DIY Heat Bank/Thermal Store system? (longish)

Our current tank is an old 22 gal. (100 litre, ish) one and after fitting a new bath which is a shade bigger than the old one, is only just enough to fill it hot, so this years project is to fit a new tank. (One of this years projects, oh will it ever end!!!)
Current way to heat it is from conventional back boiler (non condnesing with hopefully a good few years left in it yet) on a standard Y-Plan system with a 3-way valve feeding the tank and CH loop (5 rads, but I want to add 1 or 2 more). The tank coil for this heater is a "top-loader" presumably replacing an old immersion heater. (system fitted by previous owners) There is also a range cooker (Stanley DWH) with a back boiler feeding a conventional coil in the tank. The top-loading coil doesn't seem too efficient to me - the boiler cycles a bit quicker than I'd like when trying to heat the tank up, and it takes a long time to get the whole tank really hot...
So rather than just fit a conventional (2 coil) tank, I'm thinking of moving to a heat bank/thermal store system and after enquiring, realise I can't really afford a DPS/Pandora system, (was quoted close to 1500) so wondering about DIYing it...
I'm not a fan of pressurised systems - it's true that I do a lot of SCUBA diving where I encounter tank pressures of over 250 bar, but I try to avoid it if I can... So, the idea is to start with a big well-insulated coil-less tank with loft F&E tank. I'd love to put the tank in the loft, but the hatch isn't big enough )-:
Bathroom Hot & Cold will be fed from tanks in the loft (via existing Stuart Turner pump), kitchen will be gravity fed (as it is currently - HW tank is on same floor as bathroom, loft is one up, kitchen one down) - mains pressure is high where I am (8 bar!) but flow rate is low and that won't be fixed unless I dig up a lot of tarmac/concrete to see where the restriction is)
The tank will be directly heated from the existing conventional back boiler assisted by the cooker with small back boiler and eventually supplementary heated from roof-top solar panels.
So Q1: Can I just parallel up the heat sources like this, and just tap into the top and bottom of the tank round its periphery with 3 inlet & outlet pipes to the various heat generators? Each one will be pumped (Although the cooker right now is gravity fed)
(I've heard the words "Dunsley Neutraliser" banded about, to combine heat sources like this, but as I understand it, a thermal store will just act like a big DN...?)
Would there be a problem with the cooker, back boiler, central heating system (and solar) all sharing the same water and F&E tank? (The down side is having to drain the lot if there's a problem, but with isolation valves in the right places, I think I can live with that)
Q2 is how to get the heat out - A plate heat exchanger would seem to be the most efficient - can these be bought independently of a tank and DIY installed? A pump on the tank side and a flow-switch on the other side is all I need, presumably.
They seem to have diffusers on the inlet & exit inside the tank (going to the heat exchanger) - does the tank have to come with these fitted, or are they just sections of pipe with holes drilled horizontally?
The only other conundrum is how to heat the radiators... Do I tap off the tank with another pump, (+TRVs, thermostats, timers, etc.) or use a conventional Y-plan system off the back boiler?
This system will have more pumps that I can shake a vacuum tube at, and the tank end up looking like a colander!!!
I suspect the major cost is the plate heat exchanger and control units. (And maybe the tank, and dozen essex flanges I'll need to plumb into it!) Basically if I can save 500 on the bits, I'd put it towards the solar panels. (Although fitting them will require scaffolding, to bridge over a lower glass roof, so it might work out to be even less attractive, but thats another problem for another day ...
I guess the only issue with a non-pressurised system like this is that the F&E tank will have to be above the solar panels, but I can worry about that later...
And any ideas where the best online places I can use to get the basic parts? Essentially tank and heat exchanger...
Thoughts/comments welcome!
(Eg. how many building regs I'll be breaking!!!)
Thanks,
Gordon
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It won't be expensive. A suitable exchanger costs around 80 quid. The cooker can (and probably should) stay on gravity circulation if the heat bank is sufficiently above it with wide enough pipes. Having reliable gravity circulation is better for the health of your average cooker back boiler, especially if solid fuel.
The controls for this system are relatively simple. You shouldn't need any external control boxes. It only gets a little complicated when it needs to make decisions like "should I run the gas boiler, if I'm already getting solar heat". Even this can be simplified by having the solar thermostat lower than the boiler one. Then the boiler only comes on when the tank has depleted somewhat. Unfortunately, this would lead to needing a larger cylinder, as you could only rely on there being hot water above the boiler stat.
If you don't like the colander effect, you might get away with T-ing off from fewer flanges, although you should pay attention to mixing pumped and unpumped circuits in this configuration as unpumped circuits are sensitive to neutral points and might get reverse or parasitic circulation.

You could use your existing tank if large enough, but otherwise pretty much any cylinder will do. I'd buy an indirect one. Even if you don't use the coil now, you may wish to use one in the future and it will do no harm. You can get hot water cylinders anywhere, really.
The plate exchangers are available from GEA Ecobraze. A 14 plate 100kW is fine, although you can go for more if you like.

This can also be solved with an additional heat exchanger and pump (are you counting the pumps yet?!) or by using the indirect coil if you haven't used it for the boiler.

None, provided the cylinder is prefoamed and any gas, oil or electric heating appliances are thermostatically controlled. Also, if radiators are driven directly from the heat bank, then their pump needs to be thermostatically controlled to prevent primary circuit wastage which would result in the boiler firing when neither central heating or DHW demand it.
Christian.
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The cooker is gas, and they recomend a pump - right now without the internal additional baffles, it cuts out on long runs at high temperatures (eg. roasting), and thats with a separate heat-sink radiator in paralel with the tank coil! However, there are quite a few bends in the 28mm pipework, so ...

I'm not overly woried about the controller - if it needs it, and I can't find anything suitable, it's not outside my abilities to put together something with a PIC and a handfull of temperature sensors and switches to control the various pumps, etc.

Actually, you've given me a good idea - my current tank is 100l capacity, small, and has no foam insulation - it's got 2 standard jackets wrapped round it in a desperate attempt to try to minimise heat loss.. A good reason to replace it, if nothing else.
However, a tank with a coil will then let me connect the solar into it at a later date and minimise the colander effect.

Thanks.
If I use the indirect coil, I'll then need to either pressurise the loop and include an expansion vessel, or have a seaprate F&E tank for the Solar. I had a look at a friends "professionally" installed solar system recently and there seemed to be 2 expansion tanks in the loop which I really couldn't work out... It was also running at 2.5 bar which I though excessive, however...
Pump count: Boiler, (cooker), Solar, Heat exchanger, CH = 5... (Plus the existing Shower pump! I'll end up spending more on electrickery driving the pumps than I'll save in gas...)

I'm presuming you are menaing room thermostats here... Hm. with some inginuity, I could zone off each room (or at least upstairs/dwnstairs) if I wanted to, using thermostats in each zone and solenoid valves - the bathroom towel radiator being un-valved, but I'd also need a thermostat in there too. Probably no bad thing.
Thanks. Looking at tanks now, they are not that expensive, with foam insulation, etc.
Tank: 100, heat exchanger 80... plus bits & pieces, pumps, etc. still, a lot of change from the 1500 I was looking at!!!
Cheers,
Gordon
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Doesn't the cooker connection require a heat dump radiator ?
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Yes - It's already got one.
Thanks,
Gordon
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Well that's the most important reg - the one for safety - met. If you look on the Albion website for the Mainsflow you can pick up a lot of info on the possible configurations even if you are building your own.
Also if you are buying a prefoamed tank, which is pretty essential for a heatbank as it runs hotter, you may as well wrap it in the old blankets as well to slow down heatloss further.
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The cooker does have its own thermal cut-out too (which works!) so if the worst came to the worst, it's not going to boil the tank! (The old coal-fired Rayburn used to!)
I had a look at the Albion site and they have a tank with a big internal coil to heat the water, rather than an external heat exchanger. I'm sure the external unit will be more efficient, but the internal one would certinaly simplfy plumbing, etc

Indeed - when this beast is built, there isn't going to be an un-insulated pipe to be seen unless absolutely neccessary...
Gordon
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From a fitting point of view, no difference.

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pipe to be seen unless absolutely neccessary...

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