The back of the thermostat has W,Y,G,O,Rc,R,B not used, slots on the back. My old thermostat only has two wires labelled R and another labelled C.
There is no wiring instructions for two wires, I have a furnace that heats water.
Can you advise on where these wires go?
Thanks

You just need a simple two wire thermostat the one you have is for a a/c and heat system. These types require different wiring and generally need a constant power wire which you may not have.