range rating so the boiler only uses up to a certain amount of power.
That's what I assumed you meant, but wasn't sure. Unless you have a good reason to limit it, leave it to the Vsmart to decide what power it needs - leave it on auto. That setting is there for more basic control systems where all they can do is on or off demands for heat, whereas the Vsmart can tell the power just how much heat is required, from its calculations.