ok, now you need to set it correctly to ensure it allows the correct amount of water through, you can't just open it fully and away you go, it needs balanced, does the cylinder have a thermostat on it?
By the way this system used to be gravity fed when heating on both rads and water heated but new boiler upgrade and heating now zoned and pressurised that's why amendments had been made would this have any bearing on balancing the water cylinder as heating now should only be heating water when switched on for water??
If it's set up as an S Plan (2 separate valves) then it's certainly better and will be controlled through the stat.
Depending on the cylinder type though, especially when it's an old cylinder then the flow through the coil would be set to minimise the re-heat time and keep boiler cycling to a minimium.
It isn't just a matter of pull this, open that, shut the other... A system is comprised of components that have to work in harmony... An old system relies on this more so... As long as components still work and can be manipulated then it will work.
Proper engineering is only compromised by lack of maintenance.
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