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


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You have become used to turning the knob on the thermostat and instantly feel a change in temperature throughout your home. Changing the thermostat temperature no longer affects your home’s interior means the furnace might be facing a problem. There are a few reasons why this may happen, and it is often due to a problem that is easy to fix. 

When the furnace doesn’t reach the set temperature, make sure there isn’t too much dirt on the furnace filter, check to ensure the compartment door for the blower is closed properly, make sure the pilot light is lit, and check the furnace for faults. 

You will be able to identify why your furnace does not reach the configured temperature in this post. Additionally, we help you determine how to fix the temperature issues with each of the potential causes. 

The Filter Is Blocked and Dirty

As time goes by, you should realize that it is common for particles to collect in the furnace. This is why regular maintenance is highly advised – as HVAC specialists can clean the entire furnace when they service the system. When you haven’t serviced the furnace for some time, then the problem could be a dirty filter. 

The filter helps to keep harmful particles and residue from entering your home. If these particles enter your home, it could affect your entire family’s wellbeing, leading to allergies and other issues when there is too much residue building up on the filter. The airflow through this part of the furnace will be affected. 

This means less of the heated air can enter the ductwork and flow through your vents – also leading to the conclusion that your house will not reach the temperature you configured on the thermostat.  

How To Fix

Fixing this issue starts with an inspection of the filter. The good thing here is that it is relatively easy to fix this problem by yourself. You will be able to easily see dirt and debris collecting on the filter – and cleaning it shouldn’t be a challenge.

You do need to open up the compartment where the filter is located. You will find this close to the blower module in the furnace:

  1. You should be able to inspect the filter before removing it. Use a flashlight if you need extra lighting in the environment. 
  2. See if the filter is clean or dirty. If there is dirt on the filter, then this might be causing poor airflow through your ductwork. 
  3. Remove the filter by unscrewing it and then unhooking the component from the furnace. 
  4. Use running water to remove the dirt that has accumulated on the surface of the filter. Once clean, wait for the filter to dry – you want to avoid getting the interior of the furnace wet. 
  5. Place the filter back into the furnace, fasten it, and close the compartment door. See if this helps to improve temperature regulation when you configure the thermostat. 

Use the video below if you don’t know how to clean the filter:

The Blower Compartment Door Is Tripped

Most furnaces come with a compartment door. This door is part of the system’s main compartment and houses some of the furnace’s most crucial components. Depending on the furnace model you have, there may be a witch installed in the door. 

When the door is closed, the switch is turned off. When the door is open, the switch is tripped. In a scenario where the switch is tripped, the furnace may come on, but it won’t function normally. When changing the temperature on the thermostat, the system remains tripped because the door is not closed. 

This is a safety feature that comes with your furnace. The system only operates when it is certain you will not face the dangers associated with an open door. Kids in your home, for example, may stick their hands into this compartment if the door is not closed. 

How To Fix

Among the reasons that exist, this is probably one with the simplest fix. If the furnace doesn’t reach the set temperature due to a tripped compartment door switch, you will start fixing the issue by closing the door. 

Make sure you hear a click sound when closing the compartment door. This is proof that the door is properly 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 doesn’t detect the door is closed, you may have to consider getting a replacement switch. 

The Pilot Light Is Not Lit

Some furnaces still use pilot lights. This is generally the case if you have an aged furnace installed in your home. The newer models turn to electric ignition modules that help to create a spark. The older models used a flame to ignite the burners – this flame is known as the pilot light. 

If the pilot light is not lit, it cannot create ignition. In this particular situation, you will find that your home remains cold no matter the temperature you configure on the thermostat. This is because there is 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, and other ones have this particular part hidden away inside a compartment. 

You need a lighter for this step if you have to light the pilot light manually. Use a lighter with a long tip – this ensures you get a better reach while lighting. 

The video below shows you how:

Problems With the Thermostat

Do not overlook the possibility that the issue may lie with the thermostat. If you find that heated air is blown into your home, but it is not at the specific temperature you configured on the thermostat, then take a closer look at this module. 

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 has gone bad. Look for LED lights that indicate power. It is a bit easier to see if a digital thermostat has gone bad – it will likely not show anything on its screen. 

How To Fix

Start by testing the thermostat’s power. You should use a multimeter for this step. The multimeter should test the wires bringing power to the thermostat and the wires that connect the thermostat to other parts of the furnace. If power comes into the thermostat, but nothing goes out – then you have a faulty thermostat. If power comes in and goes out, then the problem likely lies elsewhere. The power supply could be at fault if no power is coming to the thermostat. 

Summary

When the debris builds up on the filter, or wiring problem between the thermostat, and the rest of the furnace, a temperature change will not reflect throughout your house. This is sometimes a problem that requires the filter to be cleaned, but there are times where a more advanced fix is needed to get the temperature control working again. 

Professionals can quickly detect the cause behind this problem and provide a fix that will help you regain better temperature control in your home. Use the form below to connect to an expert.

Steve Rajeckas

Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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