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

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 those polar vortexes 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 more 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.

Before I proceed to explain the possible issues and solutions, I need to make sure you understand the basic furnace vocabulary well. 

Here are some technical terms I’m going to use:

How Does A Furnace Work?

Another thing that needs to be understood well before proceeding to solving furnace problems is how this system works. Therefore, let me explain its heating cycle.

When the indoor temperature falls below the thermostat’s established temperature, it commands the control board inside the furnace to start the heating process. The ignition switch turns, the gas valve opens, and the draft fan activates – that’s the start of the combustion process!

Then, as the furnace’s heat exchanger warms, the flue gasses escape through the exhaust pipe.

Then, a blower fan pushes room-temperature air over the heat exchanger, where it absorbs heat before being circulated through your home’s air ducts.

Now that you understand how the heating process works let’s explore why your furnace might be blowing cold air.

Furnace Maintenance Tips

As a rule of thumb, it is better to prevent problems than to fix them, so I’m giving you a few tips on how to maintain your furnace to lower the risk of its malfunctioning:

• Replace the Furnace Filter Regularly: This air filter protects your furnace from dust, animal hair, and other contaminants. Choose a furnace filter with a high MERV rating, which indicates how many microns a furnace can collect, and replace it regularly (on average, once every three months).
• Vacuum Dust Off the Pilot: Don’t let the pilot gather too much dust or debris. Otherwise, the color of the pilot light may change and give you false information.
• Clean Drain Pipes: Condensate lines are important for removing moisture from the unit. This keeps the furnace dry and free of mold and mildew. Hence, always remember to keep your drain pipes clean.
• Schedule Regular Checkups: Make sure to schedule an HVAC maintenance service at least once a year to prevent your unit from developing more serious issues in the future. You can always call Phyxter to schedule a professional service: +1 (778) 488-0690.

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

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

Most people don’t understand 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.

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.

✅ 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, call a local contractor and ask if they can find affordable air filters for you.

How to Check the Air Filter

1. First, shut off the main switch.
2. Remove the cover of the filter compartment in your air handler and remove the dirty air filter.
3. Replace with the new air filter.
4. Make sure the furnace air filter is compatible and facing the right direction.
5. Wait some time for your furnace to cool down, and then restart it.

Check The Thermostat Setting & Batteries

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 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.

How to Adjust the Thermostat Settings

1. Check your thermostat and review the fan setting. 
2. Ensure the switch is set to AUTO, not ON. 
3. When set to AUTO, the fan will only run during a heating cycle, preventing cold air from being pushed through your vents between cycles.

If the settings are correct, the batteries have been replaced, and the thermostat still doesn’t work, it’s most likely broken, and you need to contact an HVAC technician.

The Furnace Service Switch is Turned Off

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.

The Flame Sensor is Dirty (High-Efficiency Furnaces)

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

To clean it, you’ll need a 1/4″ hex head nut driver, an emery cloth, and a paper towel.

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, which causes the furnace short cycling.

If this is the case, then follow these steps:

How to Clean a Dirty Flame Sensor

1. Locate the toggle switch near the furnace or shut off the dedicated circuit breaker.
2. Remove the furnace access panel.
3. Locate the flame sensor near the burner assembly (it usually has one wire connected to it).
4. Disconnect the wire from the sensor’s back.
5. Unscrew the single screw holding the sensor in place.
6. Carefully slide the sensor out.
7. Hold the sensor by its ceramic or porcelain base, avoiding touching the metal rod.
8. Gently rub the metal rod with an emery cloth.
9. Wipe the rod with a clean paper towel to remove any residue.
10. Carefully slide the sensor back into place.
11. Secure it with the screw you removed earlier.
12. Reconnect the wire to the sensor.
13. Replace the furnace access panel.
14. Turn the power back on to the furnace.
15. Test the furnace to make sure it’s working properly.

Remember, if you’re unsure about any step or encounter any issues, it’s best to contact a professional HVAC technician for assistance.

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

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.

If you want to try to fix the pilot light on your own, follow these steps carefully

How to Fix the Pilot Light

• Power off the furnace.
• Find the pilot light assembly and set the reset switch to OFF.
• Wait 10 to 15 minutes to let any escaped gas dissipate.
• Turn the reset switch to PILOT. While holding it in this position, use a lighter or match to ignite the gas at the pilot end.
• When the pilot is lit, switch it to ON.
• Power up the furnace again.
• If you cannot relight the pilot or it keeps going out, call for furnace repair.

A Faulty Thermocouple Might Be The Problem

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.

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

The Electronic Control Might Be Messed Up

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.

How to Fix the Electronic Control

1. Turn off the main switch and restart your furnace.
2. 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.
3. 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.

There Might Be a Problem with The 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.

Here’s a step-by-step guide on how to clean a gas valve in a furnace:

How to Clean a Gas Valve

1. Shut off electrical power to the furnace at the circuit breaker.
2. Close the main gas shut-off valve near the furnace.
3. Remove the furnace access panel.
4. Locate the gas valve (usually near the burner assembly).
5. Carefully detach any electrical connectors from the valve.
6. Use a wrench to loosen and remove nuts or bolts securing the valve.
7. Check for signs of wear, damage, or debris buildup.
8. Pay attention to valves, seals, and connections.
9. Use a soft brush or compressed air to remove dust and debris.
10.  Be gentle to avoid damaging sensitive components.
11. Inspect the valve body and connections for any signs of corrosion.
12.  If significant corrosion is present, consider replacing the valve.
13. Apply a thin layer of high-quality lubricant recommended by the manufacturer.
14.  Focus on moving parts of the gas valve.
15.  Carefully reattach the gas valve to its original position.
16.  Reconnect any electrical connectors you removed.
17.  Turn the gas supply back on.
18.  Use a gas leak detector or soapy water solution to check for leaks around connections.
19.  Turn the electrical power back on.
20. Run the furnace through a heating cycle to ensure proper operation.

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 gasses 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!

There is Accumulated Water Around the Air Handler

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.

You can try to clean condensate drain clogs by following these steps:

How to Clean the Condensate Drain

1. Power off the unit at the breaker.
2. Find the condensate drain pan.
3. Use a wet/dry vacuum to get rid of any water sitting in the pan.
4. Take out the pan and clean it with mild dish soap and water, then replace it.
5. Trace the condensate drainpipe from below the pan to its exit outside your house.
6. Assemble your wet/dry vacuum at the drainpipe opening and suck out any clogs.
7. Power up the system again.

If it doesn’t work and your furnace is still blowing cold air, you need to contact an HVAC service provider.

Your Furnace is Overheating

Another reason for a furnace blowing only cold air is that it might be overheated. Don’t worry – this doesn’t necessarily mean the system is broken. When the system overheats, a safety feature shuts it off to protect you from potential hazards like a carbon monoxide leak, a fire, or even an explosion.

Even if the safety switch shuts off the furnace, it may blow cold air for some time before the whole system finally powers off. 

The Main Reasons Why a Furnace Might Overheat

• Mechanical failure: Furnaces have multiple moving parts, which can malfunction at times, especially in older units. Overheating is one of the most frequent issues arising from mechanical problems. If you think that might be the reason, it’s recommended that you call a professional HVAC technician to analyze and solve the problem.
• Dirt buildup: A clogged filter is one of the most common causes of a heater blowing cold air. Even if you replace the air filter, dirt and debris can still accumulate within the unit, leading to overheating. If your furnace looks dirty, you can try to clean it yourself, but I strongly advise you to contact an HVAC service provider for a more thorough and careful cleaning.
• Old age: As furnaces age, their performance weakens. While parts may wear out, a well-maintained furnace should work well for about 15 years. However, without proper maintenance, it could fail much sooner, so it is crucial to schedule it regularly. If your furnace is old and frequently overheats, its age might be the issue. Call an HVAC professional who will determine whether it can still be repaired or if you need a replacement.

You May Not Give Your Furnace Enough Time to Warm Up

A furnace contains many moving parts that take time to start working. When you turn on the furnace, it needs a moment to fully activate. 

If you notice that the heater is blowing cold air, try waiting a bit longer, especially if it’s the first time you’ve used it this season.

Your furnace should begin blowing warm air within 15 minutes. If the air still feels cold after this period, there might be another issue with the system.

You Might Have Leaky Ducts or Vents

Leaky air ducts or vents can cause hot air to escape from the furnace, resulting in the circulation of cold air.

If this is the case for you, call an HVAC technician for a leak inspection. This issue shouldn’t be addressed without proper knowledge and experience.

The Final Note

If you’ve checked everything and your furnace is still blowing cold air, it’s time to bring in a professional.

Homeowners in the North and Central Okanagan can count on Vernon Air Conditioning, Plumbing & Electrical Services for fast, reliable furnace repairs. We’re the highest-rated local contractor for a reason—our team delivers expert service you can trust.

📞 Call our Vernon, BC office today to schedule your service and get the heat back on.