70s estate chalet bungalow, uprating to near-Passivhaus - EWI incl over roof, airtightness, new 3G windows. In this case, no MHRV, instead humidity-controlled ex fans to wet rooms, and opening windows.
Extg 80s rads and HW cyl from end-of-life floorstanding gas boiler into masonry stack prob gas-lined.

30m2 avail for optimal-angled unshaded southish mix of PV and/or solar thermal - Dartmoor, therefore insolation 15-20% higher than nat average, except it's Tavistock, where it always rains.

Possibilities:
1) as the boiler will be doing little work, just leave it incl flue, plumbing etc
2) remove; new tiny balanced flue boiler nearby, plumbing alterations etc
3) solar thermal incl cyl also connected to 1) or 2) (5% VAT, RHI)
4) PV (5% VAT, FIT)
5) immersion heater in the new solar thermal cyl, on usual PV/grid mix
6) box of tricks to dump xs PV to the cyl when avail instead of feeding-in to grid; cyl made bigger/store
7) 'dry' woodburner where the old boiler used to be (20% VAT), with
8) either uprated 8m liner in the extg stack (20% VAT), or
9) abandon or demolish the stack, 8m of new twinwall flue (20% VAT)
10) or 'wet' woodburner with back boiler connected to the solar thermal cyl (5% VAT, RHI, incl to 8/9))

With 3), 4), 5/6), 8/9) and 10), we have to wonder whether 1) or 2) are still necessary.
Esp if the cyl became an even bigger thermal store feeding the extg rads as occasional backup.

What does the team recommend, for an all-round balanced energy scheme, in the circumstances as described?

I'll make a separate thread of that.

Capital is available but it wants to be the medium-to-long term economic best-buy incl considering reduced VAT, and FIT/RHI income.

Heat loads - haven't bothered to calc, let's say fabric/vent loss reduced to 10% of a trad 70s 3-bed home with 1G and 2" in the loft, or 25% of same with plastic 2G, cav fill and 4" in the loft. DHW and electric same as ever. Gains - av 1.25 occupants, no dog, lightish cooking, early to bed!

Posted By: fostertomHeat loads - haven't bothered to calc

Think that needs to be done don't you.
Electric is cheap on the cap ex, emissions free at point of delivery, very flexible and getting greener slowly.

Replace the solar cylinder with an Ecocent (has solar coil) - ST backed-up by electric DHW at average COP 2.5

Replace extract fans in showers/kitchen with humidity-controlled grilles and ducts to Ecocent intake - ventilation heat recovery from at almost no additional capital cost. [edit: I'm a pillock. This won't work if with ST - in sunny weather the heat pump would never run, leaving the wet rooms wet.]

Low level fresh air intakes to bedrooms (via long-ish pipes with bends).

High level air paths from bedrooms/sitting room to wet rooms.

If solar gain is good, main Ecocent intake (always open) from utility room (?) with radiator from spare Ecocent coil in same room; pump with humidistat/thermostat drives radiator; if room is cold or damp rad heats up; indoor drying room at little extra capital cost.

If solar gain is poor, Ecocent main intake from outside (or attached garage, or greenhouse?).

Get rid of old boiler and flue.
Replace with new wall mounted balanced flue for CH only.

Any cash left over, spend on PV.

BTW - RHI premium payment is only available if wet woodburners are MCS registered and there is only one I know of - Broseley Evolution 26 (which would probably be too big at an output to room of 10kW).

Dimengineer - that's a 'one with everything' solution! What I'm seeking is something creative to work with what we've got, get gd result for minimum cost. Here's the current re-statement:

'The biggest and least painful saving possible at this time, to trim the overall building contract, wd be to simply leave the extg. old boiler, loft header tank, rads, pipework, stack and flue/liner undisturbed, as occasional backup spaceheating, and partial water heating. Advantage of retaining the client’s drying cubd!

Then add solar thermal for main water heating, prob requiring new HW tank? and/or PV for main water heating as well as general electricity, esp now that
http://www.yougen.co.uk/blog-entry/1970/How+to+use+excess+solar+electricity+for+water+heating/
EMMA alternatives now coming through. Is it still justified to install two separate solar systems – would just PV do it these days?

Then wait and see how much use the old boiler gets a) for spaceheating b) for water heating, see where to go from there, because it will need replacing soon enough.

So that is a redefinition of what needs to be optimised, incl VAT rates, FIT and RHI.'

Any comments on that?