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Why is My Furnace Not Blowing Hot Air?

Image: a couple bundled up in blankets on the couch, cover image for Why is My Furnace Not Blowing Hot Air?

It’s no fun when your furnace is broken. Furnaces are responsible for keeping us warm and comfortable in the winter, so when they stop working properly, it’s time to act. Here are some tips to help troubleshoot why your furnace isn’t working correctly. The good news is they’re usually easy to fix with some furnace maintenance.

Thermostat Problems

Several things can go wrong with your thermostat. So, it’s a great place to start your checks.

Fan is Set to On

You set your thermostat to the desired temperature and leave it to warm your home. But what happens when your home reaches that desired temperature?

Image: a man adjusting a thermostat.
If you’ve left the setting to ‘on,’ your fan will continue to blow air, but at a colder temperature. To prevent this from happening, turn the fan to auto. When the temperature is reached, the fan will shut off.

Someone Keeps Adjusting the Thermostat

A person’s body temperature depends on several factors and may change during different seasons or times of the day. And if family members keep changing the thermostat setting, that could be the cause of the cold air. Keep an eye on your thermostat and look out for anyone altering the temperature setting.

Image: a hand adjusting an older thermostat.

Battery is Low

If your thermostat is powered by batteries, it should have an indicator light that alerts you when the battery needs replacing. Always call a furnace maintenance technician if unsure how to check or change the battery.

Air Filter is Clogged or Dirty

Check when you last had a furnace tune-up to determine whether this is the case. If it’s been a while, remove the old air filter and clean it with a vacuum cleaner. Then, replace it in the furnace and restart the unit again. If this doesn’t work, you may need to replace your furnace filter.

Image: a clean and dirty air filter side by side.
We recommend changing the air filter at least twice per year. Once in the spring before you start the air conditioner and once in the fall before it’s time to turn on the heater. For those who live in homes with long-haired pets, you may need to change your filter more than the usual amount.

High Limit Switch Failed

The high limit switch is a safety device that shuts down the furnace when it detects excess heat. If this happens, you will hear a clicking sound inside the furnace.

If you hear clicking from inside the furnace, turn off the power and call for furnace maintenance as soon as possible.

Clog in the Condensate Drain Line

A condensate line clog is one of the most common reasons a furnace stops working. This can happen when dirt, rust, and other debris build up on the line. When this happens, it can cause problems with your furnace’s operation and even cause damage to your system.

Issues with the Fuel Source

A furnace won’t work without fuel. So, it’s important to check this first. Look at the meter on your gas line or check the gauge on your oil tank. If your furnace is fired by electricity, there could be a broken fuse.

Broken and Leaking Ducts

If you have a leaky duct, the cold air from outside can get into the ducts lowering the temperature. This can result in lower energy efficiency for your heating and cooling system because it has to work harder to maintain a comfortable temperature.

Image: a pile of flexible piping used for ductwork.

Pilot Light Isn’t On

The pilot light is a small flame that sits on top of your furnace. It is there to ignite the gas for the rest of the burners. The pilot light is controlled by a thermocouple, which senses when it is hot enough to ignite the gas. If your pilot light isn’t on, your furnace will blow cold air out.

Burner Issues

If your furnace has been running and suddenly stops working, it may be because of an obstruction in the burner tube. If you have a gas furnace, take off the front panel on your furnace and check the flame pattern. It should look like a blue flame burning cleanly without any smoke. If it’s yellow or black, there’s probably something obstructing the flame from burning properly.

Flame Sensor Issues

The flame sensor detects if there is enough oxygen going into the combustion chamber for the burners to ignite and burn fuel properly. If anything blocks the flow of oxygen into the chamber or if there isn’t enough coming out of it, no ignition will occur, and cold air will blow out instead. The most common culprit here is a dirty or clogged flame sensor that needs to be cleaned or replaced.

Always Call a Professional for Furnace Maintenance

Furnaces are complex machines with many moving parts that require special skills and knowledge to operate safely and efficiently. A trained technician knows how to safely inspect, repair, clean, and maintain all furnaces (gas, oil, electric), including those that run on heat pumps or geothermal systems. They also know how to identify potential problems before they become expensive repairs.

Image: a person changing an air filter as part of professional furnace maintenance.

Don’t let your home get cold this winter. Instead, keep your furnace in top condition with a furnace tune-up with the experts at Adeedo.

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