Here are 4 reasons why your furnace is blowing cold air

Your furnace is all that stands between your family and the cold night air outside. Most homeowners don’t really realize how cold their home can get in the winter until their furnace stops working or runs into problems. Your furnace blowing cold air is the last thing you need on a chilly, January night.

f you’ve noticed cold air blowing out of vents when the heat is on, you may have a problem with your furnace. In this article, we’ll walk you through the most common causes of these furnace problems, as well as what you can do to diagnose and fix these temperature issues.

Start by troubleshooting your furnace problems

Noticing a change in your home’s comfort? Are things starting to feel a lot colder, even though you have the thermostat set higher than ever before? Let’s start with some basic tests to determine what the problem is ahead of calling in an HVAC expert.

Use the hand test

If you feel as though your furnace isn’t working properly, start by feeling what’s coming out of your vents or registers. After a few minutes of the furnace running, put your hand up to the register. If you feel your furnace blowing cold air—or the air coming out of the vent is at room temperature—that could indicate that there’s a problem.

Look at your thermostat

Before you go any further, make sure that your home’s thermostat is set to the correct temperature and mode. Obviously, if the thermostat is set to “cooling,” you’ll need to switch it over to its heating mode. If you have a programmable thermostat, take this time to review your temperature settings—many homeowners later discover that their thermostat’s automated temperature controls were not configured correctly during initial setup.

However, the fan setting could also be at fault. If your fan setting is set to on (instead of “auto”), the fan might be running even when the furnace itself isn’t. This leads to cold air being pushed out of the ductwork and into your home. Reset the fan setting to “auto” if this is the case.

Check your furnace

If everything with the thermostat checks out, go into your garage or into your basement to look at the furnace. Does it turn on at all? When it does turn on, are there any strange noises or smells that are out-of-the-ordinary? If so, go ahead and turn the system off. You’re at the point where you’ll want to call in an HVAC professional for a closer look.

Is your furnace blowing cold air? Here are 4 reasons why.

Below, we’ve outlined the four most common causes of this malfunction, and what each indicates about your gas or electric furnace.

Not every furnace malfunction is caused by one of these four issues. In fact, there are many potential causes of furnace breakdowns and problems. Your best bet, as always, is to call in a professional who can look at your furnace and determine what the root cause of the problem is—and how to fix it!

Your furnace isn’t turning on

This is straightforward enough. When you turn on the thermostat and set it to heat your home, does the furnace switch on? If temperatures in your home are falling and the furnace still isn’t starting up, something’s wrong.

First, double-check if the system has power. While some gas furnaces can operate without electricity, many require an active electrical connection. For obvious reasons, if you have an electric furnace, you’ll need it to have power to operate. If your electric furnace is blowing cold air, it has power, but there may be something else that is preventing it from operating properly. If you’ve run through the thermostat test described above, you’ll need to take the next step of bringing in a professional for repairs.

If you have an older gas furnace blowing cold air, check its pilot light. Most gas furnaces built from 1990 onward have pilotless ignition systems, but an older furnace may still use a traditional pilot light to start the heating process. If there is no pilot light flame, you’ll need to relight it. Either follow the manufacturer’s instructions or—if the pilot light isn’t relighting or staying lit—call in an HVAC professional to help you.

Flame sensor malfunction

In a gas furnace, the flame sensor is an important monitoring device that protects your furnace and your home. As its name suggests, this sensor detects whether or not the natural gas or propane flowing into the furnace is igniting. If not, it cuts off the flow of gas: if the gas isn’t being ignited as part of the heating cycle, it could lead to gas buildup or gas leaking into the air of your home.

However, flame sensors don’t last forever. Over time, they can crack or otherwise become damaged because of the intense heat inside of the furnace. Or, they can be covered in soot that prevents them from working properly. In most cases, a malfunctioning flame sensor will incorrectly determine that there is no combustion when, in fact, your furnace is working normally. As a result, it shuts down the system.

The good news is that flame sensors are relatively inexpensive to replace. Go ahead and call your locally trusted HVAC contractor for an inspection.

Your furnace has overheated

You might be surprised to learn that furnaces can “overheat.” After all, they’re built to create heat—is there really such a thing as too much of it? The answer is yes. All furnaces have internal temperature monitors that shut down the system when things get too hot.

If you notice your furnace not blowing hot air, or only running for a few minutes at a time before shutting off, it could be a sign that this precautionary mechanism is shutting the system off.

In a vast majority of cases, the issue boils down to airflow. All furnaces need proper air intake to operate. If there is too little cold air coming into the system, the heating cycle doesn’t work properly and the furnace begins to heat up. Eventually, this is a runaway cycle that causes the furnace to shut itself off so as to avoid internal damage.

Most airflow problems are caused by either the air filter or the registers in the home. Double-check that your air intake filter is clean and installed correctly. An extremely dirty air filter can actually block inbound air from reaching the furnace. Then, look at all the registers in your home. If a majority of them are closed, that could also cause airflow obstruction.

Leaky air ducts

Every home, no matter how old, is bound to have a leak or two in the air ducts. As the home settles and shifts over time, the air ducts can develop small cracks, fissures, pinholes, or other forms of damage. They might even pull apart and separate! As your furnace and blower push heated air through these “Swiss cheese” ducts, much of the heated air is lost before it reaches your living room, bedrooms, or kitchen. As a result, your furnace has to work harder and harder to heat your home to the desired temperature.

Of course, air duct leaks aren’t just a one-way street. As heated air escapes, cold air from your attic moves in. This might be why you’re feeling cold air coming out of your ducts: your furnace is working fine, but the heat it’s creating isn’t reaching the inside of your home.

If your ducts are in an accessible place—such as a crawlspace or basement—perform a modified version of our hand test from earlier. Run your hands over the duct surface to see if you feel a temperature difference. If your ductwork has a leak, you’ll probably feel it right away.

Take note of where all these leaks are, and then call in an HVAC technician. Heating professionals have access to specialized heat detection software and tools, allowing them to pinpoint all the places where ducts are leaking and need to be repaired.

Call in a professional

Never try to fix your own furnace! Beyond the technical challenge involved, you’ll likely void its manufacturer’s warranty and you run the risk of causing permanent damage to the system. That’s if you can get the system running again: although we’ve outlined a few reasons why furnaces blow cold air, we’ve really only scratched the surface when it comes to potential furnace issues. A vast majority of homeowners do not have the training or tools needed to properly diagnose heating problems, fix them, and ensure the furnace is then working safely again.

In other words, you need to call in a local professional. Look for a company with experienced, certified technicians. When you call them, ask about their prior history working with furnaces of your type, as well as how soon they can be out to your home for repairs.