I needed to replace fan and limit control (White-Rodgers, model 5D51-35), because the fan switch was broken (fan would constantly run) and heat would not stay on.

I took a picture of the control, the way it was wired and labeled the wires. The control had:

  • two wires running to the limit side
  • one wire running to the lower fan side.
  • a jumper copper wire running from the upper limit side to the upper fan side.

When I took the part into the supply store the sales person told me the jumper wire was not supposed to be connected. I installed the new control without the jumper and the heat will come on, but the fan will not come on. The instructions indicate if you have a low volt or millivolt do not use jumper. How do you know if it is a low volt or millivolt system. (I'm using a 120v system.)

Do I install the jumper line because the fan will not come on. (jumper line was connected on old control)

  • 1
    Is your system line-voltage (120v?) If so it is not low/millivolt. – Steven Feb 20 '13 at 20:45
  • Wheres the picture, it would help to see it. – Chimchuimann Jul 23 '14 at 5:49

The limit switch is a normally closed switch that opens on a temperature rise. The fan switch is a normally open switch that closes on a temperature rise.

Think of them as 2 separate switches that are side by side in a common enclosure.

Each switch needs a power supply and each switch lets that power run through it under the proper conditions.

They both sense the same temperature inside the furnace. So when the burner kicks on and the temperature inside the furnace rises it causes the fan to come on. But if the temperature inside the furnace gets to high (blower motor fails, blower belt breaks, filter dirty so air flow is low etc) then the Limit switch will open and cause the burner to shut off.

You need the jumper that will put 120 volts on both INPUT sides of the switches Fan/Limit control.

That way as the furnace heats up, the normally OPEN fan switch will close it's contacts to turn on the fan motor and the normally CLOSED limit contacts will remain closed to let the power through the safety circuit and only open in the event the furnace gets to hot and will cause the gas valve to close.

Now it is important to note that from your description of your issue that you have a high voltage limit control. Some furnaces use a low voltage limit control, in that case you do not use the jumper.

I just noticed that your question is 2 years old....duh me..... well I hope this helps anyone who has the same issue.

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