Having seen a great system recently, I am looking again at a solar thermal & stove & accumulator system for primary DHW and space heating in my post-insulation house. The difficulty is that there are very limited number of places I can put an accumulator (either under the stairs or in a corner of the enlarged kitchen space - we are gaining floorspace of 12 sq metres, so it means losing one of those), and nowhere upstairs.

Is that a problem? How far can the accumulator be from the solar thermal panels?

I know it is well insulated, but I would still be loathe to put my glorified hot water tank outside my well-insulated house. Also, the small garden will be made smaller by the extension. I guess it could be put underground, but is that getting too complex? If the distance between solar thermal panels and tank doesn't matter, then under the stairs seems the best place - the under stairs space will have to be redone anyway, due to new slab floor. So, we can take out stud wall completely, put in accumulator there and any other bits of 'utility room kit' (manifold for UFH if we go that route, meters, circuit board etc...). So the real question is whether that's fine - most people who have solar thermal (on its own) seem to put their tanks in the loft, but is that just convenience?

Putting the tank outside is a bad idea. Any tank, especially a large accumulator, has significant losses and you really want that heat to stay in the house.

Panels to tank distance does matter. The shorter the better, as in fact the pipe between them is the main source of losses and inefficiency in the system. On the other hand up to 20m of pipe (10 each way) is typical and people have setups with up to 40m of pipe, which works fine so long as the pipe size, pump and insulation are all properly dimensioned.

Jeff, 1-2C doesn't sound like much but you have to think about the steady-state: The temp gain across your panels may only be 10C, so if you are dropping 2 in the pipe on the way to the tank, 7 in the tank and 1 on the way back to th panels then that's 30% of your collected energy escaping in the pipe transfer and not going into the tank. It's usually not that bad, but you can see how small drops are significant. The temps in this example depend on the pump speed, but the energy transfer is largely independent of that (within wide bounds).

We've had a discussion recently about DHW being outside - a 30m run in fact - and I made no bones that this was too far, too long, and wouldn't work. This is different, and is a grey area rather than black or white.

Yes wookey, I agree that any store would clearly be best inside, but sometimes it can't be. Does one knock the project on the head as a result, or downsize the TS due to space issues? Well, losses from TS and UG pipes supplying heating are - as a percentage of that heating - far less than losses and dead legs with DHW. And inconvenience, if noticeable at all, is markedly less than waiting for a tap or shower to run hot. You simply have to accept that, despite exceptional lagging (always a good idea in mitigation of said losses), a system with an external TS (or accumulator - choose your terminology folks) will be a few percentage points less efficient due to heat loss.

Whether that's acceptable or not is your individual call. Bear in mind that a bit too much thermal storage is better than too little. And plus points of external TSs are less logs to bring indoors, ergo less manual handling, ergo more time to get your GBF fix...