First a question - How do you source your wood for the boiler, bought in or own forest. If it is bought in how do you buy it, boiler ready or do you have to cut and split. These questions alter the economics of option 1. (which ever one I presume you have enough space to season for 2 years)

I wouldn't use a heat pump to heat the TS to run the CH. I would run the CH directly from the HP to save the TS losses and increase the size of selected rads to cope with the low temps provided by a HP?

An oil boiler could directly heat the TS and could be set up to kick in if the TS temp. dropped to a predetermined level.

The PV system potentially takes the DHW out of the equation but in the winter would the PV system give enough for the DHW and leave anything over for the CH?

If you can use the PV for the DHW then all you need is to sort the CH. If you can get enough PV then up size the rads and go for a heat pump and bin (sell?) the biomass.

Any chance of increasing the insulation etc. to reduce the spend on heat pumps/PV

You appear to have an almost identical system to me and I too am considering my next step as my ageing, but still perfectly functioning, boiler plods on. Like you I've researched lots of alternatives. I also have plenty of solar plus solar thermal for DHW
Q. Does your accumulator tank have other indirect heating inputs, e.g. coils, immersions etc.?
Q. How old is your log gas boiler?

owlman
I have a 6 kW immersion installed to heat the top 1/3 of the TS for emergencies. This is where I might divert any excess PV generated power once my DHW is up to temp in the summer.
I also have an unused DHW coil at the top of the accumulator that could be relocated to a lower port on the TS and repurposed to heat with solar thermal, but I haven't progressed that at all.

The log gas boiler is 2009 vintage, still going strong. Pretty bulletproof really.

djh
Thanks for the insight - I suspected the PV would not contribute in the winter, but I was unsure about capacity in the summer. Sounds like I can mothball the main heating system(s) for the summer months and let the PV deal with the DHW.

I'm up in N Yorkshire so PV experience may be different from yours, but I concur with djh that even with my 10kW array, in winter it's not enough for reliable full blown heating of a 2.5 ton accumulator. Obviously there are odd days when you can get a reasonable amount but ultimately you'll end up pulling most in from the grid.
With your 6kW immersion, (I assume that's three phase), you'll have to separate it into 3 x 2kW single phase feeds,- easily done BTW.
Log Gas Boilers as you must know are ferocious water heaters, with fire grate temps close to 1000 degrees C they heat water at a rate that standard gas or oil boilers I don't think can match, simply because they, ( gas and oil ) are designed for a different form of heating regime. This forced air "Bonfire in a Box" has few equals in that respect. I looked into hooking up my accumulator infrastructure and CH with a simple pressure jet oil boiler, even phoned the manufactures but in the end ditched the idea because I felt I was too far out on a limb control wise.
I also have a back boiler open fire in the living room, an old leftover, but which for aesthetic reasons I didn't want to loose, so I hooked it up to a gravity towel rad, (great), and a pumped loop to the accumulator coil, which is next to useless, but was cheap to do and worth a try.
So CH wise where does all this leave me;
a. Solar: is out,- except for manual sunny day boosts. supplementary resistive heating e.g. UF cables, storage heaters, towel rads, are an option but can't be relied upon in winter.
b. Automated pellet boiler; not enough storage area, but also I'm doubtful of their efficiency with such a large accumulator.
c. ASHP/GSHP Air to water; not a big enough water heating temp capacity, and having just re-furbed the place I'm not prepared to change all the rads.
d. Oil boiler; ( no gas in my village ) a conventional CH system which would mean ripping out a fully functioning and very responsive current CH system.
e. Air to Air heat pump; slightly unconventional in the UK but possibly cheap enough to suck it and see, and it could be constructed/sized to keep chill of the place in winter and thereby place less pressure on the log gas system. Summer cooling may also be something to consider as a bonus should you need it. The downside is that I still can't use my solar even in summer as the pump is an inductive load and solar diverters will only divert to resistive loads. Still I guess with the pump running the load becomes part of the house baseload so all may not be lost as far as PV is concerned. This is the route I'm following at the moment.

Posted By: Peter_in_HungaryDo you mean PV? - I thought PV produced (less) in the winter and it was ST that dropped off dramatically.
Edit - I just did a calculation (pvgis), a 4kWp array would produce half my DHW in January (about 160kWh), all of it in Feb. and Oct. and from Apr. to Sep. about 150% with a bit more mid summer. I did a quick calc for S wales using south facing 6kWp system and got the same in the winter months but more in the summer.

Yes I do mean PV. Dunno about solar thermal.

I can't speak for where you are; I'm just reporting what my 4 kWp PV does in east anglia where I am. YMMV.

Posted By: Peter_in_Hungary

owlman - are you saying that an oil or gas boiler could not properly heat a TS in the absence of the wood burner running?
My initial thought was to plumb a conventional boiler in to the TS and use this when wood was not available. If the conventional boiler won't run a TS then it should still be possible to plumb in such a boiler into the CH to run in place of the TS feed.

Hi Peter,
Personally, I think, as you say, you'd have to run it as a separate entity with manual shut offs to isolate the existing biomass system thereby just leaving the CH flow and return to connect to your oil/gas boiler and run the two in tandem.
I don't think a bog standard domestic oil/gas boiler has the capacity to cope with a large accumulator. To me there seems to be several issues;
a. The boiler water temperatures may not be high enough, and to fiddle with the boiler stat to accommodate batch burns may render any warranty void. Most biomass systems start to load around the 70-75 degree mark and in my pressurised system I ran tests pushing the accumulator store up to 92 degrees, I normally load it at about 75-80. My gut feeling is oil/gas boiler couldn't cope with that. the result could be that you'd have to settle for a lower running temperature. With CH and/or PHx demand calling at the same time, a domestic boiler would struggle to keep up.
b. Linking a gas boiler to a biomass loading valve may prove interesting to say the least. so that may have to go.
c. The pipework runs between the boiler and accumulator, at least in my case, ( i.e. 35 and 42mm ) may need re-configuring.
d. I'm sure it could be do-able but maybe, my instinct now, only with a commercial grade oil/gas boiler.

Thermo controlled non-return valves I guess could work in the way you describe, but quite a few links in the chain. Alternatively, you could charge the accumulator and keep that storage capacity as an immediate back up, running the "manually" isolated gas fired CH when you need the automation aspect, e.g. in your absence. Then switching to the store if/when you desired.
I guess it depends to some extent on your lifestyle and how much you want to automate everything.

I don't know how efficient or reliable, presumably motorised, NRVs would be. Having said that I use a motorised, thermo controlled, variable valve to regulate water temperature in my system. However, it's never fully shut off just variable. Shutting off pressurised water may require a bigger beast, but I guess they are out there, try Acaso.

From JRP’s , original post, it seems as though his absence is the need for an alternative. As such whilst not ideal and assuming there is a bit of notice as to when he comes and goes. An lpg or oil boiler in tandem plumbed with valves to isolate the store (as suggested above), would perhaps be the simplist option. Set up for oil/gas when away, back to logs on return, worst case scenario is you lose the energy in a recently charged store and have a need for multiple burns on return if the store has dropped too far ( ie cold).
Depending on how dhw is provided, pv to a dhw cylinder may cover hot water requirements through the summer and offset some gas/oil use other times.
Depending on the length of time the heating is from gas, it may be possible to run from manifolded cylinders ( assuming that is still acceptable) solving problem in short term at reasonable cost, allowing additional insulation , pv, st, gshp, ashp to be explored in the future.

Thanks for the link on PV output PiH.
It has calculated 5.45kWh in December (not much excess power probably - the house is occupied through the day so there is always a base load). The 27.4kWh available in June should be enough to have 'free' hot water for the evenings. April through August are above 20kWh, which should be enough to provide a useful boost.

Update
I am getting a quote for a GSHP now. Interesting discussion on the practicality of combining the biomass burner with a new GSHP with the contractor. He also concluded that it is either/or - it is not possible to integrate a HP system with the existing TS because of the discrepancy between the 80oC+ output from the biomass boiler and the 45oC from the HP.
We debated the benefit of running two entirely separate, parallel systems with the GSHP feeding the heating circuit directly. But every time we thought we had come up with a way to do it, we realised that there was a way for it to go badly wrong with a single component failure or human error.
So I will brace myself ready for the quote (promised for next week). It will have to include increasing radiator sizes. If I decide to go ahead there will be a second-hand biomass system on offer.

renewablejohn
from my research, the wet UFH will make switching to a GSHP or ASHP at a later date a more realistic possibility. Good forward planning!