Why Is My Furnace Blowing Cold Air? Check Out These Common Causes.

Why Is My Furnace Blowing Cold Air

πŸ”₯ Why Is My Furnace Blowing Cold Air?

As the winter has settled in, nothing can be more annoying than dealing with a malfunctioning furnace that doesn’t heat your home. If you continually face thoseΒ polar vortexesΒ in the northern hemisphere, your worry could be well justified.

After all, a heating system needing repair and cold weather is never a good match. Now let’s get to why you’re here, you have a furnace blowing cold air, and you’d prefer warm air instead.

Before you troubleshoot, you’ll need to understand a little about how a furnace works and how the heating cycle goes through before heat is distributed throughout your home.

I’m going to list the major components, briefly describe what they do, and give you some insight into what could be wrong and how you could temporarily fix it yourself.

πŸ”₯ The Top 4 Furnace Failures That Are Easiest to Fix Yourself

  1. Dirty Air Filters
  2. Thermostat Batteries
  3. Furnace Service Switch
  4. Flame Sensor

πŸ”₯ Is Your Furnace Blowing Cold Air Because of a Dirty Filter?

Woman holding pleated air filter

Something most people don’t understand is that the furnace’s heat exchanger needs a specific amount of airflow (CFM) to flow across it to remove the heat generated by burning natural gas, propane, wood, or oil.

Without removing that heat, the heat exchanger will get too hot and cause the high-limit switch to lock out the furnace.

And assuming that the blower motor and the control board haven’t failed, then the likely culprit for tripping the high-limit switch is a dirty filter.

βœ… PRO TIP: Find a service that delivers single furnace filters on a set schedule, and change the furnace filter as soon as it gets delivered.

You might be thinking that it’s not dirty yet, but the price to change a filter yourself is much cheaper than hiring a contractor to come out and troubleshoot your furnace just to find that it was only a clogged filter.

If you don’t go with the above option, then call a local contractor and ask them if they can find affordable air filters for you.

🧰 How to Check the Air Filter

changing out a Furnace filter

First, shut off the main switch.

Remove the cover of the filter compartment in your air handler and remove the dirty air filter.

Replace with the new air filter.

Make sure the furnace air filter is compatible and ensure it’s facing the right direction.

Wait some time for your furnace to cool down, and then restart it.

Related Reading: Your Air Filter Guide. What MERV Rating Do I Need For My Home?

πŸ”₯ Check The Thermostat Setting & Batteries

Smart thermostat

I’ve seen it a hundred times. A customer calls us because their gas furnace is blowing cold air.

I show up at the house for a furnace repair call, and the first thing I check is the thermostat.

In early fall, this happens all of the time.

The setting hasn’t been switched from AC to HEAT!

Simple mistake, but paying us to set your thermostat seems silly.

The next thermostat issue we notice is a blank screen…simple enough. The batteries are dead.

We change the batteries, set the thermostat to HEAT, and then to help the homeowner get some value for their money, we perform a basic furnace inspection and maintenance to confirm that their furnace is operating safely and efficiently.

The last issue you could have with your thermostat isn’t even an issue.

It’s just a misunderstanding.

Some people think that their furnace is blowing cold air when in reality, the thermostat settings have the FAN setting to ON instead of AUTO, and they feel cold air when the furnace isn’t even running.

Turn the temperature setting up, and boom, heated air starts to flow.

Related Reading: Thermostat Not Working? Read our Quick Fix It Guide!

πŸ”₯ The Furnace Service Switch is Turned Off

Furnace Shut Off Switch

I’m not going to be able to write much for this as it’s pretty simple.

The furnace service switch looks like a light switch and is usually located just inside the furnace room, so you can see how easy it could be to turn it off when you go into the furnace room to get something else.

So, before you call a professional HVAC contractor because your furnace is blowing cold air, check to make sure that the power is on to your furnace.

πŸ”₯ The Flame Sensor is Dirty (High-Efficiency Furnaces)

Furnace Flame Sensor

This is probably the most complicated repair for gas furnaces in this “Easy” category.

You’ll need a 1/4″ hex head nut driver, some emery cloth, and our blog article on how to repair a flame sensor.

But before diving into that, I want you to understand whether you need to go there.

Below I will explain what it looks like to have a dirty sensor.

If your furnace goes through its usual combustion process to the point where the flame lights for a short time, goes out and then does the same thing again a couple of times, then you most likely have a dirty sensor.

If this is the case, then you need to remove the flame sensor (it’s located near the furnace burners) and gently clean it with some emery cloth.

We have an entire article on this so make sure to check it out.

Related Reading: How to Clean a Furnace Flame Sensor in 5 Easy Steps!

πŸ”₯ The Pilot Light Has Gone Out (Mid-Efficiency Furnaces)

Furnace pilot light

If you have a gas furnace, you will have a pilot light assembly that ignites the furnace burners.

If the pilot light doesn’t stay lit, your furnace burner will switch off.

In that case, you will need to relight it.

However, a malfunctioning light can be dangerous as gas can accumulate and when the flame sparks again, you could cause an explosion.

So you must be careful while trying to relight the pilot light, or your furnace blowing cold air will be the least of your problems.

Please read our article on common causes and relighting your pilot light if this is the case.

🧰 A Faulty Thermocouple Might Be The Problem

Furnace thermocouple

A thermocouple or flame sensor (which looks like a copper rod near the pilot flame) is responsible for detecting that the pilot light is burning.

In response to temperature change, it sends an electrical current to the gas valve to feed gas to the light.

If the light is working fine, you need to check the thermocouple.

If the flame sensor is visibly corroded or you have an ignition problem, you need to change it out.

πŸ›‘ NOTE: Some furnace issues are harder to diagnose and repair which we have listed below.

πŸ”₯ The Electronic Control Might Be Messed Up

Furnace LED Indicator

Most modern furnaces boast electronic control, and electronic components are not always 100% right.

If you have checked everything so far and your vents still blow cold air, you need to think about the controls.

Turn off the main switch and restart your furnace.

If the cold air problem persists, you should hand the problem over to a professional HVAC technician.

If the LED in the furnace control panel is blinking red, you’ll need to count the number of blinks and compare that number to the diagnostic chart on the furnace access door.

If the blinks don’t match up with anything or if the blinks say that everything is normal, then you might want to call an HVAC contractor to come in and take a look.

πŸ”₯ There Might Be a Problem with The Gas Valve

single stage gas valve

Without gas, you’ll never get anything other than cold air out of your furnace.

A faulty or blocked valve will stop the gas supply to your ignition system.

If you know what you’re doing, you can remove the valve to clean out dirt and debris, though we seriously recommend using a furnace repair professional for this task.

πŸ›‘ SAFETY NOTE: We understand that you want warm air and don’t want to spend an arm and a leg on an HVAC professional, but working with combustible gases can be extremely dangerous and should be left to furnace repair professionals.

One other thing to consider is the gas supply.

A furnace technician can confirm that the gas company is providing the correct gas pressure to your home.

Related Reading: Suspect a Furnace Gas Leak? Here’s What To Do!

πŸ”₯ There is Accumulated Water around the Air Handler

Condensate Drain Line

HVAC systems usually produce a substantial amount of condensation and humidity.

There should be a drain attached to the furnace to remove the excess moisture.

If there’s a blockage, the overflow kill switch will shut the system preventing water damage and thus causing your furnace to blow cold air.

If you find water accumulating near the base, the condensate drain lines need to be cleaned.

Once you’ve cleaned and confirmed that you have proper drainage, reset the furnace by cycling the power off and on, and then you should have hot air in no time.

πŸ”₯ You Might Have Leaky Ducts or Vents

residential ductwork

Leaky air ducts or vents can cause hot air to escape from the furnace, which means blowing cold air.

If this is the case for you, call in an HVAC technician for a leak inspection.

If you have checked all of these boxes, you should now have hot air blowing out of your vents.

If not, your furnace may need a professional to look at it.

If your furnace is blowing cold air and you live in the Central or North Okanagan region, then it’s time to schedule a service call with Phyxter Home Services!

We are the highest-rated local contractor in the region to take care of your furnace repair problems.

Contact our Vernon BC office today!

Jim Dill

Jim Dill

Jim is one of Phyxter's local HVAC gurus and is a proud co-owner and the HVAC Manager of Vernon Air Conditioning, Plumbing, and Electrical Services in the Okanagan Valley.

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