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The ICG PRV is set to 3.0bar, what is the other PRV set to??

That's a good idea, you might then take a few readings with various HW only draw off flowrates and ditto from the balanced cold. Why is there another PRV installed?, is it because the mains pressure exceeds the max (I think) pressure ratio of 3:1, ie, a max mains pressure of say 8 to 9 bar...

It is, but when you shut a tap taken off the balanced cold and when/if the pressure builds up very rapidly due to the water flow stopping abruptly then the PRV will close very rapidly, the pressure is then trapped in the mains up as far as the PRV but any pressure that may have built up in the...

Have you got two PRVs installed?, one on the mains + the ICG PRV?.

I would suggest installing it before the PRV because the PRV closes when the upstream pressure is > 3.0bar so the arrestor wouldn't be of any benefit if installed on the balanced cold, (if the hammer is in the mains leg), the UVC EV should act as a arrestor to relieve the pressure in the...

Can only try and explain this a bit theoretically because I don't really understand water hammer where seemingly the kinetic energy is converted to pressure energy and even though water is incompressible would expect a very slight increase in "pipe" vol. A 0.16L arrestor precharged to 3.0bar...

OK, so its associated with the balanced cold only, (the kitchen tap is taken off the mains as you said) There is a check valve (NRV) in the ICG as shown below, this means that the EV if teed off the ICG port is only seeing the UVC pressure and will rise when the cylinder has been heated, EVs...

There is nothing between that T (for the EV) and the cold water inlet to the UVC??. If you open the outside tap which is fed directly from the mains instead of one of the balanced ones but in the same sequence that produces hammer does it have any effect?

Those corrugated coils can trap air in the corrugations but I suppose shouldn't cause water hammer.

Hope your cylinder coil isn't like one of these .....

You said in your other thread that the mains pressure spikes from its normal 4.5bar to 8.0bar during the water hammer, you also say that the shock seems to be close to or at the UVC PRV, if so, I would install a 2L shock arrester close to the PRV end or maybe even better, T off from the balanced...

Zilmet make 2L ones but they are quite expensive, maybe because they are stainless and can withstand 15bar pressure?

Is that arrestor taken off the T before the ICG one of those tiny ones?, if so a 2/5L maybe set to 2bar might be better if you have the room to install it.

This is where the EV is normally taken off but the T take off should be fine IMO.

Once again, what make/model of UVC :unsure:

There's a lot of info on @Taconite5 s "Water Hammer" thread.

Push in the shaft with a screwdriver (pump stopped) this should give more than a trickle then. Obviously if pump shat not rotating with pump "on" then new pump required

Thats a pretty powerful pump and will circulate say 0.8m3/hr, 13.3LPM , at a head of 5.8M on speed3, which quite likely will cause pump over on even a clean OV system especially in a bungalow, suggest changing to speed1, this pump will still circulate 0.8m3/hr at a 3.0M head which should satisfy...

What make/model pump is installed?

Something else to try, run the CH at its present temperature, then turn the set point (target) temperature down rapidly by say 10C, this will trip the burner but the circ pump will continue to run and the zone(s) valves will remain open, see if the noise remains the same.