Why Is My Furnace Blowing Cold Air Instead Of Heat?

Here are some of the leading causes for this issue:

Your thermostat has the wrong settings

Thermostats allow us to stay comfortable with ease. However, these devices can run into some problems at some point. If you have a heater blowing cold air instead of hot, your thermostat could be set incorrectly. Luckily, fixing this issue takes a matter of seconds.

As the seasons change, it’s easy to mess up your thermostat’s settings. This is especially true when fall turns to winter, which is when the furnace is cranked on for the first time. If you notice that your heater is blowing cold air, make sure to check the settings on your thermostat. Although it’s unlikely, you may have forgotten to move the switch to the ‘Heat’ position.

When looking at your thermostat settings, ensure the switch is in the ‘Auto’ position. If it's switched to the ‘On’ setting, the furnace’s fan will still blow air even when the unit is off. This can lead to some problems. To ensure a well-functioning system, make sure the thermostat is set to ‘Auto.’ Based on the thermostat’s settings, the furnace will come on at the same time as the fan. This is the best, most eco-friendly way to stay comfortable all winter long!

Your furnace still has to heat up

A furnace involves a lot of moving parts that don’t work instantly. When the furnace is turned on, everything needs time to fire up. That said, if you’re noticing that the heater is blowing cold air, try waiting a little longer. This is especially true if this is the first time the furnace is turned on this season.

Your furnace should start blowing warm air within 15 minutes. After that, if the air still feels cold, there could be another system issue on your hands.

Dirty air filter

An air filter is an essential piece of an HVAC system. When these are clean, these filters provide clean, fresh air for your home. However, when they’re dirty, your comfort could be in jeopardy. In fact, a dirty air filter can lead to a heater blowing cold air.

When this filter is too dirty, it blocks airflow over the furnace’s heat exchanger. This can cause the system to overheat, which trips a high limit switch—leading to a unit shutdown. This is a safety measure to prevent a fire or further complications.

If you’re unsure about the condition of your air filter, give it a visual inspection. If it appears overly dirty, go ahead and replace it. You can find affordable air filters at most home goods stores. If you need assistance, call a local HVAC professional for help.

No pilot light

If you’re asking yourself, “why is my furnace blowing cold air?” go ahead and check the pilot light on your furnace. Today, most heaters don’t have a pilot light, but they're found on many older models. This pilot light is a small blue flame that’s used to heat up the system. Without this light, the furnace has no way of heating up. This could be why you’re feeling the furnace blowing cold air.

To fix this pilot light issue, you’ll need to use a lighter to reignite the flame. While you can handle this yourself, choosing a professional may be the best decision. This will ensure everything is done professionally. Once the pilot light is lit, the system should be ready to create that warm air!

The system has overheated

When a furnace only blows cold air, there’s a chance the heater could have overheated. But don’t worry, this usually doesn’t mean the unit is broken. When the system overheats, there’s a safety feature that shuts the system off. This is a good thing—it could prevent a fire, carbon monoxide leak, or even an explosion.

Even if the furnace’s safety switch powers off the unit, it may temporarily blow cold air. However, the entire system will eventually turn off. 

There are a few different reasons why a furnace overheats, which includes:

Mechanical failure

As a furnace involves several moving parts and processes, a malfunction can happen at any time. This is especially true if you have an older unit. While a mechanical malfunction can lead to many issues, overheating is one of the most common.

Dirt buildup

A clogged filter is one of the leading reasons for a heater blowing cold air. But even if you swap the air filter with a brand new one, dirt and debris can still be found within the unit. This could eventually lead to overheating. If your furnace looks pretty dirty, call an HVAC provider for a professional clean.

Old age

The age of your furnace can also be why it overheats. As your furnace gets older and older, its performance will start to decline. However, while parts may wear out, your system should still last for about 15 years. But if you don’t follow preventive maintenance tips, it could die out much sooner. If your furnace is old and constantly overheats, its age could be the problem.

Leaky air ducts

Your HVAC system usually includes air ducts, which are scattered throughout your home. If you keep asking yourself, “Why does my heater blow cold air?” it could actually stem from leaky air ducts. On some occasions, warm air from your furnace can escape through holes in your ductwork. Unfortunately, it’s best to leave this issue to the professionals. Finding where the leak is can be hard enough, but sealing it can be even more challenging. By calling an expert, they can fix the problem in no time!

Other thermostat issues

While a thermostat’s settings could be why your furnace only blows cold air, other thermostat issues could be to blame. For instance, there could be a wiring issue. This is especially common if the thermostat installation was a DIY project. On the other hand, you could have a thermostat that isn’t compatible with your specific heating system. If that’s true, this could definitely cause the cold air issue. If you suspect a thermostat problem, be sure to call an HVAC tech for assistance.

Clogged condensate line

Some furnaces have condensate lines that transport water away from the unit. But when there’s a clog, the furnace won’t even ignite. This leads to cold air being pushed through your vents. You may also have issues with the condensate drain lines, which could also be why your heater is blowing cold air. The best way to fix this problem is to call an HVAC professional for help.