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Furnace Blowing Cold Air: Common Reasons & Ways to Troubleshoot

Why Is My Furnace Blowing Cold Air?

Is cold air blowing out of your vents when the heat is on? Does your furnace turn on and off over and over, but even when your furnace turns on, there’s no heat?

Before you call Morris-Jenkins, there are a couple things you can check yourself first, if you wanna try to get to the bottom of things.

You can always call Morris-Jenkins! We’re here seven days a week til midnight and can get your furnace fixed fast. This common heating problem is something we see all the time—don’t panic! Let’s take a look at a few possible reasons.

Can a Dirty Filter Cause Heat Not to Work?

It sounds too easy to be true, but if you have a clogged air filter, it won’t allow air to pass through it. According to one of our HVAC technicians, “Filters are designed to collect dust and debris that would otherwise damage your unit. Over time, all this dust can lead to a clogged air filter, which won’t allow enough air to pass through. If your filter isn’t drawing in enough air, there’s less flow in your furnace.” Your furnace is a controlled fire burning inside your home. In furnaces, most heat exchangers, depending on brand and size, get up to 140-170 degrees Fahrenheit. Without the cooler air of your home flowing through it and keeping the temperature steady, it will continue to burn hotter and hotter, reaching a temperature of 180-200 degrees. Your furnace is equipped with several safety systems like a flame sensor and a high limit switch to shut the fire down when it begins to overheat.

When this happens, even when the fire is off, the blower will continue to blow air through your house, but there’s nothing heating it. That’s where the cold air is coming from! Have you ever burned your finger before? Did you blow on your finger afterward to cool it? Your blower works in a similar way, but the heat it blows off of your heat exchanger is what warms a room. When it cools down, the blower will continue to blow for a while, even if it’s no longer warming your house. When the temperature gets back in a safe range, the fire will relight, and the problems will just continue until eventually your unit locks down and the fire no longer relights. That’s why your furnace isn’t blowing hot air!

Basically, your clogged air filter isn’t allowing your furnace to cool down to maintain a safe temperature, so the safety sensors shut the fire off when it overheats. If this is the case, there’s good news! Your furnace is doing its job to keep your family safe!

The solution to this problem is easy—you’re overdue for a filter change!

Related: “7 bad smells that could come from your furnace”

Two air filters in a box on a tile floor.
A dirty air filter (left) as compared to a clean air filter (right). Source: Flickr

Furnace Blowing Cold Air After Filter Change

Not all filters are created equal. Have you ever looked at your filter? There’s a rating called a MERV (Minimum Efficiency Rating Value) assigned to every filter. Morris-Jenkins recommends filters with a MERV rating of 8. The MERV rating determines how compact the pleats are in the filter. A higher MERV rating means tighter pleats. Companies advertise high MERV ratings as having longer lives, but this is simply untrue.

Remember when we said a clogged air filter doesn’t allow enough air through? This will cause your furnace to overheat, and safety features like your flame sensor and high limit switch shut the process down. Just like a dirty filter, a higher MERV rating (such as MERV 13) can easily damage your furnace by not allowing air through because the pleats are too tight together.

In order to determine if your filter is the problem, turn off your thermostat, change the filter, and turn it back on. If that doesn’t work, or if you go to replace your filter and it looks clean (and the filter isn’t too high of a MERV rating), this is not what’s causing your furnace to blow cold air. If that’s the case, we highly recommend you schedule an appointment with a certified Morris-Jenkins technician who can properly diagnose the problem so your family can stay warm this winter.

Recommended: “Filters 101”

A variety of air filters are stacked on top of each other.
Not all air filters are created equal.

Troubleshooting a Furnace Blowing Cold Air

While most of the calls we receive for a furnace blowing cold air revolve around the filter in some way, there are other reasons why your furnace isn’t blowing hot air like it should.

Your Thermostat

A duct is covered with foil and duct tape.
Rats, cats, and other animals can get into your attic and tear up your ductwork, making your home cold!

Do you program your thermostat?

Many people have programmable thermostats but never take advantage of the benefits; instead, they leave it set on auto. If you do program your thermostat, be sure that you didn’t leave it on “fan on” mode. Setting your thermostat to “fan on” is great when you have house guests or when you’re vacuuming (your unit will pull any floating debris that might be kicked up into the air while you clean). When you don’t need it anymore, it can make it seem like your furnace is broken.

When your thermostat is set to “fan on,” that fan is going to keep blowing non-stop. Because of this, even when your furnace isn’t running, your fan will. It’ll make your home feel colder than it is.

Damaged Ductwork

One thing many people overlook is the ductwork. Damaged ductwork with holes could be the reason why you’re feeling cold air blow when your furnace is turned on. We’ve covered in the past how animals in your air ducts can live in and create holes that could potentially cause even bigger issues as well.

Undersized Ductwork

Have you ever sipped soda from a coffee stir?

If you have, you know what it’s like to have ductwork that’s too small. If your ductwork is too small for your home and your unit, your unit will struggle to move air through the ductwork. This affects the return air side and the supply side. Talk to your technician about replacing your ductwork to better suit your unit and home.

Clogged Condensate Line

Typically, heating and cooling systems only produce condensation when you run the air conditioner. If you have a high efficiency furnace, however, it’ll create condensation even when running the heat. Often, the condensate line won’t be insulated. Part of your condensate line runs outside (to what’s called the “termination point”). If it gets cold enough outside, water will freeze at this termination point outside, and eventually it can freeze all the way up the line. Even if the entire thing doesn’t freeze, the frozen blockage at the end can cause water to build up in your condensate line. This trips your flood safety device, shutting off your furnace.

When you have a high efficiency furnace, there’s even more reason to stay on top of filter changes! If you aren’t changing your filters often enough (or you’re using the wrong kind), debris can get into your condensate pump. This can clog the condensate line. If your condensate pump fills up with water, there’s a safety switch that’ll get triggered, shutting off power to the furnace.

What to Do If Your Furnace Is Still Blowing Cold Air

It’s always stressful when your furnace turns on but there’s no heat. Sometimes, the solution is as simple as changing out a clogged air filter. If a new filter doesn’t do the trick, give us a call and let us know there’s cold air blowing out of your vents when the heat is on. We’ll send one of our experts out to make sure you have hot air coming out of those vents by bedtime!

You wouldn’t take your car to just any mechanic. You’d want to take it to someone who’s certified to work on your make of car.

Why let just anyone work on your unit?

Furnaces can be dangerous if not serviced properly. Sometimes, it’s best to call in the professionals when you’re stumped. Morris-Jenkins has years of experience servicing and troubleshooting HVAC systems in the Charlotte area. Give us a call or fill out a form today—we’ll make sure your family stays warm!

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