Timber has approximately 5 kWh/kg
So your 33,000 kWh/year demand will require 6.6 tonnes at 100% efficiency, probably get about 75% efficiency so getting close to ten tonnes of pellets. Find a price for pellets and a pellet store.

As for sizing the store, that is tricky as 2/3rds of the year you don't need it to buffer the space heating. So consider separating DHW and Space heating. They do different jobs at different times at different temperatures.

Use a PV calculator to get a good estimate of what you can generate, and then think about how much you can realistically use.

I find splitting the summer and winter DHW supply to be quite successful. It is especially pertinent in Spring and Autumn when you don't want to be keeping a full buffer of hot water just for DHW, and when your woodstove may provide all your space heating requirements.

RC bear in mind that under the RHI, if you do use the WB for heating or HW you will have to be metered, which apart from being an extra cost will reduce your RHI deemed payments.

WB's are often better as space heaters alone in an RHI situation, that way if you use lose less fuel through better insulation you still get the deemed amount regardless, otherwise you will only get the metered amount of heat used capped at the EPC value as a maximum.

The main reason that back boilers and stoves didn't make the RHI cut is because there is little to prevent a user form burning coal, which is obviously fossil fuel.

It is a grey area and you could challenge it, but it seems likely that they would require metering to preclude fossil burning on the stove.

Re: silo sizing. Mine is effectively in an old garage. The total area for the silo is 2.4m long, by 2.4m wide by 2.4m high ... which I thought would be good for 5-6 tons, but turns out will probably struggle to fit 3 - 4 tons!

I wish now that I had sunk the augur ... just one half morning with a digger scooping out a 600mm deep trench (2.4m long x 1m wide) would have nearly doubled my capacity and made the world of difference ...

My gut feeling, and it's just an opinion mind, re woodstoves is just have the stove as a space heater by all means, and forget about the boiler bit. By the time you've got, and kept, the boiler hot enough to circulate the HW to a coil in a large store, enough to give meaningful results you may well "fry" in the room where the stove is located.
If you insist on a boiler model and want to transfer a bit of heat elsewhere in the property, then simply put it on couple of heat leak rads e.g. towel rads., on a gravity loop.
Just keeping the stove as a simple room heater is my preferred option though. The stove is cheaper too, and with no convoluted pipe runs to install.
The better boiler stoves are expensive because they are not just a normal wood stove that the manufacturer has decided to adapt and put a wrap around water jacket inside. They, the better ones, have a more efficient flue gas internal structure to give better results, but they come with a price tag to match, because they are designed from the outset to do that job.
So, IMO keep it simple, use the pellet boiler to do the winter CH and DHW, with the wood stove for aesthetics plus additional home heat, reducing your buffer/pellet boiler demand.
For Spring and Summer use the wood stove to take off the chill, with solar DHW with immersion as back up.
In summer it's straight forward Solar DHW all the way, either ST or PV, both work fine.

Thanks for all comments so far. I am looking into the storage as I only originally planned to sacrifice 2-3 m3 of the building. If pellet deliveries are at a 4 tonne minimum then we are going to need to rethink.

I think we have very much come round to thinking as owlman has suggested above. Amazing how plans can all change so quickly! As it stands, I think we will look at installing a standard wood burner rather than a boiler, immersion back up/top up via solar PV and switch from the current oil boiler once we have git our head round the biomass/RHI/pellet storage demands.

Key decision to help progress is the store. My thoughts at present are to look at a smaller 400-500l store, we would have PV immersion input, potential solar thermal if desired and the boiler. Therefore as no longer looking at wood stove with back boiler will we be ok with the smalller store? Hoping that this way in the summer the PV will just keep the top of the cylinder hot to allow for DHW via the heat exchanger. (with boiler as back up).

Any thoughts?

Restorationcouple - we have a similar set-up here. We have a wood pellet boiler, solar thermal panels (FP), a 300 litre thermal store and a separate 180 litre DHW cylinder. Originally we just had the thermal store to supply the C/H system and DHW but it didn't work at all well. We have a 3 ton pellet store but (as has been pointed out above) we dare not let that drop much below about 0.5 ton as the flow will cease; hence we get 2.5 ton deliveries. We have built up a good relationship with our pellet supplier and he understands our situation. We have been buying from him since his was quite a small business and it has now taken off, but as he makes several deliveries in our area he doesn't mind us having less than 3 tons which is their "declared" minimum delivery. You may find a similar situation where you are.

Posted By: SteamyTeaJeff
Would you be willing to let us know what price you pay for pellets, and how they have changed since you started buying them?

Yes ok. Back in 2009 we paid £170 per ton and for our latest delivery (last week) we paid £220 per ton (bulk, blown delivery in both cases). (FYI it has been £220 per ton since April 2012). There is no real justification for large price rises IMO but my worry is that the price of pellets will inevitably be "index linked" to heating oil prices.

Great post Owlman (esp. the bit about frying inside while while waiting for DHW!) ... wish again that I had been able to read that (and act on it) a year ago before I went through much convoluted plotting as to how to leverage the maximum out of a wood stove ... e.g. many hours spent researching Broseley stoves + back boilers etc.

Luckily someone pointed out to me the £1,000+ cost of 15m of district heating pipe, and how much 'free' wood I'd have to burn for DHW to pay that back ...

I will have a chat with the plumber as he is here tomorrow and see if we can get some more accurate figures for heating demands. JonG, are you meaning heat loss from store or house?

Can anyone else comment on the single thermal store with heat exchanger v two cylinder options?

JeffB, What was the cause of the poor DHW performance, was it via coil or heat exchanger?

Posted By: JonG</cite>If possible we will always go with a HW cylinder and buffer as separates.

This is more or less what I've done. Logwood boiler,- 2500l store for CH and in winter DHW via an external PHx. The mains water supply side of the PHx is directed via a SS pressurised solar cyl, so in winter with minimal solar gain it acts as a mains pre heat before entering the PHx. In summer a simple change lever directs it, the hot mains, straight to the DHW supply and the PHx is then redundant. In the spring and autumn if we don't get quite enough solar then I top up with Immersion.

JeffB is your DHW cylinder fed from the coil/heat exchanger in the larger store first?

Unfortunately the plumber has said that our mains pressure is not high enough anyway so DHW via a heat exchanger will not be possible. However this does give another reason for us to look at 2 cylinders. Am I right to think that vented will be better if pressure is poor or does that have no relevance?

Posted By: restorationcoupleJeffB is your DHW cylinder fed from the coil/heat exchanger in the larger store first?

No, direct from the rising main. I have thought about diverting the CW supply through the thermal store in the summer months as a preheat, as the TS is acting as a heat dump for the solar thermal panels. Haven't done anything about it though as I have lots of other projects on the go!

Hi

Re : "JonG, interesting you raise this. I have trawled through what PDF articles are online and it all refers to fossil fuel back up heat sources. It says somewhere that open fires are fine however no mention whatsoever of back boiler of wood stove? Is this mentioned elsewhere? Also it permits an immersion heater as backup, can I presume that an immersion run from spare power from solar pv would therefore be ok?"


Are ImmerSun PV exports allowed to heat my immerser in my DHW tank currently heated by my Biomass pellet boiler? Is this deemed a back up source?

Thanks

J