I have a bit of a unique situation. Older building that heats and cools by pumping hot/cold water through pipes. My unit has an air handler that amounts to just blowing air over the hot or cold coils. This air handler has an old thermostat that I want to replace with a digital thermostat. Obviously, a change from the hot/cold sensor being used now. So all I want to do is install a fan control system, like a White Rogers 90-113 in place of the current set up to turn the fan off and on, single speed. I think all I need from the current setup is the two wires to the fan motor, and the thermostat wires. Am I oversimplifying, and if not how to I wire the fan controller to the fan motor and the fan controler/thermostat?

Hi guys! I’m trying to replace my old manual Line Voltage thermostat brand Erie with 4 wires with Honeywell TB6575A1000. Red and blue wire are for fan low and med speed, both on new and old thermostat. So I’m absolutely sure connecting these two by colours if perfectly fine as they match even by wiring diagram. However black and white are giving me hard time, as on new thermostat black should be hot and white should be neutral, but on old one white is connected on spot marked as “line” on diagram which I assume should be for 120V. When I measure it with voltmeter it shows constant 120V if unit isn’t of or on. When I measure black on old thermostat it has 0V unless you turn it on and then it has 120V or so. Moreover, black seems to be connected to high speed fan spot on diagram, but this unit has only 2fan speeds working, although switch is made for 3. Should I just connected black to black, white to white or other way around. For red and blue, I’m confident those are ok. What are the consequences of wrong wiring in this case? I have attached all the photos. Fan coil seems to be Luxair L350. Many thanks for your help!