Configuration of unvented cylinders as heat store / solar etc

Howdy..

I know how much you folk love a dodgy plan so here it is:

I have exported two unvented cylinders to my house in SW Spain. One is a 210l indirect Oso Solarcyl with 2 coils, the other is a 300l direct Megaflo with no coils. At the moment I'm only using the OSO with the immersion for hot water but my plan is to eventually fit radiators, solar panels and buy an air source heat pump.

In winter it's sunny in the day but cold at night. I don't need anti-freeze in the solar panels because it won't get cold enough to freeze.

My plan is to use the Oso indirect for DHW, heated from solar in one coil and heat pump in other coil, with immersion to raise the temp above heat pump max when required.

The question is how to configure the 300l Megaflo as a heat store? Should I just use it as an auxiliary store for the DHW (by connecting and pumping the secondary returns) therefore using the DHW stored water as heat store as well, or should the Megaflo be a heat store for the primary (radiator) circuit?

Yes I know it's an irregular way to do things but I am G3 qualified and got the cylinders for free so clearly am looking for a cheap Heath Robinson approach. The primary radiator circuit will be sealed system at 1 bar and the solar circuit will be open vented,

Sounds very complicated Axel.

But don't overlook that the Megaflow becomes an indirect cylinder with the addition of a pump and plate HE.

Tony

You say it will not freeze but I remember being stuck in snow overnight in Turkey overlooking the Med and the car's water pump froze and cracked. As well as core plugs getting forced out from the block.

Near the sea, never drops below 0.
I'll think of incorporating plate HE into design..

Famous last words................................

Near the sea, never drops below 0

Near Cadiz it doesn't.

When I was in the blizzard in Turkey I could see the Med.