My central heating is sealed radiator system with an external pressure vessel and pump, megaflo hot water, heated by a Potterton Suprima boiler. The boiler's getting old and I'm expecting to have to replace it shortly. All the advice seems to be that a sealed system should have a system boiler. That makes it more expensive and too big to fit in the cupboard. Can the boiler be replaced with one designed for a vented system, or won't that work these days?