Why Furnace Is Running But Not Blowing Air?

There are not many scarier moments in a homeowner’s life during the winter than when their furnace is turned on, but no hot air is coming out of the supply vents.

You start thinking of how much money this will probably cost, as well as the fact that it’s freezing outside and you’re going to freeze. Luckily, it’s probably not as dire as you think. 

There are several reasons your furnace might be running but isn’t blowing any air. These reasons can include:

  • Air filter is clogged
  • Pilot light is out
  • Thermostat is incorrectly set
  • Gas supply is turned off
  • Air ducts are clogged
  • Furnace needs servicing/replacement

Many of these are easy fixes that you can take care of yourself, but for a couple of them, you’ll probably need to call a professional.

If you’re interested in keeping your furnace running smoothly throughout the winter, keep reading for more information about each of these issues and how you can try to fix them yourself. 

Why Your Furnace is not Blowing Air

A furnace isn’t a complex piece of home machinery. It’s the main component of a forced-air heating system.

Typically, it usually uses natural gas to heat air, then uses a blower to move the air through air ducts to different parts of the house.

If your furnace is powered on and running, but you don’t feel air blowing from your vents, there are a few issues you should check.

As mentioned above, there are quite a few reasons that a furnace may be on but not blowing hot air.

Many of these reasons aren’t cause for alarm, and you can either try to fix them yourself or call a specialist for a service call.

Here’s a quick overview of the different problems you can check. The rest of the article will go into them in more detail.

IssueReason
Air filter is cloggedDirt and dust clog the air filter and blocks return airflow to the furnace
Pilot light is outPilot light can’t ignite the blower because it’s dirty or malfunctioning
ThermostatThermostat is incorrectly set or is malfunctioning
Gas supply is offGas supply valve is turned off by mistake
Air ducts are cloggedSupply ducts get filled with dirt and debris and block airflow
Furnace needs servicingSome other issue is causing the problem, and you’ll need a professional

Air Filter Is Clogged

Changing your air filter regularly is an easy home-maintenance project that can keep your furnace in working condition.

The air filter helps remove dust and other particles from the air before it’s recycled through the house. They get dirty very quickly, and professionals recommend changing them often.

If your filter is filthy, it may inhibit returning air from your home to the furnace. If you check your filter and it’s dirty, try changing it for a new one.

Even if filters say they’re rated for six months, the best rule of thumb is to change them every three months.

Whether you change them, more or less often will depend on a few factors:

  • Your house is particularly prone to dust
  • The density of the filter (denser filters should be changed more often)
  • How often your furnace runs (colder areas will change it more often)

Your air filter doesn’t only affect whether your furnace blows air; it can also shorten your furnace’s life if you let it get too dirty.

A dirty filter forces the furnace to work harder to suck air into it, which will affect how long it will last.

If you want to avoid costly repairs or replacements, you should change your air filter.

Pilot Light Is Off

The pilot light is a small ignition device that ignites a furnace’s burner, which then is used to heat air for redistribution throughout the house.

This pilot light is always on, and it’s an actual flame on older furnaces, but on newer furnaces, it’s an electric ignition system and only lit when it needs to be. 

There’s a small aperture where you can look to see if your pilot light is out or if it’s having problems on most furnaces.

If you’re stuck on how to check if your pilot light is having issues, check youtube. You can likely find a video, like this one, for a furnace that’s similar to yours. 

Pilot lights can be pretty complicated. There are a wide variety of different types, so if you’ve determined that your pilot light is malfunctioning, your best bet is probably to call a professional HVAC technician. 

Thermostat Is Incorrectly Set

Another potential issue if your furnace isn’t blowing air is that your thermostat isn’t set correctly.

You should check a few things on your thermostat to see if it could be the problem behind your malfunctioning furnace:

  • Make sure that your thermostat is set to “heat
  • Make sure the temperature is set to higher than the temperature in your house 
  • Check the fan to make sure it’s set to “auto” instead of on
  • Change the batteries if it runs on batteries
  • Check the wiring to make sure all wires are secure

Unless you want to replace your thermostat or mess with the wiring yourself, you’ll probably want to call a professional to come to check it for you. 

Gas Supply Is Off

Although you may think it’s too obvious of a solution, there are quite a few service calls for furnaces that have a simple fix: the gas valve is turned off.

gas valve

If your furnace isn’t blowing hot air and you’ve had work done on your natural gas lines or your furnace lately, you should check your gas valve to make sure it’s on. 

Locating your gas valve may be tricky enough, but it can also often be difficult to be able to tell which way is off or on.

A general rule of thumb is that if the handle is parallel to the gas line, gas is flowing, and if it’s similar, gas is not flowing. 

One significant thing to note is that you should immediately turn off the gas supply and call someone if you smell gas.

As you can imagine, gas leaks are hazardous and can lead to fires or explosions, and you shouldn’t take the chance of something terrible happening. 

Air Ducts Are Clogged

If you’ve already checked all of these other items on the list and your furnace is still not working correctly, you may have clogged air ducts.

These are the metal tubes that bring heated air from your furnace to the vents around your house. The signs of clogged air ducts are as follows:

  • Different temperatures in different rooms
  • Poor air quality
  • Increase in home heating costs

Before you call someone for an expensive duct cleaning, it can be useful to spend some time narrowing down the issue yourself.

You can set your fan to blow and go room to room, testing the airflow coming from each vent.

Before you go testing, you can remove the vents to get a better feel for the airflow. 

If you notice significantly less air coming out of one vent, you may have a blockage in just that one branch of your air duct system.

Continue checking the rest of your house in the same way until you’ve figured out where your blockages are.


Cleaning your air ducts yourself is possible if you have the right attachments, but if you’ve got one clogged duct, you’re likely close to having others clogged as well.

Calling a professional to clean out all your air ducts might be somewhat expensive, but it’s worth it to keep from over-taxing your furnace and having low air quality. 

Furnace Needs Servicing

If all else fails, it’s probably time to have your furnace checked and/or serviced by a professional.

A reputable HVAC company or technician can send someone out to your house to take a look at your furnace and, hopefully, keep you from having to pay for a whole new furnace. 

Of course, if you determine that you need an HVAC repairman’s services, you should look for the following things to make sure they’re trustworthy and won’t rip you off:

  • Number of years on the job
  • Whether they’re insured
  • Whether they’re licensed
  • Online referrals and reviews
  • Price for a consult

Unfortunately, most places will charge you just to come out and take a look at your system, but they’ll usually roll that consulting fee up into the cost of labor and repairs.

That’s definitely another good question to ask when you’re calling around for quotes, as not all companies will cover that fee in the labor costs.

Closing Thoughts

Although you might feel some understandable alarm when your furnace stops blowing hot air, there are a few troubleshooting measures you can take before you call an expensive professional.

Taking the time to check the thermostat, the pilot light, the air filter, and your ducts can all help you save some money and, hopefully, get your furnace back up and running again.