simply check. There are a couple of things you can try to do.

If you have an older gas furnace you will have a pilot light that stays constantly lit. This pilot light’s sole purpose is to ignite the gas that comes into your furnace’s burner tray. Look under your furnace and see if your pilot light is burning bright blue. If it is then you are getting gas to your furnace.

For systems without a pilot light go stand near the furnace and listen for the furnace’s air system to come on. If the air system comes on but the furnace does not light, take a second to sniff the air around the furnace. You should briefly smell a little gas after the furnace fails to come on.

Check Your Air Filter

It takes two components to make the flame that your gas furnace produces.

The first of those, of course, is the natural or liquid propane gas that comes into your furnace. Air is the other key ingredient that is used to regulate how hot the flame is that your furnace produces.

The air that feeds your burner system is pulled into your furnace through the same intake vents (sometimes referred to as registers) that bring in the air.

This air first passes through a filter. If a register is blocked or that air filter is very dirty it will shut down the airflow to your system. When this happens the system will detect it and not let the burners’ fire up.

There is a pretty simple resolution to this problem. Just take out the dirty air filters and either clean or replace them. You have to also make sure there are no boxes, furniture or other clutter blocking the air from getting into your air intake vents.

Check the Ignition Source

We mentioned that older gas furnace systems have a pilot light and newer ones ignite electronically. If these fail to work your furnace will only blow cold air because there is no heat source to ignite the gas coming into the furnace.

  • For furnace burner systems that use a pilot light, you simply need to relight that pilot light. This is best done using a long match. Once the flame gets close to the pilot light flame holder it should light very easily. If the pilot does not light after a few tries you have a bigger problem.
  • When a heater tries to ignite using an electronic ignition source you will hear some rapid clicking noises when that heater first tries to come on. That is a sign that power is going to the igniter and it’s also a sign that the gas valve is opening. You should then be able to look under your furnace and see a red glow on the tip of the igniter itself. If there is no red glow to your igniter tip after hearing the clicking noises, then chances are you need a new igniter for your furnace.

More Complex Troubleshooting

These are gas furnace troubleshooting steps for when it only is blowing cold air that is a little harder to do but can still be done by anyone even with a novice mechanical background:

Bad Thermostat

Your gas furnace will blow air through the system periodically, even if you do not have the heat on. It will circulate air constantly on certain air settings too. If you turn the heat on and this circulated air is not warm after a few minutes you may have a thermostat problem.

To check this, turn your thermostat all the way up and also turn off the constant air circulation setting. After that go stand near your furnace. You should hear your heater’s airflow system come on and you may hear clicking noises that indicate the heater control system is telling the burner it’s ok to come on and produce heat. If there’s only silence then there is a good chance you have a defective thermostat.

To replace a defective thermostat you will most likely have to replace the entire control head found on your wall. Read our Thermostat Buying Guide and Reviews for more info.

Check for Major Holes in Ductwork

Just like you will have trouble heating your home if the doors are open, so too will have problems generating the warm air flow from your furnace if you have a major hole or a seam has come undone in your ductwork.

This is a fairly rare occurrence on ductwork that’s not exposed to outside elements but you cannot rule it out if you have a flame at your furnace and it’s still blowing cold air.

Most ductwork in your house will either run through your attic or in a crawl space under your home. You will have to grab a strong flashlight and go into these spaces to actually examine all the exposed lengths of ductwork in these areas.

How do you fix it if you do find a sizeable hole or seam opening in the ductwork coming from your gas furnace?

You could replace those sections of ductwork that are causing the problem but that may prove to be difficult and expensive. A simple but still effective fix is to just seal any holes or separated ductwork seams up by covering those exposed areas with a professional grade duct tape.

Make Sure The Condensation Drain Line Is Not Plugged

This one only applies if you have a modern high-efficiency gas furnace. These systems heat so efficiently in their burner area that it results in much condensation being formed. So much condensation is produced that they actually have a catch pan and dripline to help remove it.

There is a sensor built into this line that will not allow your gas burner to fire if it senses that dripline setup has a blockage.

This catch pan and dripline setup are located directly under your gas furnace’s burner tray. If your furnace is blowing cold air you can try gently cleaning out any blockages in this dripline. Once the blockages are removed the sensor will allow your gas furnace to fire up again if that’s what was causing the problem.

When All Else Fails, Call Your Local HVAC Service Company

If you have tried all of the above steps without success you probably want to stop trying to fix the problem yourself. That’s unless you are very familiar with how a gas furnace works.

The reason for this is that further troubleshooting to figure out why your gas furnace is blowing cold air takes advanced furnace knowledge and the right troubleshooting equipment.

Not to mention, that if you do not know what you are doing and you continue trying to make repairs, you could actually make things worse. That could be a very costly mistake on your part. So at this point call a trusted HVAC service company and have them send one of their skilled service techs to your home to help.

While that service person is at your house working on your gas furnace, it’s also a great time to inquire about getting routine maintenance done once a year on your gas furnace heating system. Many times proper routine maintenance can help you avoid ever having problems like your gas furnace blowing cold air in the first place.

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