In message l.net,
Dave Liquorice writes
On Wed, 28 Oct 2020 09:28:59 +0000, Tim Lamb wrote:

The actual issue is that the diverter valve momentarily blocks the pump
flow leading to water hammer which opens the boiler relief valve thus
losing system water:-)

Have some relays that switch the pump off when the divert valve is
being motored?
System boiler, internal pump.

An alternative solution is to fit a bypass valve: to maintain flow
during the transition. Soldering access is a pig!

That might not stop the hammer unless reasonably open...
The idea was to divert the flow through the hot tank coil with an
automatic bypass valve (Honeywell DU144)

Will adding another expansion vessel do anything? After all you have
one already and they have quite high interia due to their size. It
would have to connected close to the divertor valve an on a short
pipe as well.

Tucked away in the boiler, I don't know which leg it is on. Also 8L is a
bit meagre for very cold weather when more heating circuits are running.

Surely one of the small "water hammer arrestors" would be better?
Screwfix have several, can't persuade Toolstations site to cough
anything up.
Pass. What I hear as water hammer may be the boiler safety valve opening
and re-closing. Plumbed in to the condensate pipe so hard to tell.


--
Tim Lamb