CH control valves

[garrytheskate]

Saunier Duval SB30e boiler
Drayton MA1 3way Valve
Sunvic SZ2301 2way Valve

On my system between the 3 way valve and the HW cylinder there is a 2 way valve. The problem is more times than not there is a hell of a bang from the 3 way valve when HW is called for as if the 2way is fighting the 3way. Is there a problem if I isolate the 2way and just use the 3way to control the CH and the HW.
Both the valves have been replaced.
As for what causes the bang I can only assume there is a wiring problem
Regards

 

[twgas]

The usual reason for having both a 2 port and 3 port would be the addition of an unvented cylinder to a Y plan system and as a consequence you must not interfere with the 2 port valve.

 

[garrytheskate]

Mick
Thanks for that. How should the 2 way be wired in? when I took the old one off it had 4 wires, 2 of which were taped up together doing nothing. These I think were from the internal switch
regards

 

[twgas]

I should have asked on my other post, do you have an unvented cylinder?
If you do then I advise you to get a G3 certificated engineer to check/repair your fault, the wiring may be manufacturer specific.