Well I've repaired a lot more that I didn't fit than that I did, but the failure rate on new ones has been high. Most of the Vaillants diverters of course, though they seem after the design gets changed, and quite a lot of fans, pumps, leaks, cracked pcb's.
Quite a number of problems to do with being pressurised, which tends to be combis of course. Diverters generally are more troublesome and more expensive (like pumps) to fix than a 2 port valve would be. Problems with dirt in systems start with much cleaner water than conventional systems would be fine with.
Cheap construction lets a lot of combis down - (eg Halstead fronts!). With combis costing about the same as a much simpler conventional boiler we shouldn't be surprised.
Quite a number of problems to do with being pressurised, which tends to be combis of course. Diverters generally are more troublesome and more expensive (like pumps) to fix than a 2 port valve would be. Problems with dirt in systems start with much cleaner water than conventional systems would be fine with.
Cheap construction lets a lot of combis down - (eg Halstead fronts!). With combis costing about the same as a much simpler conventional boiler we shouldn't be surprised.