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Why is My Furnace Blowing Cold Air?


Image: couple bundled up on the couch with warm drinks, cover image for Why is My Furnace Blowing Cold Air?

Furnace maintenance is one of those things that many people put off until winter hits, and they realize that something is wrong. If your find your furnace blowing cold air, here are a few tips to help you find, and potentially fix, the root of the problem.

Thermostat Problems

Several things can go wrong with your thermostat, so it’s a great place to start when checking your heating system.

Fan is on the Wrong Setting

Your furnace fan runs whenever the thermostat calls for heat. You can change it so that it only runs when necessary. This is called “auto” mode and will prevent your fan from blowing cold air when the desired temperature is reached. It’ll also save you money on your heating bills. Oftentimes, we see homeowners set their thermostat to “fan,” instead of “auto.”

Image: a man changing the thermostat settings.

The Thermostat Keeps Getting Altered

In most cases, people won’t change the thermostat settings because they don’t want to make someone else in the home uncomfortable. However, if you live with other people, you may want to keep an eye on who is pushing the thermostat buttons. It is also a good idea to move the thermostat high enough on the wall so that kids can’t get to it.

Image: a hand is changing a thermostat's settings.

Battery is Low

When a thermostat battery runs low, it can cause the thermostat to stop working. This is not a sign of a problem with your heating and cooling system or your home’s wiring. It’s just a dead or waning battery. As a batter starts to decline, so will the thermostat functions. Make sure you replace thermostat batteries at least once every year.

A Clogged or Dirty Air Filter

If you find your furnace blowing cold air, you may have a dirty or clogged air filter. To make sure, remove the air filter and inspect it. If it’s clogged with dust, clean it with compressed air or wash it in warm water with mild soap or just buy a new one.

Image: a clean and dirty air filter side by side.

It’s recommended that you change your air filter twice every year. Once when it heats up in the spring, before you turn on the AC for summer, and once before temperatures dip in the fall before it’s time to keep the furnace on 24/7.

High Limit Switch Failed

High limit switches are designed to protect the compressor from overheating by shutting off power to the compressor. This can happen when there is too much heat from the compressor or a blockage in the line.

If your high limit switch does not work properly, it could cause problems with your furnace. If your high limit switch stops working, contact an HVAC technician for furnace maintenance. If left alone for too long, this problem could cause more damage.

Clog in the Condensate Drain Line

The condensate drain line is a narrow tube that runs from your furnace’s heat exchanger to a drain pan or other point where it can be released into a drain. If this drain line becomes clogged with debris, then your furnace won’t work properly because it won’t have any way to release its excess moisture. Your best bet is to call an HVAC technician to clean your drain line.

A Problem with the Fuel Source

You could be out of fuel, so your furnace has stopped working. Here are some things you can do before calling in a professional:

  • The circuit breaker or fuse box. If you see a blown fuse or tripped circuit breaker, replace it with a new one of the correct amperage.
  • The gas supply line to make sure it’s not leaking or crimped.
  • Look to see if you have oil left in the tank.

Leaking Ducts

The biggest culprits for leaks are usually old or damaged ductwork that isn’t properly sealed against leaks from within or without your home. However, these leaks can occur at almost any point along the HVAC system.

Image: metal tubing used for ductwork.

Pilot Light Isn’t On

The pilot light is a small flame that ignites the gas coming through your furnace. If your furnace doesn’t have a pilot light, it uses electricity to ignite the gas.

Check to see if the pilot light is out. If so, you’ll need to relight it and reset the thermostat.

Burner Issues

Burners are designed to burn fuel at a very high temperature. Over time and with use they can become clogged with dust and other debris. If you’ve owned your furnace for a long time and it burns out frequently, check the burners to make sure they’re not blocked by anything.

Flame Sensor Issues

If the flame sensor is dirty or worn, it will not be able to detect the flame correctly and will cause the furnace to shut off. This can result in poor performance and an increase in energy costs. You may be able to clean your flame sensor, but if in doubt, call in a professional. You may need the experts to install a new sensor.

Always Call a Professional for Furnace Maintenance

Furnaces are complex machines that can break down at any time and put your home at risk. It’s dangerous to mess with gas or electricity unless you know what you’re doing.

Image: an HVAC technician changing the furnace's air filter.

Keeping your furnace in good working order is essential, so don’t forget to schedule annual maintenance and tune-ups. A qualified HVAC technician will ensure your furnace stays reliable and, more importantly, safe.

Make sure you stay warm this winter. Keep your furnace working in top condition by scheduling a furnace tune-up with the pros at HELP.

Posted in: Heating

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