Why is my furnace blowing cold air?
November 2, 2021 •Reid Geiler
After a long summer, the Midwest is finally starting to see some autumnal temperatures. As the air grows cooler, residents will make moves to prepare for winter. They’ll make their beds with winter blankets, pull out their collection of sweaters, and turn on the heat for the first time in months. A few unlucky families will find that their furnace is blowing cold air.
Cold air from a furnace is the last thing you want when the nights are getting chilly! But fortunately, this issue is often easy to resolve. We’ll walk you through what to do if your furnace blows cold air, starting with the simplest troubleshooting.
Thermostat fan setting
When was the last time you adjusted your thermostat fan setting? You might have made a switch at some point between now and the last time you turned on your heat. Maybe you wanted to turn on your HVAC’s fan to circulate the air and forgot to switch it back.
The first step to troubleshooting an issue with unwanted cold air is to check the fan setting. If the fan is set to “On” rather than “Auto,” then it will blow continuously. That means that even when the furnace is not actively heating air, it’s going to circulate air through the vents. As a result, it may go through periods of blowing out cold air.
This can be helpful for distributing warm air more evenly. But, if you only want warm air to blow out of the vents, you need to switch the fan setting to auto. That way, your furnace will only blow out air that it has heated.
The thermostat setting isn’t the only straightforward thermostat-related fix to a furnace blowing cold air. It’s also possible that your thermostat needs to be replaced. If it’s not a matter of the fan setting, ask yourself when you last replaced the thermostat. You may want to investigate the condition of the device if it’s been a while. Try changing the batteries and consider replacing the thermostat all together if that doesn’t do the trick.
If you’ve recently replaced your thermostat but did it yourself, it could be that you picked the wrong kind. Not all thermostats are compatible with all HVAC systems. When DIY troubleshooting fails, call Geiler for help with finding the right thermostat to fix the problem.
You might want to read Why is my thermostat blank?
Dirty air filter
Air filters are an essential part of a furnace’s functionality. They prevent particles of dust and dirt from entering and damaging the HVAC system. Filters also keep these particles out of the air in your home, which reduces allergens and keeps the space more comfortable.
Furnace air filters may need to be changed as often as every three months, depending on the living conditions. When a filter becomes too dirty, the consequences can be a pain. Sometimes they’re as inconvenient as a furnace blowing cold air on a cold day.
A very dirty air filter blocks the air from passing over the furnace’s heat exchanger. With air movement blocked, the furnace begins to overheat. This triggers the limit switch, which stops the heating cycle when the furnace reaches a certain temperature. As a result, the furnace only blows cold air, all because of a dirty air filter.
Once you’ve checked out your thermostat, the next step to addressing cold air from the furnace is looking at the air filter. It’s relatively easy to locate the air filter on a furnace. Most air filter compartments are quickly identifiable, but if you need help you can search your furnace model online or call the professionals at Geiler. You’ll know which replacement filter to buy by looking up the furnace model online, or by checking the current filter. To replace the furnace air filter, simply slide it out and slide in the new one.
As soon as the new filter is in, the furnace should have a chance to cool down and restart. This could stop your furnace from blowing out cold air. If not, it’s still smart to change the filter, so you haven’t wasted any time! Now it’s time to move onto the next potential cause.
If your home is heated with a furnace older than, say, 2010, then the issue may be the pilot light. Older gas furnaces use pilot lights to ignite the furnace burners that heat the air. If something has caused the pilot light to go out, the burners can’t turn on and neither can the heat.
When previous troubleshooting hasn’t stopped your furnace from blowing cool air, it’s worth looking into the pilot light. This takes a few steps and starts with turning off the furnace. Once it’s off, you can use your furnace’s user manual to find the pilot light assembly. Turn the reset switch to the “Off” position and wait 15 minutes. While you’re waiting, find a lighter or some matches. You’ll use these to light the pilot light.
Once it’s time, turn the switch to “Pilot” and use your lighter at the end of the pilot to light the outgoing gas. When the pilot is lit, you can switch it to “On” and turn on the furnace.
If the pilot light was out, this will almost certainly fix the problem. Your home should now have warm air from the furnace. But, if you have trouble lighting the pilot, keeping it lit, or simply feel uncomfortable with the project, you can call your local HVAC professionals for assistance.
When it’s time to call a pro
In many cases, fixing cold air from a furnace is simple. Other times, it takes the investigation of a professional.
Geiler has been serving the Cincinnati area for over 130 years. We’ve helped heat countless homes over the decades and have seen every cause of furnaces blowing cold air. We are confident in our ability to address issues with HVAC systems in all seasons. Whether you need help with a pilot light or need to investigate the potential of leaking air ducts or an inadequate gas supply, we can help!
You might also want to read:
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Is my furnace too old?
How often should I change my AC filter?
Should I leave the AC fan on?