My Furnace Is Blowing Cold Air When The Heat Is On (Here's Why)

Ryan Womeldorf
by Ryan Womeldorf

When you live in a cold-weather climate, your furnace becomes one of the most important tools at your disposal during those cold months. Your insulation can only keep so much of that cold out before your furnace needs to intervene and warm things up.

So, when you go to turn the furnace on and are met by cold air, it can be a frustrating experience. Not only are you left with cold air in place of warm, but you’re left with questions. Namely “why is my furnace blowing hot air? It could be due to an empty oil tank, issues with computerized controls, or a problem with the thermostat among others.

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Reasons Your Furnace Is Blowing Out Cold Air

When your furnace starts blowing out cold air instead of warm, it is important to find the culprit. There are plenty of minor reasons for this issue but a few serious ones as well. Troubleshooting the issue is the first step towards a fix.

In some of these instances, a quick do-it-yourself fix is more than possible. But in other instances, you won’t have much choice but to call in a professional. They will not only be able to recognize the issue far quicker but will implement a faster repair, too.

Electric Heaters

The most common cause of heating issues with an electric furnace is due to the air filter. When the air filter gets dirty, it can block the necessary airflow to the heat exchanger. Basically, this leads to the furnace overheating, even potentially breaking the heat exchanger.

Start by finding the air filter and cleaning or replacing it. If that doesn’t resolve the issue, it may be best to have a service technician come out to assess the problem.

The Wrong Thermostat Settings

Sometimes, it’s as simple as having the wrong thermostat settings. In an office setting, in particular, there can be a bit of a war over what the temperature is. In those battles, the thermostat fan can get set from AUTO to ON.

What does this mean? Well, the fan will keep on running even without providing actual heat. When you change the setting back to AUTO, you should have heat again. At home, this is a far more avoidable mistake.

Other Issues Related to the Thermostat

If that simple fix doesn’t resolve the problem, don’t count out the thermostat just yet. If you have recently installed a new thermostat, start there. It is entirely possible that you may have chosen a thermostat that simply isn’t compatible.

Even if that isn’t the case, try the simple things. Check the batteries, for starters. More often than not, a simple fix is the right one. There is the chance, however, that you get unlucky and the thermostat problem is actually a much bigger issue.

Computerized Controls

The thermostat is not the only computerized component of your furnace. For furnaces with an electronic control panel, in particular, you may need to reset the system first. Try turning the entire furnace off through the power switch, wait a couple of minutes, and then restart.

Think of it as rebooting your computer system to fix some sort of glitch in the software. Unfortunately, if that doesn’t fix the issue, you may need a pro to come out and check out the issue further.

Pilot Light

Much like your water heater, older furnaces have a continuous pilot light. From time to time, just like the water heater, the pilot light can go out. When that happens, your furnace is unable to heat up as it normally would.

The bad thing is that something as simple as a gust of air can cause your pilot light to extinguish. This is one of those potential do-it-yourself fixes if you have done it before and are comfortable in doing so. If you’re not comfortable, a professional can come out and resolve the issue in short order.


Though there are components in place to protect your furnace from overheating, it can still happen from time to time. This can happen for a few reasons, the most common of which is a dirty or clogged air filter.

It is imperative to change out your air filter on a regular basis so that it doesn’t become clogged and result in overheating. If your furnace isn’t blowing out hot air, try swapping out the filter and then restarting the entire unit.

Clogged Oil Filter

This only applies if you have an oil-fired furnace. From time to time, the filter can get clogged up with debris and dirt, the same as an air filter would get clogged on a gas-fired furnace system. When the filter becomes clogged, the ignition may be affected, and your furnace may stop blowing hot air.

A do-it-yourself fix can be implemented but it is important that you have at least a little experience working with furnaces. The fix can be quite messy, and it isn’t the same as changing out an air filter. It might just be easier to leave the entire thing to the pros.

Faulty Thermocouple

Maybe you can keep relighting the pilot, but it eventually goes out. When this happens, the thermocouple couple could be at fault. When that happens, it can lead to the furnace blowing cold air instead of warm.

The thermocouple is a sensor, and it is meant to control both the ignition and gas valve. It can be a pretty easy fix, even for inexperienced do-it-yourself types. Even a couple of YouTube videos can get you prepared to change out the thermocouple.

Oil Furnace Issues

There are some other issues that can take place within the furnace that may not be as apparent but are a problem no less. It can be difficult to spot these issues, so call in a professional to inspect and maintain your furnace to be safe. Even yearly maintenance can go a long way towards extending the life of your furnace.

  • Empty Oil Tank. Should you not have an oil-fired furnace, this won’t apply to you. But if you do, there is a chance that it could simply be out of fuel. When that happens, the blower will keep circulating air even after the burner stops producing the requisite heat.
  • Gas Valve. When the pilot light won’t light at all, it could be as simple as having a faulty gas valve. Another instance could be if you haven’t had the system cleaned in a long time, the valve can become stuck due to debris and dirt within. It is important to not only have your system checked but cleaned as well.
  • Dirty Oil Gas Burner. When the furnace burner gets ignored, grime and dust can accumulate. When that happens, it can lead to trouble with igniting. It cannot be emphasized enough that having your furnace cleaned from time to time can help to alleviate more issues than you realize.

Other Potential Issues

  • Leaking Ducts. The cold air blowing into your home may not actually be due to the furnace itself but the ductwork instead. When there are holes or cracks in the ductwork, air can escape through them taking the heat with it. Ductwork will require a professional fix in most cases.
  • Electronic Ignition. In newer furnaces, there is a device known as an electric ignition. This could be in the form of a hot surface ignition or an intermittent pilot. Your system may need some minor adjustments or for this part to replaced altogether.
  • Comprised Flame Sensor. If the furnace starts out by blowing warm air but quickly turns to cold, the flame sensor could be at fault. One that has become caked in grime or worn out, causing the burner to continuously shut off.

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Regular Maintenance is a Must

Though it has been said numerous times through this post, regular maintenance on your furnace is of the utmost importance. For those unfamiliar with furnaces, spotting issues as they develop may not be all that easy.

By bringing out a service technician yearly, you can stay on top of potential issues with your furnace. Staying ahead of those problems is the key to preventing expensive repairs or even an entire furnace replacement.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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