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Lennox Furnace Not Blowing Hot Air? 4 Common Causes (+ Fixes)

A Lennox furnace not blowing hot air can be troublesome when colder weather rolls in. Fixing the problem might be as simple as flipping a switch, but you’ll likely need to open up the furnace to find the cause. 

To fix a Lennox furnace not blowing hot air, make sure the air filter has been cleaned recently, see if there are problems with the thermostat configuration, check the thermostat wiring for issues, and make sure the pilot light is burning.

In this article, I’ll explain each of these common causes of a furnace not blowing hot air. I’ll also walk you through how to fix each issue. Let’s get started.

Air Filter Needs Cleaning

Whenever there is a problem with the supply of air or heat from the furnace, it is good to start with the air filter. This is one of the furnace’s main components – it plays a role in ensuring dust and other particles in the furnace do not get blown through the vents.

By removing these particles from the air, the indoor air quality of your home improves.

As the filter blocks off particles, they start to accumulate on the surface of this part. The filter has very small holes and often consists of multiple layers. The particles can collect on the inside as well.

When there is a large collection of particles and debris on the filter, it may start to block air from the blower fan. This means heated air cannot push into the vents and heat up your home. 

How To Fix

If your furnace has a dirty air filter, then the solution should be relatively simple. You will simply need to clean the filter and put it back into the furnace. Not all filters can be reused continuously.

If you do not have one of the reusable options, then you should buy a replacement filter before removing the existing one:

  • Locate the filter in your furnace. Some furnaces come with a filter that is situated outside of the main compartment. It is, however, also possible for the filter to be installed on the interior of a compartment. 
  • Remove the filter from its casing and check the surface for a layer of debris. If you see particles on the filter, then you want to clean it. 
  • Some filters can be cleaned by using some running water. This will help to rinse all of the collected particles from the filter. 
  • You may need to replace the filter if it is worn out or has suffered damage. 

Here’s a video showing how a furnace filter is cleaned:

Problems With Thermostat Configuration

You’ve pressed the power button on the thermostat and expect the heat to start entering your home in just a few minutes. Sure, this is how a furnace work, but there are times where a few additional configurations are needed. 

The thermostat acts as a central control station for your furnace. Each setting that you configure on the thermostat will affect the functioning of the furnace. There are a few settings that may be behind a Lennox furnace not blowing hot air. This is why you should consider investigating all the settings on the thermostat. 

How To Fix

Your thermostat should be kept on during this process. You want the furnace to run to see if a change in settings on the thermostat rectifies the problem you face with hot air distribution:

  • Take a look at the thermostat. Check all of the device settings – you need to see what the temperature is set to.
  • Look at the fan setting. This will be position at “on” or “auto” in most cases. The ideal position is auto and not on. However, When turned to the “on” position, the fan continues to run even when there is no heat cycle. This can sometimes cause cold air to be blown into your home. 
  • If you have an HVAC system with a furnace and AC, then make sure the thermostat is set to “heat” and not “cool.” 

The Pilot Lights Need To Be Lit

If your furnace is a couple of years old, it might be a model that still uses a pilot light. The pilot light is a very small component but essential for the burners to do their job. When you turn the furnace on, the pilot light is used to create a flame – the flame is made through the gas passing to the furnace’s burner. 

Sometimes, the pilot light fails to work normally. This can be after a very long summer, for example. While the furnace was turned off, dust collected on the pilot light. Now, it has problems lighting – even when you use a lighter for manual ignition of its flame. 

How To Fix

Manually lighting the pilot light is a possible solution to this issue, but you should look into problems with the pilot light itself if this fails. If the furnace has been off for some time, there is a big possibility that the pilot light is simply dirty. This means it needs to be cleaned – once cleaned, you will be able to get it to light using a standard lighter you have lying around:

  • Consider using a can of compressed air when cleaning the pilot light. This ensures you can get particles out of the small areas of the pilot light. 
  • A vacuum can also work. This will suck particles out of those hard-to-reach areas of the pilot light. 

Replace the pilot light if you find that cleaning it does not help you light it manually. 

Broken Wires in Thermostat Wiring

The thermostat communicates with the furnace. When you turn the thermostat on and set a specific temperature, it signals the furnace to ignite and heat up air for your home. If there are problems with the wiring that runs from the thermostat to the furnace, it cannot effectively communicate. 

A broken or burnt wire can cause communication problems – which means the furnace may not turn on. A break in a specific wire may mean the furnace can turn on but adjusting the temperature yields no furnace results. 

How To Fix

You need to inspect the wires that connect the furnace and thermostat to determine the issue. Take a look at the wires that come out of the thermostat. Follow these wires all the way to the area it connects to the furnace. 

See if you can find breakage in the cables. You should also see if there are signs of damage. Perhaps the wire is twisted at a specific point. In this situation, the wires inside may have broken, even though the insulation is still in good condition. If you find a twit in the wires, consider opening up the insulation and seeing if the inside wires are broken. Check for cuts in the insulation as well – this may indicate problems with the wiring inside. 


Wires that have broken off, incorrect configuration on the thermostat, a dirty air filter, and dead pilot light are possible reasons for a Lennox furnace not blowing hot air. Understand where to look and how to diagnose the process, but consider leaving the fix to the pros. 


  • Jake Alexander

    Jake is a freelance writer from Pennsylvania who enjoys writing about science and sports. When he's not writing for Temperature Master, he can be found watching the NFL or playing basketball with his friends.

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