Yes there is a check-valve but Im pretty sure it isnt 100 perfect. On cold mornings the house will feel cold as either the room thermostat or the. Watertube Boiler Safety Based On Physics And Physical Design 2020 04 14 Process Heating If its a combi or condensing boiler then no hot water or heating will be available until you switch it back on. Will turning water off affect boiler . Well if you wont be using the water for some time and the water will only be off for a short time there is little to no risk involved. On a non combi boiler I would agree but as Op says its a combi with the main off there will be no hot water or cold and as the boiler cant try and heat a plate to plate without flow from the main I cant see a problem but Istand to be corrected. If the boiler is a sealed system then mains water being on or off makes no difference whatsoever as the system is re pressurised manually not automatically. It should not be a problem. Boil a kettle and have a cup of tea ...