We have been installing many Glow-worms (probably close to a thousand) over the last five years, both non condensing and now condensing, and system cleanliness is everything in getting the combis to work trouble free. We still get the odd condenser that leaks internally, and boy do they rust quickly with that acid. We've had to bin one at 2.5 years old.
At home I have a 13 year old Ideal Turbo 2 condenser which has had two faults in that time 1) fan after 8 years 2) corroded siphon tube after 11..other than that it still runs like a train (it goes clickety clack all the time
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I have a Vokera Combi in another house I have..and that has been 100% fault free over five years& gives brilliant hotwater, but we have a full water softener to save the plate to plate..and it works!
We have just started to fit Buderus combis on one contract, and we are all pretty impressed so far with the quality of the kit....time will tell as ever.
As for fitting a secondhand Worcester CDX..I can see the temptation, but its just too dodgy, and the installer is unwise to put himself in that position. I'm not that impressed with Bosch..All the things that go wrong on my Alfa seem to have the word Bosch written on them!
I do a fair bit of fault finding, and sometimes its a doddle, and sometimes its a nightmare. Anyone who claims they fix everything every time Ho Ho Ho. How about the condensing combi that worked fine except when it rained..when it stopped..usually after the rain had stopped..It turned out the soakaway was filling up and backing up the condensate drain...obvious once you know, but by heck it took some figuring out to make the connection.
Its all fun
Alfredo