IIRC the maximum differential pressure for a v4043 is 0.6bar - I'm thinking other makes are probably similar. Your mains will almost certainly be more.
I don't see any problem with building a thermal store but I think bringing in combi preheat etc brings a level of complexity to this above and beyond what is required.
My 160litre store needs keeping nearly completely hot for a (p-shaped) full bath's worth of hot water, so I'm thinking that your 300litre store will need keeping at least half hot for the same. I can see with solar you'd expect to need part of the store cold sometimes (ready to add CO2 heavy heat later) so I think the idea of running the rads off the store is a non starter.
Also you may want to consider the relative positions of the coil and immersion element, as using the pv you'd want the immersion at the very bottom and the coil halfway up, I suspect they will not be that way round on your cylinder.
Hi Mogget,
I see our posts crossed, thank you for taking the time to research the pressures.
I must however disagree with your conclusions.
The maximum differential pressure of the valves wouldn't really apply as the tap or shower would be turned off first, which would build up the pressure in the pipework either side of the valve so no differential pressure, if the changeover happened mid flow then the water in the pipe downstream wouldn't drop to zero immediately either otherwise you would just have a dribble at the shower.
I would be using a normally open valve for the PHE valve and a normally closed for the combi anyway so you wouldn't get a situation where the downstream pipe was empty and the upstream fully pressurised with both valves closed, even after a drain down.
The rads and UFH are run from the lower half of the tank which is where the tappings are, the top of the tank is reserved for the DHW.
As this is solar PV we don't need to keep the bottom of the tank cold as you would with solar thermal and the position of the immersion is fine further up the tank as the elements get much hotter than the 80`c which is the maximum the tank would actually be set to.
As the coil tappings are near the center of the tank, the heating (which I`ll be circulating at about 40`C max) wouldn't affect the DHW at the top of the tank much at all.
Having the immersion at the bottom would be counter productive as it would be heating the cold water and expecting it to rise without heating the rest of the cold water around it by convection, the recharge times would be horrendous.
With the immersion around halfway up, the top of the tank would be heated quicker and the heat would work its way down the tank and stratify nicely.
