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Very often your furnace will blow the cold air at the startup because it is just being warmed up. Your furnace may also not ignite due to the lack of gas delivered (someone could turn it OFF by accident), and all you have to do is to turn a handle on the gas pipe.

There could be several possible scenarios to the cold air being blown out of your vents:

  • You are getting cold air all the time. No hot air at all.
  • You are getting cold air some of the time. You are actually getting hot air, but cold air is circulating as well.

Cold air all the time

If you are getting cold air all the time, that generally means that your furnace has just been shut down and only a fan is blowing. In this situation, the best thing you can do is to reset it and look at which part of the cycle it will fail.

If it starts the process but shuts down very quickly (within 90 seconds), you could be looking at the problem with a dirty filter. If the burners light up and shut off after a while (and this repeats several times), you could be looking at a problem with a dirty flame sensor.

If your burners, don’t come on at all, you may have a faulty igniterAny problem besides changing a dirty filter in your furnace should be handled by a qualified professional only!

If at any point your furnace shuts down, but your fan keeps on blowing, you will be getting COLD AIR being blown around your house!

Resetting your furnace may actually solve your problem and get your furnace functioning correctly again! So, how do we reset our furnace?

  • Step 1. Turn ALL power off to the furnace for 15-30 secs.
  • Step 2. Turn your furnace back ON.

Cold air some of the time

There are situations where your blower fan will be blowing even when the furnace is no longer heating up. This means that along with hot air, you will have cold air circulating around your house.

The first thing to check is your thermostat setting. If your fan button is in the “ON” position, you will have a blower fan running constantly. You need to put it on the “AUTO” setting to get the fan working only when needed.

If you are on AUTO setting and your blower fan is still working, there is also a possibility that something is stuck at the circuit board level. In the video below, it is recommended to give your furnace a little jolt:

Furnace Error Codes

Another thing you should do when troubleshooting your furnace is to check the error code of the blinking light (more common in the newer models) and WRITE IT DOWN! Why is this important? First of all, it will give you a clue as to what the problem with your furnace may be.

Second, if you start shutting the power OFF (or a furnace disconnect switch will be triggered), the code will be reset and you will not have access to it any longer. That will make troubleshooting a lot harder:

So, what is this code? Similar to Morse Code, it will give you information on the problems with your furnace. It is also called the “FAULT CODES” and has short or long flashes (that could also be color-coded).

First digit – short flashes

Second digit – long flashes

The meaning of the codes is usually located inside the front furnace panels.

** Important! Code meaning varies depending on the model and manufacturer. You can check your furnace manual to see the meaning of each code.

Furnace flame sensor

So, how do you use the code mentioned above? If you get seven or eight flashes (from the furnace example code above), you may want to clean your flame sensor first.

This is the most common problem with furnaces shutting down and short cycling:

If the control board does not sense a normal flame operation, it will shut the whole thing down after several tries. In this case, the simplest solution will be to take out your flame sensor (or flame rod) and clean it with a ScotchBrite pad.

This is what the flame rod (also called thermocouple or flame sensor) looks like:

You may notice some black or white residue on your flame sensor upon removing and it could be scrubbed away rather easily with any scrub pad (using sandpaper, even fine, is not recommended). Now you can put back a nice and shiny flame rod (or sensor) and see if it solves your problem.

Furnace filter

Another common problem behind furnace blowing cold air instead of hot is a clogged or improperly installed filter. If your furnace detects insufficient airflow, it will shut down to avoid overheating.

Here is a proper way to insert your air filter:

Filter’s “arrow” has to be pointed in the direction of the airflow.

If your air filter is inserted correctly when was the last time you changed it? A clogged air filter is another problem that you may face when you are troubleshooting your furnace.

You should always keep several clean filters at hand, so you can change them as needed. Here is a video on how to insert filter correctly:

Keep in mind that if your furnace detects any problem with the airflow, it will shut itself down. Furnace filters should be replaced every 3-6 months.

Limit switch

The limit switch is a sensor that monitors air temperature at the heat exchanger. Its primary purpose is to avoid overheating problems that could lead to a cracked heat exchanger (another article).

When overheating is detected (limit switch triggered), it will cut the gas supply and shut off the furnace. At the same time the blower fan will keep on working to cool the heat exchanger down and this is how you will get cold air circulating in your house.

In this case, it actually temporarily solved some kind of problem and further inspection will be required by an HVAC specialist. Even if furnace inspections sound like fun, if you have never done it before, leave it to professionals!

If your limit switch goes bad, it can do all kinds of things with your blower fan and even shut down your air handler before it gets a chance to warm up. One of the symptoms of a faulty limit switch is the operation of the blower fan without any heat coming through.

This part should be changed by a professional as well.

Igniter and pilot light

In the older furnaces, there is a pilot light that doesn’t go out. If it’s out, your furnace will never ignite. It can be reignited by a specialist (or yourself if you know how).

In newer furnaces electronic igniter replaces the pilot light:

If the igniter fails, heat will never be generated, and the blower fan will be circulating cold air as well.

Circuit board fuse

If this tiny little fuse blows, it can cause a blower fan to run while nothing else is running! This is NOT in all furnaces! In most cases, nothing will work at all!

To check the fuse, you will need to open a lower panel and find something that looks like a car fuse:

But it is NOT a car fuse! It’s a low voltage fuse that is designed for 24-volt systems (car fuse is 32 volts). Here are some fuses for furnaces from Amazon if you are having trouble finding any – 3-amp (paid link) or 5-amp (paid link).

If it’s blown, replace it with a new fuse of the same value, close the panel, turn the power back ON and test your furnace.

** Warning! Make sure you turn the power and gas supply OFF when working with the furnace and its components!

Attention! This article is for informational purposes ONLY and is NOT a replacement for a professional advice! You will need to visit school's website for details and updates, as well as consult your local HVAC specialist for appropriate solution to your problem.


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