How do I calculate the boiler size to replace an old Baxi back boiler with a gas combination boiler in a two bedroom bungalow which meassures 10mx7m? there will be one bathroom with shower and one kitchen, the building is approx 30 years old with plactic double glazed window and will have 100mm+ loft insulation, the foof is pitched and tiled. Also any suggestions as to which boiler make? Many thanks Rob.