Skip to Content

Furnace Won’t Reach Set Temperature? 4 Common Causes + Fixes

When your HVAC system works correctly, you should feel an instant temperature change when you turn the dial up. But if turning up the heat still leaves your home freezing cold, that means your furnace won’t reach the set temperature. There are a few reasons why this can happen, and it’s often something you can fix quickly. 

Your furnace won’t reach the set temperature because the filter is clogged, the blower door safety switch is tripped, the pilot light isn’t lit, or the thermostat is malfunctioning. Fixes include replacing the filter, closing the blower door, lighting the pilot light, and checking the thermostat.

This post will teach you to identify why your furnace won’t reach the temperature you set on the thermostat. I’ll also explain how to fix each issue. Let’s get started! 

The Air Filter Is Clogged

It’s common for particles to collect in the furnace over time. This is why you need to follow a regular maintenance schedule. HVAC specialists can clean the entire furnace while servicing the system. If you haven’t replaced the dirty air filter for some time, your furnace won’t reach the set temperature.

The filter keeps harmful particles and debris from entering your furnace and home. If these particles enter your home, it could affect your family’s well-being, leading to allergy symptoms and respiratory illnesses. Moreover, you’ll get noticeably less airflow.

This means less heated air can enter the ductwork and flow through your vents — your house won’t reach the set temperature.  

How To Fix

Start by inspecting the filter. The good thing here is that you can fix the problem relatively easily yourself. You can see the dirt and debris collected on the filter — cleaning it shouldn’t be a challenge.

However, you must open up the compartment where the filter is located. You’ll find the compartment close to the blower module in the furnace:

  1. Turn off the furnace.
  2. Refer to the owner’s manual to identify the air filter.
  3. Inspect the filter with a flashlight to determine if it needs replacing or is just dirty.
  4. Clean or replace the filter according to its state or age. Depending on the type, filters need replacing every 30–90 days. If your filter is new but dirty, run it under water and let it air dry.
  5. Put the filter back into the furnace.
  6. Check if the furnace is reaching the set temperature.

 Watch the video below to learn how to clean the filter:

The Blower Door Safety Switch Is Tripped

Most furnaces have a door on the main compartment where all essential components are located. Depending on the furnace model, there could be a safety switch sitting behind the door. 

When the door is closed, the switch is turned off. When the door is open, the switch gets tripped. A tripped safety switch means your furnace won’t come on or function normally if it does. Changing the thermostat temperature won’t fix this.

That’s because this is a safety feature that comes with your furnace. The furnace can only operate when the door is closed.

How To Fix

This is probably the most straightforward fix of all the possible reasons behind your furnace not reaching the set temperature — simply close the furnace door.

You should hear a click when the compartment door is fully closed. This is to confirm that the door is connected to the safety switch. Now, turn the furnace back on and see if your problem is solved. 

Note that the safety switch can become faulty over time. If it can’t detect that the door is closed, you may have to get a replacement switch. 

The Pilot Light Isn’t Lit

Older furnaces use pilot lights instead of electric ignitors. Your furnace relies on the pilot light’s constant flame to ignite the burners instead of a spark coming from the ignitor.

If the pilot light isn’t lit, combustion can’t take place. In this particular situation, you’ll find that your home remains cold no matter the temperature you configure on the thermostat. This is because there’s no flame on the burners. 

How To Fix

The specific procedure for getting the pilot light to have a flame depends on your furnace. Some furnaces have a visible pilot light; others have the part hidden away inside a compartment.

Once you find the pilot light, you have to ignite it manually. Use a lighter with a long tip to reach the pilot light.

The following video will walk you through the process:

There Are Problems With the Thermostat

If you tried all steps from above and the furnace still won’t reach the set temperature, there’s could be an issue with the thermostat. If you find that heated air is blown into your home but isn’t reaching the temperature on the thermostat, take a closer look at the thermostat. 

There are a few problems that may affect a thermostat. If you use a thermostat with a knob, it can be hard to tell if it’s broken. Look for LED lights that indicate power. It’s a bit easier to know that a digital thermostat isn’t working — it won’t show anything on the screen. 

How To Fix

Start by testing the thermostat’s power. You should use a multimeter for this step. Use the multimeter to test the wires responsible for delivering power to the thermostat. While at it, check the other wires that connect to other parts of your HVAC system as well. 

If power gets to the thermostat, but nothing goes out, you have a faulty thermostat. 

If a signal comes in and out, the problem likely lies elsewhere. The power supply could be at fault, for example.


When debris builds up on the filter, or you have a problem with the thermostat, you won’t feel a temperature change. Most of the time, this issue is something you can fix yourself, but there are times when you need a more advanced fix to get the temperature control working again.


  • 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.

As an Amazon Associate, we earn from qualifying purchases. We may also earn commissions if you purchase products from other retailers after clicking on a link from our site.