Hi Chris, there are several on this site who have log fired wood gasification boilers with their associated accumulator storage tanks, plus various methodologies for delivering DHW from that heat store.
In my case it's a 42kW boiler a 2500l accumulator, with a plate heat exchanger. I designed and fitted my own system with lots of, mostly manufacturer, help Sweden, Austria, Germany etc. It seems things haven't changed, inasmuch as as the UK plumbing trades generally don't understand these sort of systems. It says a lot about basic hydronic heat transfer teaching I guess.
The old adage "less is more" springs to mind from your plans. You have three tanks of various sizes when one, maybe larger, tank and a decent large capacity DHW plate heat exchanger, ( despite your reservations ,they work extremely well), and you would save space and lots of superfluous pipework. You could also incorporate two or three immersions into the tank, and a solar thermal coil. Better still would be point of use DHW, especially if you're fortunate enough to have a 3 phase supply.
At a quick glance of your plans, you may not be aware that dedicated Log gas boilers like mine have a large valve called a Loading Valve which in the initial boiler firing staged effectively short circuits the boiler hot water flow back to the boiler until operating temp is reached, and only then bit by bit. This is to prevent an inrush of cold store water into the boiler thus affecting burning efficiency. I see no reason why such a device wouldn't work for your big burner. I can vouch for the assistance I got from the Swedish manufacturer of mine.

Posted By: owlmanAt a quick glance of your plans, you may not be aware that dedicated Log gas boilers like mine have a large valve called a Loading Valve which in the initial boiler firing staged effectively short circuits the boiler hot water flow back to the boiler until operating temp is reached, and only then bit by bit. This is to prevent an inrush of cold store water into the boiler thus affecting burning efficiency. I see no reason why such a device wouldn't work for your big burner.

The plan is for the WBS to run on gravity and gravity systems don't use loading valves. Part of the loading valve function is to pump from stove to TS. Also gravity systems tend to the self regulating. when the water temp drops the flow also decreases proportionally.

Posted By: philedgeHave a plan for storing wood and dealing with dust and dirt from the wood/fire- they're messy things

+1
And from your description of the property ( there'll be seven bedrooms and five bathrooms, with potentially up to 15 people ) plus its location it sounds like it might be linked to the tourist industry. If this is the case then you may want to run your plans past your local environmental health inspector (or what ever they call themselves these days) to make sure he is happy with your plan to put a dirty log burner in the kitchen - and get any positive response in writing in case he changes his area and you get a new person. Don't ask how I know!

Posted By: philedgeIf you can, duct the combustion air to the stove direct from outside otherwise you're drawing in a lot of cold outside air into the house from to keep your furnace running!

+1

Posted By: philedgeMay be have a longer term plan for heating as tree surgery/ forestry is hard work and you may not always have as much wood available as you do now??

+1
I run my heating from 7 ha of my own woodland/forest. At age 55 I got the trees felled and stacked by contractors and for the last 10 years I have had someone else convert the logs to 'boiler ready. (usually the tenant who does it for 'free' heating).

Thanks for your comments folks,

To answer a few (all very sensible) queries:

-Interesting to hear positive reviews of your DHW plate heat exchanger. A couple of people I’ve spoken to have found that the temperature can drop off halfway through running a bath and they have to stop for a few minutes to allow the water around the plate to get back up to temperature. Does that just mean their TS temperature isn’t high enough? Due to the length of the house I’d be looking at installing a DHW circulator - would that work with a plate do you think?

-In the event the thermal store gets to boiling point during a power-cut, the backup diesel generator fails, and the pump to the emergency heat dump therefore isn't operational, or if the sub-floor heats to such a high temperature that the heat dump doesn't cool the store enough, the system would just boil over into the header tank and then out of the overflow. It would be a waste of thermal energy as hot water would be spat out into the back yard, but nothing should explode (unless I'm missing something?)

-Very good point about non-return valves on the coil feeds to prevent thermosyphoning into the thermal store when the immersions are on and the stoves aren’t.

-Yes, the the UFH tank is there to be able to use an immersion if the TS is colder than the 35-40 degree UFH system. As you say, it will have to be a chunky immersion. The house will be very well insulated in roof, floor and external walls, so hopefully the load shouldn’t be too great once the system is up to temperature. Have you any other suggestions on how to electrically heat a wet UFH system? I was thinking about some sort of high-power in-line instant water heater but couldn’t find any examples of one being used in this context.

-The 30kw stove is a Henley Druid 30, nominal output 23kw to the water and 7kw to the room.

-The room is a big open plan kitchen/living/dining (12mx5m) with open stairwell to the back porch/landing and first floor hallway , so I'm reasonably happy 7kw won't be excessive - I do like a warmer room temperature than most. I was also thinking about making some removable insulated doors over the stove alcove to reduce this when needed.

-Yes, there's a 4" external air supply duct to all three stoves.

-PV panels and other renewable sources heating immersions are a definite possibility to consider - the panels are probably out of budget at the moment but fitting the immersions in anticipation would be sensible.

-The stove is at the 'living' end of the kitchen, near the door the logs will be coming in through, so mess and muck will be away from the food prep area

-I'm not planning on any commercial catering in the kitchen. One end of the house can be closed off and may be let as an airbnb/annex/cottage thing, but this will be self-catered and away from the log mess.

-My business is a forestry and arboriculture contracting and consultancy firm, and I'm increasingly using employees for the practical side and spending my time surveying and desk-based - unless the trees stop growing or the business goes south, my wood supply isn't dependent on my own physical health.

-By my calculations, raising the temperature of the 2000l thermal store from 20 degrees to 80 degrees will require about 140kwh, so about 6 hours of burning the stove at 23kw output to the water. I'm hoping that this would only be required on returning from holiday when the system's fully cooled, and that normally it would just be a case of 'topping-up' the store. Under normal circumstances a full store's worth of energy wouldn't be used up in one day, and the immersions should take up the slack if there's a full house and there's a lot of demand.

-I have considered a log gasification boiler such as Peter and Owlman use. I’d much rather have something inside the main house which I can load up in a dressing gown first thing in the morning while the kettle boils. I’d like to see the flames and for the stove to be the heart of the household… but maybe I’m being overly romantic and a log gasification boiler is really what I need!?

Cheers folks, all good food for thought, any further comments very welcome!

Regarding phx, my mums heating system (40kw pellet boiler, 1000l buffer ( runs 60-80 degrees)) for an epc D/E rated annex and house respectively), uses phx to seperate the boiler/ buffer water from the seperate annexe/house heating systems ( this was to prevent contamination between old and new installs as well as in the event of a leak in house or annexe risk a ton of water escaping.
Plates were rated at 25 and 40 kw respectively and effectively work so well that you wouldn’t know they were there. ( ie, it’s as if you were flowing hot water from the buffer around the systems)