Why is my furnace blowing cold air?

gas furnace flames

It’s a cold morning or the first chilly day of the year and you crank up your furnace. Only…your furnace is blowing cold air.

Oh no!

What do you do?

Well you probably turned to Almighty Google and found your way here.

Many times, the reason your furnace is blowing cold air is easily discovered and fixed!

Let’s take a look at some of the common causes:

Why your furnace is blowing cold air

1. Your thermostat fan is on

The first potential reason that your furnace is blowing cold air is that your thermostat has set the fan to ON instead of AUTO.

In this situation, the blower will run continually day and night.

That means it blows air whether your furnace runs or not.

If you’re getting warm air some times and cold at other times, you should check this first.

It might just be that simple.

furnace is blowing cold air because the thermostat fan is set to on

2. Your Thermostat is set to Cool

Another reason your furnace is blowing cold air could be that your thermostat is set to COOL.

If you live in an arid climate like El Cajon, you might find that it’s cold one day and hot the next. Or 50 degrees in the morning and 90 in the afternoon.

Since you constantly go back and forth between your hot and cold air, you might discover you left the setting on COOL.

Or this might be the first time you use the furnace for the winter.

And instead of switching it to HEAT, you just raised the temperature on the thermostat.

3. Your air filter is dirty

One of the more likely culprits is your air filter.

When was the last time you had it changed?

We recommend that your change your air filter at least once every 90 days while your furnace is in frequent operation.

Chances are, you probably haven’t changed your filter recently.

Maybe in years!

If that’s the case, it might be filthy.

A dirty air filter prevents good airflow, causing the heat exchanger to over heat and trip the high limit switch which disengages the flames.

This is done to prevent overheating, causing your coils to crack and leak poisonous fumes.

When this happens, the air handler might still be blowing air, but it’s not being heated before coming to your home.

furnace is blowing cold air because the air filter is dirty

4. Your pilot light is out

Similar to the issue above, if you have an older system with a pilot light, and that flame is out, the heat won’t engage.

Because the gas didn’t ignite, the valves closed and cut off the gas supply to your furnace.

Follow the instructions on your unit for relighting the pilot light, or call your local HVAC technician.

5. Your Flame Sensor is Dirty

Another common reason your furnace is blowing cold air is that your flame sensor is dirty.

This results in your furnace not heating up the coils.

How a furnace works is that a spark or flame ignites the gas that heats the coils in your unit.

If those flames fail to ignite, the furnace needs to know to close the valves so you aren’t pouring natural gas into the furnace.

That’s where the flame sensor comes in. Its job is to detect a flame, and if it does, it keeps the furnace running.

But if you have a dirty sensor, it won’t work and will close the valves. Even if a flame is present.

When this happens, your coils aren’t heating up and neither is the air passing through your system.

6. Your gas line is off

Similar to all of the above, if the gas line is off, blocked, or has a leak, the flames won’t ignite.

No flames means no hot coils.

No hot coils means no hot air.

If you suspect an issue with your gas line, call your gas provider immediately.

7. Your condensate drain is clogged

Yet another reason your furnace might turn off is a clogged condensate drain.

The air in a furnace works in a cyclical pattern.

Your furnace heats the air and send it to your home. When the air cools down, it goes back into the return vent to be heated once more.

After being in your home, the air has picked up some humidity.

When then humidity hits the hot coils in your furnace, it evaporates into a gas and settles on the sides of your unit.

After it has had time to coil, it forms water droplets which run down to the bottom of the furnace, where a drain takes it to the outside.

If that drain is clogged, your furnace will trigger a safety switch, turning of your furnace.

8. Your furnace hasn't warmed up yet

While it might be pretty simple in theory, often in the “heat of the moment” (that was a joke) we assume the worst.

But the reason your furnace is blowing cold air might not be a problem at all.

If you’re furnace is just turning on, the coils need time to heat up. That means the air passed through your system at first won’t be super hot.

You also need to factor in that the air in your ducting is likely cold and stagnant and has to be moved out first.

Give your furnace a few minutes to warm up and if it’s still cold, you might consider checking some of the items above.

vent for furnace is blowing cold air

9. Your ducting has leaks

Usually a furnace blowing cold air due to ducting leaks would also be in conjunction with lack of air flow.

If you feel like the air coming out of your vents isn’t as strong as it used to be, this might be a problem.

When your ducting has major leaks, then the hot air is mixing with the cold in your walls or attic.

Need Furnace help?

If you live in San Diego and you’ve checked all the items above but your furnace is still blowing cold air, give Precision Temperature a call.

Our friendly technicians can help you diagnose and fix your furnace woes.

Call 619-588-5321 to schedule an appointment.

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