Banging central heating/h/w system

[white90]

Moved into a new house on the 11th Jan
since then when the heating/water are on
there is a loud banging when the thermostat on the cylinder calls for heat
the Y valve motors over and BANG!
I have bled the radiators lots and lots
bled the HW system via the Braukmann vent
but with no success
it stops sometimes when both systems are hot
worst in the morning or when the timer activates both systems in the afternoon.
I have been as yet unable to locate a pipe where the noise is eminatiing from
bit reluctant to pull up all the flooring just yet.
I do notice that when the Y valve is supplying the CH system water is still coming out of the Braukamann Vent(when the top is removed
is this to be expected?
I thought the Y valve would divert water from the HW circuit altogether.

thanks for looking any advice great fully received.

 

[white90]

Further investigation
the banging only really occurs when the 3 way valve goes from feeding the CH to combine the HW and CH
the Braukmann ea122 Valve appears to work ok
no water is going up the vent pipe

 

[stoobydale]

Sounds the same as the problem that I am having. Posted in another thread, the reply's have suggested that I balance the rads. Maybe you could try the same.

 

[white90]

Fixed
the 3 way valve was not sealing when the h/w circuit was meant to be off
new valve prob fixed

 

[stoobydale]

HI, How did you find out that the valve was not seating? This could be what my problem is.