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Furnace Blowing Cold Air? These Common Issues May be the Reason

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When your furnace is blowing cold air your warm and inviting home can turn into an icebox in a matter of hours! And if it's the dead of Winter, this can be a serious problem that needs immediate attention.

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But before you call for help, you might be able to do some simple troubleshooting and save yourself a little money by fixing it yourself. In fact, the problem may be as simple as changing your filter! Changing your filter is a good starting point, but if the problem persists we have other options for you to troubleshoot.

Electric Furnace Blowing Cold Air

If you own an electric furnace and it's blowing cold air there's a few things that could be causing the problem. Check to see if one of these troubleshooting tips fixes the issue.

Fuses or Circuit Breaker

The first thing you should check is the fuses or circuit breaker. If you've recently had an electric power surge or any other power issue, the fix could be as simple as replacing a fuse or flipping the circuit breaker.

Leaky Ductwork

Leaky ductwork can cause serious issues for your heating system. There's a lot of issues that can cause your ducts to leak. Seals breakdown, the constant pull of gravity, rodents, incorrect installation, and even age can play havoc when it comes to delivering hot air.

Ductwork is often installed in areas such as attics, garages or crawl spaces where there isn't temperature control. When this is the case there's a good chance that the material will degrade and begin to leak.

Your furnace may or may not be blowing cold air if your ductwork is leaking, but you may have noticed it more challenging to keep one or two rooms warm. Another telltale sign is if your utility bills are higher than they should be based on use.

You can try to follow your ductwork and check for leaks, you may even find something obvious and easy to fix. But you might want to consider calling a professional HVAC service who has the knowledge, tools and experience to find and repair the problem. They may even need to run a static pressure test during their assessment.

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Thermostat Issues

You might be suprised to learn that one of the most common residential HVAC issues is the thermostat. Many times the problem can be resolved by restoring the thermostat to it's factory default settings. Consult your users manual for directions on restoring the settings for your specific unit. 

If this doesn't resolve your cold air problem it could be the themostat itself, but before calling a professional, we recommend trying the other troubleshooting suggestions first.

Black oil barrels

Oil Furnace Blowing Cold Air

When an oil furnace blows cold air the problem could be the result of one of the following reasons:

Out of Fuel

It may seem silly, but you'd be surprised at how many people's furnace problems are simply a case of running out of oil. This is especially common if you have an older fuel tank with a less-than-accurate fuel gauge. Before calling for repairs, check your fuel level.

If after refilling the tank your furnace doesn't turn on, it may be necessary to manually bleed and restart your furnace. Although you can do this yourself, we recommend calling a professional as there may be another underlining problem.

Clogged Filter

Changing your furnace filter is an important part of caring for your HVAC system and it should be done, at minimum, every three months.

With that said, even if you recently changed your filter you should check it again. If your furnace ran out of fuel, or was simply low on fuel, this could be your problem. The furnace will be more likely to pull in sediment from the tank, which can clog your air filter. 

Also, if your oil burner isn't functioning correctly you should check for a clogged filter.

Flame Sensor Issue

After checking that your fuel level is adequate and your filter is clean, the next area to check is the flame sensor. This is a likely cause if your furnace blows hot air, then cold.

If the flame sensor is dirty, it will produce inaccurate readings which can cause heating issues. The flame sensor itself is relatively easy to clean, but many homeowners are uncomforable working on their furnaces. If you choose to call a professional to do the job for you, this would be a good time to have your system inspected. 

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Consensate Drain Freeze

If you find that you go to bed comfortable and wake up freezing because instead of your heater delivering warm air, it's blowing cold, it might be because the condensate drain has frozen.

As your oil or gas-powered furnace heats the air, condensation is created. The condensation must be removed from the system and if your condensate drain terminates outside your home, it can freeze.

When the condensate drain freezes, it causes condensation to drip back into your system and trigger a shutdown. If your fan is set to "on," your furnace will continue to blow cold air until the drain thaws.

You can easily fix this problem by thawing the condensate, but you'll need to take long term measures to prevent the issue from happening again. The best thing to do is have an HVAC specialist route the condensate drain into your sewage drain. This will eliminate the risk of freezing.

Natural gas flame

Gas Furnace Blowing Cold Air

If you own a gas furnace, there are several potential things that can cause cold air to blow when you're expecting hot.

Your Fan is Set to On

If your thermostat is set to "on" instead of "auto," it could be causing your furnace to blow cold air. When the fan is on, the furnace will continue to blow, regardless of whether or not the air is being heated.

If this is the case, you can simply switch the setting from "on" to "auto" and your problem should be fixed.

Pilot Light is Out

The pilot light is another common culprit of gas furnace problems. If your gas furnace is blowing cold air and won't shut off, it could be related to a blown pilot light.

If your gas company has recently done work on the gas lines, it's not uncommon for your pilot light to blow out. It's also possible that the furnace's pilot light isn't well shielded. If this is the case, someone walking by the burner could generate enough of a draft to cause the pilot to go out.

If your pilot light did go out, try to relight it. If it stays lit, you should be back in business. But if it doesn't light, or it won't stay lit, you'll want to check that gas is flowing to the pilot. To do this find the gas line and make sure that the gas valve is parallel with the pipe.

This video will show you how to light your furnace pilot light.

Watch the Video

If the gas is on, but the pilot won't light or stay lit, the pilot tube or orifice may be dirty. If this is the case, you'll first want to turn the gas valve that supplies the pilot light off. Use a wrench or pliers to remove the pilot tube. Then, use a pipe cleaner or other small brush to remove any clogs within the supply tube.

Next, use fine-grit sandpaper to remove any oxidation from the tip of the pilot, and reattach the pilot tube to the furnace. Turn the gas valve to the "on" position, and try to light the pilot again.

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Faulty Thermocouple

If cleaning the pilot tube and ignition element doesn't solve the problem, and you continue to lose your pilot, the thermocouple may be defective.

The thermocouple is a sensor that regulates ignition, and if it requires adjustment or replacement your furnace will blow cold air. In most situations, it's best to enlist the help of a professional to fix this problem. 


Pilotless Furnace Blowing Cold Air

Pilotless furnaces have been an energy-saving breakthrough for the home and commercial heating industries. They are available in electric, gas and oil models. 

Depending on the type of burner you have, it isn't uncommon to experience any of the issues listed above. In addition, there are a few problems that are unique to pilotless furnaces which you'll want to look into if the troubleshooting tips above don't fix your problem.

Bad Hot Surface Ignitor

An HSI or Hot Surface Ignitor is the most popular way to ignite a pilotless furnace. Unfortunately, it isn't uncommon for an HSI to malfunction.

The HSI features a cobalt element that's prone to cracking. These components typically have a 3 to 5 year service life, so it's a good idea to keep a spare on hand for when it inevitably needs to be replaced.

Use care not to touch an HSI when troubleshooting or installing a new one. The oil from your fingers can cause permanent damage to the part.

Bad Vacuum Switch

To mitigate carbon monoxide and puff backs, an internal motor kicks on and creates a vacuum just before the burner engages.

Once the vacuum is created, the vacuum switch sends a signal for the furnace to ignite. If the vacuum does not occur, the furnace will not ignite. In other words; no vacuum means no ignition, which may be why you are experiencing cold air when you're expecting hot.

CPU Issues

As a pilotless burner owner, you'll want to keep your fingers crossed that your issue isn't related to your burner's circuitry. Excessive heat or exposure to water can damage the circuit board, which can cause your furnace to lose some or all of its functionality.

If you're lucky, the problem may be as simple as changing a fuse on the circuit board. Unfortunately, more complex issues in this department typically require an expensive replacement.

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