Pump overrun causes loud bang

[Agricola]

Hello,
I am not a plumber, but I have done some DIY in the past, so I have some experience. Following a new boiler install by a professional about a year ago, there remains an issue when the pump overrun initiates. The boiler is a Worcester Ri compact installed downstairs. the pump is installed above it and feeds out to two Drayton 2 port valves, one for C/H one for H/W. Between the pump and the valves is a tee with a divertor to provide a by-pass during overrun. this works fine when the C/H closes down, but when the H/W closes down there is a loud thump as that valve closes against the pump flow, even with the divertor set to its lowest pressure. The H/W tank is about 8 or 9 meters away, upstairs and the pipes (28mm) are laid between the upstairs joists with minimal fixing (now difficult to access).
It is as if the Drayton valve is being forced closed suddenly by the pressure from the pump, but this doesn't happen with the C/H side. It doesn't happen when the H/W closes but the C/H is still open, only when the H/W on its own is running. The pump is running at its lowest speed. I have tried to eliminate airlocks by putting the pump up to top speed, then back down.
Has anyone else experienced this issue? (Cant find it mentioned anywhere on Google). Could the valve be faulty? It is fitted the right way round according to Drayton installation booklet.
HELP !!!

 

[Gasmk1]

Hello,
I am not a plumber, but I have done some DIY in the past, so I have some experience. Following a new boiler install by a professional about a year ago, there remains an issue when the pump overrun initiates. The boiler is a Worcester Ri compact installed downstairs. the pump is installed above it and feeds out to two Drayton 2 port valves, one for C/H one for H/W. Between the pump and the valves is a tee with a divertor to provide a by-pass during overrun. this works fine when the C/H closes down, but when the H/W closes down there is a loud thump as that valve closes against the pump flow, even with the divertor set to its lowest pressure. The H/W tank is about 8 or 9 meters away, upstairs and the pipes (28mm) are laid between the upstairs joists with minimal fixing (now difficult to access).
It is as if the Drayton valve is being forced closed suddenly by the pressure from the pump, but this doesn't happen with the C/H side. It doesn't happen when the H/W closes but the C/H is still open, only when the H/W on its own is running. The pump is running at its lowest speed. I have tried to eliminate airlocks by putting the pump up to top speed, then back down.
Has anyone else experienced this issue? (Cant find it mentioned anywhere on Google). Could the valve be faulty? It is fitted the right way round according to Drayton installation booklet.
HELP !!!

it is being forced shut by the pump you are correct, try a new valve

 

[Agricola]

it is being forced shut by the pump you are correct, try a new valve

Thanks for your comment, partly right. I decided to e-mail Drayton at the same time as posting this query. It seems the valve is upside down. The instructions say the flow should be "from A to B", so my assessment was based on the A and B on the actuator casing next to the manual lever, but Drayton say that the underside of the brass valve body is marked A and B, and they are the opposite to the markings on the actuator - hence the confusion. Hopefully, when my plumber comes and changes round the valve the thumping will have disappeared. Anyone using Drayton valves (with the detachable actuator), please note !