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I have an old gas furnace without a circuit board. Just a 4064 fan limit control, 24v transformer, gas burner control and fan. I wired a new wifi thermostat in. It works great. I would like to be able to use the fan only control of the thermostat. Can I do this? Where would I connect the G wire? I was thinking I could remove the jumper in the 4064 limit controller. But if I use low voltage on the limit side will it still only allow the fan to run by temperature? There is a fan/auto switch on the 4064 that works.

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    Can you post photos of the furnace wiring, and a link to the manual for this fan limit control of yours?
    – ThreePhaseEel
    Commented May 18, 2020 at 23:49

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You will need to add a 24 volt relay that is controlled by the green wire and a common and wire the relay contacts to bring on the fan by jumping the contacts in the 4064 fan/limit control. You can use a RIB relay or similar one.This is how it was done in the "old days".

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