I would be very grateful for some expert advice - I'll try and be concise whilst not missing out any necessary details...
Boiler: Worcester Bosch 24CDI RSF combi, approx 11 years old
Problem: DHW operation is somewhat intermittent insofar as we are experiencing fluctuations of hot water temperature (hot - cold - hot - ...) be it at a tap or shower. As this is occurring the boiler operation is as follows: DHW demand correctly detected, boiler ignites, full flames for 30 seconds or so, flame gradually modulated down, flame gets extinguished, 15 second or so pause, flames reignite, and repeat...
- No fault indicator lights, nor permanent failure, just poor operation as described - CH assumed to be working fine (not directly observed given the weather/temperature) - *** CH flow pipe heats up even during DHW-only demand (I initially though it might be conducted heat through the pipes but have now decided it's more than that - proper heating of the water within). I am assuming this fact is the relevent biggy here? *** - Diaphragm was replaced ~4 years ago. I am not suspecting this as it is operating the microswitch fully/correctly/consistently. - Medium-hard water area (Bristol), mains pressure ~65PSI - Inhibitor in the primary circuit for at least the last 6 years, no softener in the mains water feed - Gas valve replaced ~5 months ago due to regular ignition lockouts - PCB fault ruled out same symptoms with a spare fitted
The heating-up of the CH flow pipe is ringing the alarm bells for me... I thought perhaps the diverter valve might be sticking.. However the pin is making it as far as the microswitch but could the divert valve still not be moving sufficiently? Am I right in thinking that it the diaphragm serves only to activate the microswitch, and doesn't actually move the diverter mechanism itself? The latter being done by DHW water flow (against the actions of a return spring)?
Given the age of the boiler (~11yrs) and the medium-hard water area I was wondering if the HW path in the secondary heat exchanger might be scaled up hence causing insufficient flow to fully open the diverter? At the same time this would restrict heat transfer to the HW and hence cause the primary circuit to overheat thus cutting the flame ...?
I hope I've not wittered on too much but still giving you enough to go on... Your help/thoughts would be very much appreciated.