Giannoni diverter valve

[cdbe]

Hi, I have just changed the diaphragm on the above diverter which is fitted to my Wickes Halstead 30/90 combi. Now fires as soon as hot water called for, the old diaphragm was shredded so no surprises there. The only problem now is that when the cold water tap is closed quickly it is operating the valve for a second (the pin momentarily rises, operates the microswitch and the boiler starts to fire - by this point the valve has closed so the pin drops and the boiler shuts down). I took the top off the valve, checked the new diaphragm was correctly seated etc and noticed that there was a small screw on top of the valve that restricts the water flow through it - by adusting this I can stop the kitchen and wash hand basin cold taps operating the valve when closed (the pin moves a bit but not enough to operate the microswitch) but when the bath tap is run and quickly closed it still happens. All taps are lever taps.

This didn't happen before I changed the diaphragm but it was so knackered that I doubt it would have lifted the pin anyway.

Clearly there is a sudden increase in pressure when the cold tap is closed which is fooling the valve? (I have had a play with the stopcock on the rising main but the cold water pressure makes no difference).

Any ideas, all I can think is that the diaphragm is a bit stiff as it is new or there is fault with the diverter valve?

 

[mrgas]

Have you bled all the air from the hot water pipes to all taps and any dead legs in the hot water system?

 

[compheat]

fit a non return valve on cold inlet to boiler this will cure the problem

 

[cdbe]

Thanks, I appreciate your replies. I did bleed the hot water and it was full of air but did not cure it. I can fit a NR valve on the water inlet if that would work (but would it not be better on the cold supply to the bath or am I misunderstanding?), I am just curious why it is happening now. All I can think is that I redid the bathroom about a year ago (including a new mixer on the bath) and that even then the diaphragm was on its way out and concealing this.