PS No, I havent not read all 10 pages to see if any measurements have been made of the dynamic flow rate or shower consumption. I rarely see anyone quoting them on forums. Few even understanding the measurements needed.
I don't blame you, of the ten pages, about 9 of them are a mix of the old arguments about legionella, personal insults accusing myself and JonasX being some chap called Drivel, and the usual arguments from self-styled experts proclaiming that it's 100% impossible for a thermal store to work.
And no, available flow rates are not mentioned.
Thanks Agile. in the example you have given what happens after 15 minutes of the two showers running? Does the water then go completely cold? How long would I have to wait to then have another shower?
PS, I have a WB 30cdi conventional boiler, if that makes any difference?
At some point the water will go cold. It probably won't go completely cold in an instant, but it will go cold. It depends on the heat transfer capacity of the reheat coil which will limit the amount of heat that can be put into the tank - while you draw out hot water from the top. Also, because of the way reheat works, it's not like the thermal store where as long as it's set up right, you'll keep hot water at the top even if the overall heat content is dropping.
The primary difference is this.
In your thermal store, it's heated top-down at up to the full capacity of the boiler. The design of the DHW coil sets the limit at which you can remove heat from the store, and the outlet will start to lower in temperature as the overall thermal content of the store reduces. If set up right, the boiler will continue to keep the top of the store hot, but at reducing flow rate as the bottom of the store cools down - the DHW temperature is a function of temperature difference and flow rate - so at the reducing differentials lower in the store, the overall heat transfer will reduce but there will be no abrupt change. If you were to draw off heat for long enough, the system would stabilise with heat output matching heat input from the boiler. Reducing the draw off rate will result in an almost immediate increase in draw off temperature since the top of the store is still going to be hot and the transit time of the DHW through the heating coil would be increased and allow for more heat transfer.
In a hot water cylinder (whether vented or unvented), reheat is by a coil near the bottom of the store. This heats the water around it, and this hot water convects around the cylinder. In still water this will result in some top-down heating, but largely it is "bulk heating" where the whole store is heating at once. Once you've drawn off all the hot water, you will then have cold or lukewarm water - and again if you keep drawing off water the system would stabilise at a temperature where the heat drawn off in hot water would balance the heat put in by the heating coil. Reducing draw off rate will not result in an immediate increase in draw off temperature as the whole cylinder has to heat up.
Many of us remember the days when reheat times could be hours and so if someone used up the water for a bath, then you would have to wait a while until there was enough water for the next bath. Modern cylinders are now much better, with high efficiency coils and hence faster reheat times. However, having a (very quick) look around, the capacity of the reheat coil doesn't seem to be mentioned much - even those offering a high capacity coil for quick reheat don't seem to give any figures.
What I can say from experience is this ... without the boiler running, a traditional open vent cylinder goes cold quite abruptly as I've found out while showering after the boiler has tripped
