Log burning 'Back Boiler' using a secondary HW tank

[Rafiki]

Only my second post here.  Oak Frame SIP panel house now erected but a lot of construction work still to do including plumbing and heating systems. House is entirely off grid  so no mains gas or electricity. We will have a large PV solar array on adjacent  modern barn, with large battery storage. Thermal Store of 300 litres  with inputs from Immersion  ( which detailed calculations show will be sufficient for 8 months of the year), small LPG boiler and a wood burning back boiler. Outputs will be for DHW and UFH.

The issue is with the wood burning back boiler which  unfortunately is  as far away from the TS as is possible- a full 17metres and on the same  ground floor. It may be possible to move the TS to the 1st Floor, but house not desiigned for this, so weight may be an issue, in any case it would still be 17metres from the back boiler. From everything I have read  back boliers are inherently dangerous due to boiling / explosion risks and reliance must not be placed on an electric circulating pump as it could breakdown. There will be an upstairs radiator / towel rail or two to act as a heat sink - but is that enough ?

My suggested solution is to install a small ( maybe 100 litres) secondary HW tank upstairs very close above the log burner and have the log burner heat the water in that tank ( which would be open vented with a header tank in the loft) then pipe the water in that tank to the TS using an electric pump. That HW would in turn heat the TS via a heat exchanger  (or directly). 

I realise that there would be some losses in the system and the pipe work from the secondary tank to the TS would need to be very highly insulated and relevant valves and stats would need to be incorporated to ensure that the secondary tank didn't inadvertantly cool the TS when the log burner not in use.

 

Thoughts welcome?

Andrew

 

( PS actually this is my daughters house and I'm not convinced that she and her husband or their plumber fully appreciate the dangers or complexity of their project)

[dpmiller]

there are also boil-prevent thermovalves available if the boiler has suitable connnections. Some require a cooling coil inside the boiler, others are a double valve that injects mains cold water at the bottom and release hot flow at the top simultaneously.

 

Our Bronpi stove was specced with one of these:

https://woodmanstore.com/product/two-way-thermal-release-valve-regulus-dbv/