Unfortunately you cant just say - water pressure's great so it's all fine - pressure and flow are linked but they are 2 different things. No- you wouldn't normally use the same supply pipe that feeds the HW cylinder to cold feed a shower - they should have their own supplies.
If the shower can flow (hot and cold) more than the inlet to the cistern can flow, then the cistern will empty faster than the supply can make it up, it really is as simple as that. Ideally you want to know, in L/min, what the feed into the cistern delivers and that's then compared to the shower flow on full.
I can't find a manual for that shower so can't tell what it will flow @ max. The delivery to the cistern is easy, shut the supply to the cistern, run a cold tap to empty it say half way then turn the water back on and fill a container at the cistern feed fully open, timed over 30 secs, measure what you have then x2.
At the end of the day it is relatively easy process to find out if it will be ok or not and your plumber should be able to work through (edit) that process pretty easily for you.
People that don't consider these types of thing properly are the ones that invariably run into issues, always best to get it checked out.
Thanks for that advice - I'm trying to get my unpractical brain around it!
Just an update and a request for confirmation/explanation.
The plumber has put in a temporary solution so we can see if this works- i.e there's sufficient water capacity in the current tank to answer the question as to whether we need an additional tank up there for the shower's own cold supply. He took a cold supply to the shower, off the current cold supply to the HW cylinder, just above the HW cylinder's supply.
So far, that seems fine. Just the two off us (2 bed cottage), and we usually only shower once a day, one after each other most mornings. No lack of water so far, and shower's water flow rate is fine. I sometimes run my shower cold, and while the rate is slower, it's still adequate. Also, when listening to the cold water tank fill up after the showers, the sound of the inlet valve filling stops after a min, or so. It seems the tank is not emptying a great deal, (inlet and outlet flow rates reasonably matched) and not needing to fill for a long period to replace the water that has flown out.
To try and ascertain the tank inlet and outlet flow rates, I ran the shower on max (after checking the handbook/manual and finding no info on this). Anyway, the shower filled a measuring bucket to 6.5 litres in 1 min. Now trying to discover the cold water tank fill rate is/was extremely difficult. While the tank is a 29 gall/114 litre capacity, the inlet is obviously set below the top of the tank rim (so the tanks hold less than 114 L) and in the awkward, limited space of the loft, with a black tank i na dark space - even with a head torch - I couldn't get an accurate estimate of the change in that capacity/level, when running the shower and trying to measure the flow rate into the tank.
What I tried, if this helps, was to measure, indirectly, the water flow rate of the cold mains supply into the tank by checking the L/min of an outside cold tap, on full flow, just down line of the tank's inlet. That was about 45 L/min. I do realise that the rate through the tank's ballcock valve will be less than this.
The plumber said that if the tank's capacity seems fine for us, he'll have to return and put in a separate cold feed out of the tank to supply the shower, requiring drilling the cold tank for this. Advice above also told me that the shower needed to have this separate cold supply, rather than our temporary fix with a shared single outlet from the tank for the HW cylinder and the shower.
Please can someone explain to me (a plumbing dunce!) why this is necessary (you'd need to explain slowly and in simple, idiot-proof language!). I'm worried that if an extra outlet (22mm?) is drilled in the tanks, this will mean 2x the flow and speed of the tank emptying (?). Also, I've read that the cold supply from the tank to the shower has to be 50/60mm below the cold supply to the HW cylinder, so that if the tank runs dry/or very low, the supply to the hot will cease first and the shower will run cold rather than scalding. From what I can see in the loft space, there's no ability to do this as the outlet is already low in the tank.
With the current, temporary fix, with one shared supply to HW and cold shower feed, if the tank runs dry/near empty, then both the HW and cold will stop simultaneously, or slow at the same rates (?).
Can the HW still run out the top of the HW cylinder if there's no, or very little, cold supply pushing in from the bottom? Or, is there a syphon effect from the top of the HW tank so it'll keep flowing, hot, when there's no cold coming in at the bottom?Again, sorry for the numpty questions. Every day's a school day as they say!
Thanks for your patience.
