Ariston Eurocombi A23/MFFI multiple faults - advice please?

[magick777]

I'm pleased to report that my air pressure issues are now fixed. With a new APS and the fan running correctly, all that remained was to clean the venturi, sealed up the combustion chamber and my central heating is all back to rights.

Thank you for the explanation on the diverter valve; if replacing the diaphragm doesn't work, I'll understand why. The logic in trying it is pretty solid, though; if I'm wrong I've wasted a £7 part and an hour in order to know that the valve needs replacing. If I'm right, I've saved a £100 part and probably a hefty labour bill to fit it. Here's hoping, and thanks to all who have helped so far,

 

[magick777]

Well, I'm pleased to report that my repairs seem to have been successful in restoring both CH and DHW on-demand, for the first time in two or three years.

The diverter valve diaphragm was ruptured, and a new one caused the pin on the valve to travel far enough to contact the microswitch, but not reliably to activate it on a low flow rate. I take this as evidence that the valve will indeed need replacing eventually. However, a 1mm adjustment to the arm of the microswitch has fixed it just fine for the moment; DHW on demand is a luxury that I'm not used to, and I'm more than a bit pleased with the result.

If and when it gets worse, having now changed the diaphragm, I'll be far more willing to believe it's the valve that needs replacing. In the meantime, you've helped me save somewhere between a few hundred quid and the cost of a new boiler (plus financing), so thank you to everyone who has helped.