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Furnace Not Starting After Summer? Top 4 Causes (+ Fixes)

When summer hits, there is usually no more need for the furnace to keep running. The furnace produces heat, which you will usually not be interested in the warm weather of summer months flow into your home. Sometimes, however, when switching on the furnace after summer, you face problems.

To fix a furnace not starting after summer, look for a tripped circuit breaker, make sure the batteries in your thermostat are not dead, inspect the ignition sensor, and thoroughly clean the furnace filter. 

If you can’t get the furnace to start up after being off all summer, then this article is right for you. I’ll walk you through common reasons why this happens and give you the most effective solutions to the problem. Let’s get started!

1. Circuit Breaker Tripped During Summer

It is generally good to start with some of the basic causes behind a furnace not starting after summer. This is why we recommend starting with the circuit breakers. 

Circuit breakers are essentially a safety feature in a furnace. When there is an energy surge, the circuit breakers will trip. This causes the power supply to the furnace to be cut. The reason for this is to avoid the power surge from damaging the control board and other parts of the furnace. 

During the summer months, there may have been a power surge throughout your home. In this particular scenario, the circuit breaker has tripped. This means you won’t be able to turn the furnace on at all, as it will not allow power to push through to its mainboard. 

How To Fix

If a tripped circuit breaker is behind the furnace not working, then the fix will usually be simple. 

  1. Start by finding the circuit breakers on your furnace. 
  2. The position tends to depend on the specific model you have. You can refer to the owner’s manual if you feel unsure about where the circuit breakers are located. 
  3. Now, you need to reset the circuit breakers. 
  4. Once the circuit breakers have been reset, you should be able to turn the furnace on without any problems. 

If the circuit breaker keeps tripper after a reset, then this troubleshooting video may be helpful: 

2. Thermostat Batteries Are Dead

Different kinds of thermostats can be coupled with your furnace. If your thermostat runs on batteries, then this is another potential problem to consider. 

You leave the furnace off during the summer months. During this time, there is a chance that the batteries could run empty. In this case, you will find that the thermostat is unresponsive. It will not power on. If the thermostat has an LCD, it will remain blank even when switching it on. 

For many people, dead batteries in the thermostat seem to be relatively common following a long summer. This is not a major issue and can usually be fixed in just a few steps. However, you should see if you can find a few tools to help test the batteries in the thermostat. While there is a good chance that the batteries could be dead if the thermostat does not come on, other problems have also been associated with this particular issue. 

How To Fix

You first want to determine if it is the thermostat batteries behind the problem you are facing. To do this, you will need to remove the thermostat from its mount. 

  1. The batteries are usually located behind the rear panel in a thermostat. You may need a screwdriver to access the batteries. 
  2. Start by testing the batteries with a voltage meter. See if you get any reading with this device. 
  3. If there is no reading, then you need to replace the batteries in the thermostat. 
  4. Make sure you take note of the specific battery type used in the thermostat. Then, get a replacement for the same model. 
  5. Place the new batteries in the thermostat and close the panel. See if the thermostat turns on when you press the power button now. 

Need a visual guide to help you replace dead batteries in your thermostat? Then take a look at this video:

3. Problems With the Ignition Sensor

The ignition sensor is sometimes also referred to as the flame sensor. The sensor is found in the ignitor of the furnace and helps with regulating the ignition process. 

There are two reasons why the ignition sensor may run into problems. First, it may be related to dirt collecting on the sensor itself. In this case, the sensor cannot function properly. In other cases, the sensor may be faulty. 

The specific approach to fixing this problem depend on your findings. If the furnace does turn on, but there is no ignition, it is a good idea to take a look at the sensor. 

How To Fix

You first need to gain access to the ignition sensor. This particular piece of hardware is found in the same location as the ignitor. You may need to unscrew a panel on the furnace to access the ignitor. 

  1. Inspect the sensor closely. See if there is any dust or other debris that has accumulated on the sensor. 
  2. Use a soft material, such as an earbud, to gently clean the sensor. Avoid using any damp cloths or chemicals to help with the cleaning process.
  3. If no dust lies on the sensor, then the sensor itself may be faulty. In this case, you might want to consider getting the sensor replaced. 

Here’s a video to help you understand the diagnostic process for a faulty or dirty flame sensor: 

4. Furnace Filter Is Dirty

The filter in the furnace is crucial for ensuring particles in the air are blocked out. The filter gets dirty over time. 

Even though the furnace was turned off during summer, dust and other particles can still enter the furnace and collect on the filter. When the filter has a thick layer of debris, it could interfere with the furnace’s functionality. 

Fortunately, many people find that giving the filter a good clean helps to restore functionality. Note that power lights may turn on in this case, but the furnace itself won’t work. 

How To Fix

  1. Remove the furnace door. Some doors are simply hooked in, while others use a screw to fasten the door to the furnace. 
  2. Remove the filter inside the furnace. See if it looks dirty. 
  3. You can usually rinse the filter off under some running water. Do not use any additional products to clean the filter – such as cleaning chemicals. These chemicals can cause damage to the filter. 
  4. Once the filter is clean, you should wait for it to dry if you used water. Remember that the furnace has several electrical components inside, so you do not want liquids to touch these parts. 
  5. After drying, the filter can be returned to its correct position inside the furnace. Ensure you fit the filter properly, as a poorly fitted furnace can also cause problems in the future. 

This video guides you on the process of cleaning a dirty furnace filter:


A furnace not starting after summer is a relatively common problem. Sometimes, this is caused by a circuit breaker that tripped or dead batteries in the thermostat. There are times where the problem is related to the electrical components of the furnace too. 

When the problem does not seem to be a quick fix, such as changing the batteries, you should consider hiring an HVAC expert. Not sure who to call? Fill out the form below, and we’ll help you connect with a local professional. 


  • 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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