Ravenheat CS80 DHW not working

[BrainyArch]

Ravenheat CS80 T - CH works ok, fully working, modulating all good, but DHW only firing up for 30 seconds before cutting out burner, then tries again in endless loop. can see burner modulating, so thinking that side of things is ok. temporary solution: reduce DHW temperature demand to 5 degrees above current temp and it works, then raise it by another 5 degrees and so on - works all the way up to max heat output. leave 5 minutes and start all over again? no difference if CH also engaged. less than year old and not old pipework so not thinking heat exchanger here. been told PCB replacement. hoping there may be cheaper solution before ordering PCB? thanks

 

[cjg]

still under g"tee call them out, to fix.