CH system design advice please (no Gas or Oil option)

[SimonH2]

At the risk of incurring the wrath of those more experienced than I, I have an idea for a system which I am certain will NOT work. The only problem is, I don't understand why it won't work.

What would happen if I plumbed the stove, the underfloor heating and a coil-less tank together? so the stove directly puts hot water into the top of the tank and a cold return from the base of the tank goes back to the stove - there is also a feed from the top of the tank to to the underfloor heating circuit (pumped with non-returns etc) where the return breaks into the cold return to the stove.

So when the CE is off, the tank is getting hot (normal precautions on water boiling etc) and when the CE pump is switched on the main flow is via the CE/stove/tank.

Now don't get mad, I don't seriously think it'll work - only I'm very curious.

Actually, you've described a very workable system. Add a heat exchanger and you've got a thermal store or heat bank which will also give you DHW. Such a setup is good for when you have multiple heat sources, and/or a heat source where source and load flow rates have different requirements. The tank acts as a neutral point, so all the connections can be setup to suit their own requirements - isolated from the other by the buffer tank.

So typical setup (adjust according to options fitted) :
Gravity circulate the woodburner - hot feed to the top of the cylinder, cold return from the bottom.
Solar thermal, coil in the bottom of the cylinder.
Heat pump - return from bottom, feed to part way up the buffer.
Boiler (whatever fuel) - feed to top, return to part way down (you normally only want to heat part of the buffer with the boiler and allow the other options to work in "their bit".
And of course, immersion heaters according to requirements. If it's a backup only then just one which is typically part way up the store so as to allow for hot water, and CH when the hot water is satisfied. If doing a lot of electric, and especially if off peak tariff in use, then two (or more) heaters - one part way up for day time topping up, another at the bottom (or part way up if allowing for solar or heat pump) for off-peak use.

Radiators - feed from high up (not the top, leave a dedicated DHW section if doing DHW), return to bottom. UFH - feed from a bit lower than rads, return to a little up from the bottom (on part load, rad return is likely to be cooler than return from UFH). With the rads, you can fit TRVs on all the rads and do away with a room stat, use a fully modulating pump (eg Grundfos Alpha 4L) which can modulate down to zero flow rate - just switch the pump on/off with the time clock.

DHW - either use an internal coil (thermal store), or a pump and PHE. With a pumped PHE loop, you ideally want a thermostatic valve that will restrict the flow so that the return to the bottom of the store is fairly cold. With a thermal store coil, it will naturally take heat from all levels.
The result si that you end up with a store that's hot at the top, and gets cooler as you go down. Things like heat pumps and solar panels will work efficiently heating the cool water in the bottom, effectively giving a lift before the boiler or woodburner does the last bit if the others can't cope.

And not a single motorised valve in sight !

For GSHP you need an area of unshaded ground approximately twice the footprint of the house, and the collectors need to be roughly 1200mm cover.

Or go very deep - which is what the house up the road has, the garden is quite small and heavily shaded.