Which boiler type

Hi, Well it all depends on some factors like: What is your hot water demand? How many bathroom(mixer showers) In larger houses where there are 2 bathrooms with mixure showers it's better to have the cylinder but sealed system not open vent cause you will have more heating water pressure and no air locks. you waste a lot of energy because most of the time you heating up the whole cylinder but not using even half of it. Having a combi boiler you will have only one boiler dealing with heating and hot water,the biggest wall hang combi boiler (worcester 42cdi combi)will give you 17.1 litre per min hot water flow rate so if you using two mixure shower at the same time you will only get 8.5 l/m flow rate from each. Having a hot wayer cylinder you always suffer from low hot water pressure from the taps specially in the bathroom. Its more efficient to have a combi boiler and you save a lot on gas as it will only works when there is a demand. If your house is a 3 bedroom semi or detached house i would say go for a combi boiler,installation cost will be higher than conventional boiler swap but you gain alot more on your gas bill. Thanks