OK zimmer are you familiar with this particular boiler model?...since your postings suggest otherwise.
As soon as the OP posted we had a reply indicating a blockage in the strainer/restrictor.
Now a novel water/energy saving feature of this boiler is the reduced flow from the hot taps until the DHW is up to temperature. This was clearly shown to be functioning correctly by the OP.
Next we have your suggestion it's the diverter valve.
Now 99% of faults regarding failure to fire up the boiler in DHW mode are down to the flowswitch assembly and NOT the diverter valve which is an entirely seperate item. The heating appears to function correctly.
A posting from poxi attempted to steer the OP in the right direction (ie see if the DHW demand LED on the modulation board comes on) however the OP has ignored this and called someone in.
The OP now says the lockout LED on the user control board comes on. If he is turning the boiler off immediately after a demand and then turning it back on it may well come on due to overheat conditions...not a fault. Or perhaps the flowswitch is failing (still my expected fault) and it has jammed on firing the burner with no demand.
Puma flowswitches either not indicating a demand or stuck on must be one of the most regular postings on this forum...a simple search will confirm that. On most Pumas (which by now are at least 6 or 7 years old) the diverters will be starting to play up..normally shown as poor central heating performance, but not normally stopping the burner firing up with a DHW demand.
Hopefully we will hear the outcome and solution; from the symtoms in the original posting I'm not convinced the diverter is the problem.
And whilst we are still on the subject.......
If some suggested solutions are either irrelevant, not the obvious likely fault or are clearly not warranted at the initial stage of investigation then why should other OP's not question there validity. It's in the interest of the OP both financially (ie to avoid swapping parts in good working order) and to avoid rambling threads that only add to the confusion.
Several other forums exist to offer the public advice on their central heating, many however, offer spurious advice, often dangerous advice, and ultimately do not resolve the problems. Perhaps the reason this forum solves a significant number of OPs problems is due to the quality of pro's (and other regulars) and the "self policing" of the postings. ie dangerous, misguided or irrelevant postings are "shot down". Experience is everything when it comes to repair and keeping an open mind to the solution should be a foremost attribute, therefore I will not dismiss the diverter being the culprit here but merely suggest it is unlikely.
Your initial postings were, to say the least a little confusing (and continue to be), you freely admit to charging customers whilst you "read the paper", you show little understanding of the powerflushing process, you charge a very low rate for installations (showing implications of cost cutting), you can install a combi in 1/2 a day, you are obviously highly strung from your irrational behavior towards others, you felt hard done by and felt belittled by others. I like to think this forum shows the better side of an industry beset by "cowboy" attitudes and poor quality of workmanship. Therefore you must excuse me if I reserve judgement on your character, suffice to say that so far it is not particularly high.
