CH pump continues to run intermittently

[Mike12345678]

Hi all after some advice about central heating.
I have a old style system with tank in loft, cylinder in airing cupboard etc. The boiler is a glowworm 50ff, Wilo gold 50 pump and 2 Drayton zone valves za5. The problem is boiler is working, lights correctly and hot water gets hot and ch gets hot but intermittently when heating is up to temp the bolier will switch off but the pump will continue to run causing pipes to bang and judder. ( boiler will then sometimes req reset) My first thought was a faulty zone valve but I have checked these and all is well. Any ideas what to check next and how would be much appreciated.
Thanks mike

 

[oilhead]

Probably pump overrun stat is trying to cool down the boiler, and if the bypass is not working, it is pumping against a dead head. Not easy for a DIY'er to trace, but 1st try to find your bypass, which may only be a gate valve between flow and return that may have been inadvertently closed.

 

[dilalio]

The Barnet by-pass doesn’t bloody work either
Thankfully, I don’t have to use it tomorrow morning

 

[Mike12345678]

Ok I will look for this. Thanks.

 

[dilalio]

Ok I will look for this. Thanks.

Barnet Bypass
https://maps.apple.com/?address=Bar...hGAnXSUAxQXawRs/wzb8529Il6R3XSUBBxWmnkz3Pzb8=
https://maps.apple.com/?address=Bar...hGAnXSUAxQXawRs/wzb8529Il6R3XSUBBxWmnkz3Pzb8=