We've installed a few hundred Vaillant boilers and we check the hoses (by squeezing them) on the annual service. If they are delaminating/leaking we get Vaillant to come out and replace them.
Because of the almost watertight casing the results of a hose failure can result in the drowning of the boiler.
Acccording to Dangermouse the newer hoses have a white marker and these are more durable; the very latest ecoTECs have a new design of flexible copper connectors, which I assume will resolve the issue once and for all.
No boilers are perfect and the earlier German ecoTECs do have the hose weakness and the fragile water sensor. Still a very good product, IMHO.
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