I understood these were the boilers to go for, so we had our Worcester Boach 24 cdi replaced with a Viessman 050 30kw, the model recommended as a suitable size replacement. The installation went very smoothly but a few weeks later, hot water failed. A Viessman engineer visited and said it was a very small blockage in the impeller of a valve within the boiler, so replaced the valve with a filter to help prevent the same happening again. He was surprised a piece of grit (or whatever it was) so small would cause the problem but there you go. We can hear the pump working when the boiler is on so I asked him why the boiler is noisier than the 20+ year old Worcester it replaced, particularly as it is advertised as quiet. He said it has a higher output than the Worcester, so is bound to be noisier and that he did not consider it noisier than it should be. His comment made me feel like I cannot book an engineer to look at it without being charged, as he already dismissed it as not a problem. Additionally, occasionally the hot water just does not come on. we have to turn the tap off for a short while and try again. An intermittent problem like that is very hard to prove and you can bet it would work ok an engineer called, so we would have to pay. I read loads about boilers before having ours replaced. Viessmann seemed the best but I'm disappointed. I wish we had picked a Worcester instead.