Furnace Starts Blowing Cold Air [3 Easy Solutions]

Share This Guide

If your furnace starts blowing cold air instead of heat, don’t ignore it. This could be a sign of thermostat issues, dirty air filters, or dirty flame sensors. These issues have to do with your heating system and if they are not fixed quickly, they can bring in other costly repairs. But the good news is, it’s probably not a big issue when your furnace is blowing cold air.

furnace starts blowing cold air

There are several other things that can stop your furnace from blowing hot air. Some of these issues are easy fixes (that you can sort out yourself). But other issues might need you to call in professionals to come and help. But before we find out the possible causes for your furnace not blowing hot air, lets first briefly find out how a furnace works.

How Does a Furnace Work?

Both electric and gas furnaces do their work pretty the same way. The key difference is that electric furnaces use electricity to produce heat while gas furnaces use propane or natural gas to heat the air for use in your home.

For an electric furnace, electricity is used to heat up the coils. When coils heat up, they will heat the air that passes through them, and the blower fan forces this air into the ductwork so that it can be distributed in your home.

On the other hand, in a gas furnace, burning propane or natural gas produces heat that passes through the heat exchanger. Air from your home that is blown into the heat exchanger is then heated up.

The furnace blower than forces this air into the air ducts to be distributed around your home. In both cases, the cooler, heavier air in your home is drawing back into the furnace and the heating cycle continues. So now let’s look at what can make your furnace to blow cool air.

What To Check First When Your Furnace Starts Blowing Cold Air

If you are like me, you don’t want to start calling HAVC experts to come and look at your furnace before trying it out yourself. We have given possible reasons your furnace is not blowing hot air. And we have also given solutions to each of them because we hate to leave you without solutions.

1. Thermostat Setting

Before going far with troubleshooting your furnace check that your thermostat is working correctly, and it has the correct temperature setting. The following are among many issues with your thermostat that can affect the way your furnace works.

 Furnace Fan Not set to “Heat” or “Auto”.

Your furnace is not always going to give out hot air when your fan is set to “ON”. When your fan is set on “ON”, fan blower will continue blowing air even when your furnace is not generating any heat (during the down cycle). During the furnace’s break, your fan will blow cold air.

Solution: If your fan is set to “ON”, change it and put it on “auto”. When your fan is set to auto, it will automatically stop blowing when your furnace goes into the down cycle. But blow again when your furnace goes into the heating cycle. Resetting your fan should solve the problem.

Low Battery

A low battery will always cause thermostat problems. If your thermostat relies on battery power, then occasionally you may need to replace that battery. A battery doesn’t have to be dead to affect your thermostat.

Solution:  If you think batteries are dying, consider replacing them right away. This should easily fix the problem without calling in HVAC professionals.

Thermostat wrongly programmed or set:

  • If you are using a programmable thermostat, you should also check if your thermostat was correctly programmed. Because you can specify the temperature of a particular day of the week, it’s also very easy to make a mistake.

Somebody else adjusting your thermostat

Another issue could be somebody else adjusted the temperature on your thermostat. Different Individuals have different temperature levels they consider comfortable. Sometimes, you or another family member would have inadvertently adjusted the thermostat

Solution: Check if your thermostat was correctly programmed or correctly set to the correct temperature. This should help with the problem, but if this doesn’t help, go the next step.

2. Furnace Blowing Cold Air- Dirty Air Filters

  If your Furnace air filters are not changed on time, they will cause issues with your furnace.  Air filters need to be changed at least once every three months to keep your furnace running properly. They remove dirt and other substances from the air before it is blown into the furnace.

But if they are dirty and clogged, they will limit the amount of air that is drawn into the heat exchanger. This can create overheating within your furnace. When that happens, the system’s safety measure will kick into action and shut off the heating system. This is all to prevent other parts of your furnace from getting damaged.

However, when the system shuts down, the blower fan can continue blowing cold air, that’s why you may feel cold air coming out of your vents.

Solution: Check your air filter. When your air filters are dirty replace them. In some furnaces Air filters can be found at the return vents. In others, they can be found right at the furnace. They are inexpensive and easy to install yourself. But make sure that you buy the correct type and size.

3. Dirty Flame Sensors

Flame sensors are part of the safety mechanism that is built within your furnace. When the flame sensors are very dirty, they can’t detect the flames in the furnace. And if they can’t detect the flames, the gas valve will close so that there isn’t any leaking of the gas.

This means there will be no combustion in your furnace, but the furnace blower fan can continue running, only blowing cold air.

Solution:  If flame sensors are very dirty, cleaning them will solve the problem.  In other cases, if they are completely dead, a replacement can do the job. For issues to do with flame sensor you will definitely need to call in HVAC professional service to come and help.


You should now be able to troubleshoot your furnace with the tips that we’ve given in this guide. So, the next time your furnace starts blowing cold air, try out our tips before calling any technicians. You’ll be surprised to know how quickly the problem will be sort out.

But remember, if you feel it is too much of work for you to do, it’s smart to call HVAC professionals to come and do the job for you. 



Share This Guide

Leave a Comment

Don't Settle With A Broken AC

Call 1-877-342-2087
To Find 24/7 AC/Heater Repair In Your Area