Just went down to the middle bathroom and tried the shower there that is an attachment to the bath and wow the water powers through!...is this because the cylinder with all the water is up one level so the flow has gravity on its side?...where as the shower in the en-suite is on the same level?
Have you tried the offending shower with the spray head removed? Just the hose from the valve still attached? Also, remove the hose and check for any inline filters/strainers that may have become blocked with debris - following that, remove the bar mixer shower and check for strainers in the inlets of the valve and give them a backwash in case they’re blocked.
It’s for backup purposes and is pretty good work practice IMO as you can still take a shower - collect hot water in the event of a breakdown with your cylinder.
Also, you keep referring to a “boiler” do you actually have a gas/oil fired boiler in the property or is your UV cylinder direct? I.e. is it heated electrically by immersion element(s)?
Edit: had to tweak my reply as things kept going awry!
Just went down to the middle bathroom and tried the shower there that is an attachment to the bath and wow the water powers through!...is this because the cylinder with all the water is up one level so the flow has gravity on its side?...where as the shower in the en-suite is on the same level?
You lose 1 bar of pressure for every 10m increase in height. So it's certainly plausible that the shower below the unvented cylinder would have slightly higher pressure. But probably not to the extent of the reduction in flow you are seeing.
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