This boiler was run for several years with a leak in the underfloor pipework and DHW was operating intermittently. The leak was fixed and the system flushed several times before adding inhibitor and all worked OK for about 6 months. Now the intermittent DHW fault has returned. The DHW will usually work if the CH is run for a few seconds first. I think there is a partial blockage in the DHW circuit which is causing the bypass to operate which in turn causes the main flow switch pin to retract, shutting down the boiler. If the CH is on first the pin is already pushed out and there seems to be enough force with DHW operating to keep the pin out. I've removed the diverter valve and DHW heat exchanger and cleaned both internally. When flushed under a tap there is good flow through both sides of the heat exchanger. I did this until the water ran clear of black particles. The fault persists after reassembling. I think that the only other place for a blockage is in the brass return group housing. I'm planning to remove the DHW heat exchanger and pump to check if there is a free passage within the brass housing for the DHW return water. I'd be grateful if anyone has any comments on my theory and approach.
Thanks.
Thanks.