I'm going to add an extra radiator to my central heating system and will need to drain and refill the system. I have a SIME combination boiler. Is there anything I need to be aware of when draining the system or is it just a matter of shutting off the supply to the boiler and opening the drain (fitted at the lowest point in the system). I'm a bit concerned about creating a vacuum in the boiler expansion tank and damaging it while emptying the system.