jim coughed up some electrons that declared:

tim thanks for the rundown - can i also ask what the twin tank stats
(& your relay stuff) give you over a single one?

Hysteresis. eg, boiler doesn't fire until the top stat gets cold and keeps
firing until the bottom stat gets hot again.

All single stats have some natural hysteresis, but in this application it's
relatively tiny, so with a single stat (near the bottom, above the solar
coil) the boiler would heat up the tank, then keep switching on and off in
fairly short cycles everytime a little heat was taken out of the tank.

This is fine for electric heaters, but not so good for boilers.

DPS will sell you a control unit that handles the twin stats, but it does
(IIRC, I discounted it fairly early on) more other things than I need, it's
a proprietary component (so's my contoller, but mine will be assembled from
DIN relays and terminals, so it's easy to repair).

also on the DPS designer thing - what are the "upper detect circuit"
and "lower detect circuit" about?

You don't want the tank becoming 100% depleted by the radiators, so the take
off for the rads is not at the top - you always want a reserve amount of
hot water to make, well, ho****er for the taps. The circuit that drives the
plate heat exchanger (for taps) is taken off right near the top, so you
always have 1/3, maybe 1/2 (depending on exact design) of a tank of hot
water ready for taps and showers.

Cheers

Tim