I have a Holly Duo Trol gas furnace -- model no. 100VF. This unit is probably forty years old. When the thermostat is set for the furnace to operate, the main burners come on for about twenty seconds and then shut off. The fans never come on. I suspect the temperature sensing switch (fan on/off switch) is faulty.

Is there any mechanical features related to this sensing device that should be cleaned prior to replacement?

Is there a simple test that I can perform to confirm that the switch is indeed the cause of the problem?

I do not believe that Holly is around any longer. Are there companies that make generic replacement parts?

Thanks in advance. tedk

 

You 1st need to determine if the switch is closing or not at the temprature. You could just turn the power off, jump out the switch then power it back up. If it starts up with a call for heat then replace the switch.

If it don't start up you will need to check for voltage at the switch.

 

Hello Mattison,

I bench tested the fan switch using a heat gun. The switch closed at elevated temperature but I did not have a way to check exact temperature.

I then re-installed the fan switch and jumped the terminals. The fan came on without a call for heat. With a call for heat, and the terminals jumped, the burners and fan came on immediately.

With the jumpers removed from the terminals and the fan switch set at 80 degrees (normal setting is 135 degrees), a call for heat turns on the burners for a little over two minutes then the burners shut off. The fan never comes on. I thought perhaps the switch was slightly out of adjustment and a lower temperature setting would make the switch activate.

Meanwhile, I am going to search for a fan switch. They shouldn't cost too much. Thanks -- tedk

 

I also have a Holly Duo-Trol gas furnace (about 40 years old). It works OK, but I need to replace the Thermostat located under the front panel of Furnace.

I just replaced the motor and i works great. Do you know where I can buy the Thermostat?

I am in Torrance California.

Thanks,

Luckey