Why is My Furnace Blowing Cold Air?

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furnace blowing cold air

Why is My Furnace Blowing Cold Air?

Category:General

December through February tend to be our colder months around here in Jackson, TN. Once you leave your house between traveling, working in cold spaces, heat at work not being sufficient, most of us freeze throughout the day. When you get home for the day, you expect to walk in and the house be warm. The last thing you want to find is that the furnace is not working. Even worse, it’s actually blowing cold air!

Most People are not even sure what a furnace is besides the generic “the thing that heats your house.” So, here is a basic understanding of just how your gas furnace works:

You change your thermostat settings to indicate your desired home temperature. This sends a signal to your furnace.
Your gas furnace control board will then ensure that all of your system’s safeties are operational. Safeties are continually monitored throughout the heating process. Natural gas ignites in the furnace and then air is delivered throughout your home via the duct work (after a 30-second delay). If at any point during the heating process a safety is triggered, the natural gas furnace is cut off and the furnace will enter its shut-down sequence. This is so you and your family stay safe.

When your thermostat measures a reading at your set temperature your natural gas furnace will shut down. Whenever your thermostat measures a reads a temperature reading below your set temperature, it will send a signal to start the process all over again. Of course, this is just one type of system (a single-stage system with a non-programmable thermostat). There are many other variations of gas furnaces and thermostats that can alter this process slightly, but this is the basic gist.

So why is your furnace blowing cold air?

Here are 3 possibilities:

1) Thermostat fan setting is set to ON
Is your furnace blowing cold air only sometimes? Is the furnace’s blower constantly running?

Check your thermostat. (That’s where a hvac technician will start first, so you should as well.) If your thermostat’s fan setting is set to ON, the furnace blower will constantly run even when your furnace isn’t heating the air.

What can you do?
Turn your thermostat’s fan setting to AUTO. Now your furnace will only blow warm air.

2) Furnace overheated
Let’s say your furnace is blowing hot air, then you find the furnace blowing cold air, but then it stops blowing any air at all after a while. What’s the problem?

It’s possible your furnace overheated. If so, the limit switch, a furnace safety device, turned the furnace burners off. The blower keeps blowing to cool the furnace down to a safe level.

So what causes the furnace to overheat? Most commonly, a dirty furnace air filter.

The dirty air filter will block airflow, causing the furnace to run longer to heat your home until it overheats.

What can you do?
Change the air filter and see if your furnace starts only blowing warm air.

Note: DON’T ignore this problem. Repeated overheating will hurt the furnace’s heat exchanger, which is expensive to fix.

3) Pilot light is out
If your gas furnace is only blowing cold air, then the pilot light may be out. Check to see if it is and try these solutions:

Relight the furnace pilot light. If the flame stays on, great!
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 (parallel/in line) with the gas supply pipe. If the switch is on, then check to see if the pilot light is dirty.

Are you still having problems or didn’t find your problem listed? No worries! We perform furnace repair services. Call Cagle Service today, 731-300-1030, and give us more details about your problem, and we’ll get back to you ASAP!


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