Posted on 4 Comments

Fuse on my control board blows constantly. How do I find a Low Voltage Short?

troubleshooting air conditioners and heat pumps

We have many customers ask how to find a low voltage short when the low voltage transformer is burning up or the fuse on the control board constantly blows? Most of the time I have found that shorts occur in the thermostat wires that go out to the outdoor condensing unit. Wear and tear of sun light, animals or weed eaters usually cause these shorts. I would suggest inspecting the thermostat wires for cracks, wear and tear. Pay close attention to where the thermostat wires go through the body of the furnace or air conditioning unit. Many times the thermostat wires will short out against the body of the furnace or air conditioner. I found this real good Youtube video on how to find low voltage shorts by ACservicetech. I hope this great Youtube Video will help you out in finding where the short is located. Thanks so much to ACservicetech for making this great video! If you have any questions please email us anytime:

You Can Search Our Site for More Troubleshooting Problems, Heating, Air Conditioning Parts and Supplies in the Google Seach box below.

4 thoughts on “Fuse on my control board blows constantly. How do I find a Low Voltage Short?

  1. I have a nutone ac/heat pump blowing the low voltage fuse. When in ac position it turns on and runs. In the heat position it turns on and after about ten minutes the fuse blows. This is a mobile home unit where everything is in the outside unit, no furnace

    1. Hi David! I am guessing that you are referring to the fuse on your control board is blowing and not the high voltage fuse blowing. If it is a low voltage fuse problem then you have a low voltage short somewhere. Most of the time low voltage shorts are in the thermostat wires. We have a post about this on the following page: Since the fuse blows while in the heating mode then this could be in any of the thermostat wires that control your heaters or any of the low voltage controls like contactors or sequencers that are in your heater’s control panel. Sorry that I can not be of much help. I hope you can easily find and fix the problem. Steve

  2. My condenser keeps shorting out low voltage wires. Yesterday I ran voltage test back n forth and found that I wasn’t getting 24 volts at the contactor but the air handler was sending it. So I just used an extra wire and swapped it on both ends. Condenser Cut right on. 6 hours later , the same thing. So I repeated the same process and used an extra thermostat wire and it fired right up again. Any thought on why it’s doing this ?

    1. Hi Daniel! I have never heard of new low voltage wires shorting out that soon unless you have a loose connection or a short to ground somewhere else in your system. I am sorry that I can not give you any advice other than check all your connections to make sure they are good and tight and you do not have any shorted thermostat wire anywhere. I hope you can find the problem soon! Steve

Leave a Reply

Your email address will not be published. Required fields are marked *