I have an old Robertshaw thermostat, replacing it with a honeywell rth221b, the problem is the wiring doesn't match. On the Robertshaw I have 4 wires, white - O red to Rc, yellow to W and Y, green to G. On the honeywell, there is no O and the w and y are on two separate terminals, I only have the one yellow wire. Anyone have a clue? Thanks

What kind of unit? Heat pump? Gas furnace? Electric furnace?

I have an old Robertshaw thermostat, replacing it with a honeywell rth221b, the problem is the wiring doesn't match. On the Robertshaw I have 4 wires, white - O red to Rc, yellow to W and Y, green to G. On the honeywell, there is no O and the w and y are on two separate terminals, I only have the one yellow wire. Anyone have a clue? Thanks
I found my error, I bought the wrong thermostat, when you replyed heatpump I look at what I bought and it's for electric AC only.
Thanks

Glad you found the problem.