I don't have a combi boiler, but it's normally on all the time, the way we use hot water in this house it doesn't seem to make much difference to the cost.

I've got British Gas' Remote Central Heating in at the moment, I suppose the better question to ask would be how DIYable is the removal of that!

I take it I can power the Nest at the existing thermostat location, it will run off the voltage supplied there ???

Cheers

Cris