Hi m8, sorry been out for a long day, just got the message.
It's pretty simple. The right hand side contains a fairly common sprung diapraghm and pushrod, which is drawn away from the hydraulic diverting section by dhw flow. As this pushrod oves away from the pushrod containing two bobbins that cause diversion of primary water, a spring at the far end pushes that pushrod to meet it. This causes the two shamfered ends of each push rod to move into a position such that the white wedge shaped tip of the dhw microswitch which is also sprung loaded to drop neatly into the v formed by the two shamfered pushrod ends. The microswitch makes contact and the pcb is told to start the pump, fan, prove it, restart it send gas through and light the burner.
A tear in the diaphragm can occur, the dhw side can be removed made easier by pump removal. The nut on top interfacing with the plate heat exchanger can be a pain to start the thread on reassembly and would require a new 1/2" washer. on puting back together with new diaphragm springs and push rod assembly (pushrod lubed with silicon lube or spittle), care to centre the diapragm witthin tehe brass parts or it will leak as the diapragm has to sit neatly centred. This is a little tricky to achieve and the aforementioned nut for the connection to the dhw plate involves some muscleing into the right position and angle. In the worst case scenario the other three unions with the plate have to be loostened to accomodate sufficient positioning.
The primary hydraulic side seldom causes a problem but a good clean and new o rings would sort it should it become a problem.
The microswitch is far and above the most unrelaiable part of the assembly. It is insufficient to test it with a continuity meter, as even though it proves with a meter it can yet be incapable of passing the required current to instruct the pcb hot water is demanded.
Chris R did put up a diagram once. I am not aware of any others.
If you have one in bits and are stuck please be more specific.
