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Problem: Why does my furnace go off on limit so much? Why do I have to replace my furnace limit switch so often?

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Problem: Why does my furnace go off on limit so much? Why do I have to replace my furnace limit switch so often?

Answer: I have noticed this year more than any other year that we are having many questions about this problem. We are selling lots of furnace limit switches. The sales are great, but I have to be concerned about why all these customers are having to replace so many limit switches? A gas furnace that is maintained and adjusted properly should rarely go off on limit unless there is a serious problem with the furnace! The limit is a safety control that prevents the furnace from overheating. If your furnace is going off on limit then this means your furnace is overheating beyond normal.  I would highly recommend that you find out why the furnace is overheating and get this problem corrected. Here are some areas that you can check if your furnace is going off on high limit:

  1. You could have a blower motor that has failed to start or is slow in running.  You might need a new blower motor.  In many cases, a new capacitor or changing the blower speed might eliminate the furnace going off on high limit.  If you think you need a new capacitor please click here to see the capacitors that we sell.  If you would like to know how to change the speed on your blower motor then there is a really good YouTube video on this subject made by AC Service Tech. Please click here if you would like to see the YouTube video on how to change blower motor speeds. 
  2. You could have a dirty obstructed furnace filter. The furnace is not getting enough air and the heat exchanger overheats causing the furnace limit to open and stop the gas from burning to prevent a fire or damage to the furnace.  I am sure that you know it is important to change furnace filters on a regular basis.
  3. You could have a dirty furnace blower wheel. If the blower wheel is dirty the wheel can not pick up enough air to keep the furnace from overheating. We have a picture below that shows a dirty blower wheel. As you can see this wheel is not going to pick up much air!

dirty blower wheel

4. You could have a dirty or stopped up evaporator coil. If the underside (on an up-flow furnace) Air conditioner coil is stopped up with lint or dirt then this will cause the furnace to overheat and go off on limit. The furnace can not get rid of the heated air so the heat builds up and the furnace goes off on limit. A stopped-up evaporator coil hurts the furnace and air conditioner’s efficiency a lot. Make sure that your evaporator coil is not stopped up so air can easily flow through the evaporator coil. We have a post about cleaning evaporator coils on the following page: Please click here if you are interested in seeing how to clean an evaporator coil.

dirty evaporator ac coil
Above is a Picture of a Partially Stopped Up Evaporator Coil:

5. You could have obstructed returns or supply registers. Make sure that all returns are not obstructed with furniture or stopped up with dirt. Make sure all the supply registers are open.

6. You could have the gas valve adjusted too high where the gas input from the gas valve is producing too much heat for the furnace to handle. Please click here if you would like to see our post on how to adjust gas valve pressure. 

7. Last but not least. You might not have the recommended temperature rise on your furnace. Every furnace should have a label that shows what the recommended temperature rise is for the furnace. If the furnace is adjusted to the manufacturer’s recommended temperature rise then the furnace should not be going off on high limit.

We have three really good YouTube videos below that explain what temperature rise is and how to troubleshoot if your temperature rise is not within the recommended range that is set by the furnace manufacturer. The first YouTube is a 2-minute tutorial that explains what temperature rise is by Tom Kleinman. Thanks to Mr. Kleinman for making this information easy to understand in a video! The next video shows how to measure temperature rise on a furnace. You do not have to have a digital temperature meter like in the video. A low-cost thermometer could be used to measure the temperature rise in your furnace. This 3-minute video is by Lennox Learning Solutions. The third video shows the top 4 reasons why a furnace short cycles by AC Service Tech LLC.  Many thanks to AC Service Tech for making this great video! If you have any questions please email me, Steve Arnold, or comment below. Our email is arnoldservice@gmail.com.  I will be happy to try and answer any questions that you have. God bless you all! Thanks so much for reading our post and visiting our site! We Would Love to Help You Out and Earn Your Business! 



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