This has been discussed before, I see, but many of the information links are now inactive, and my case may be slightly different.

I'm in northern Spain, living nearly off-grid. I'm installing a back-boilered wood burner and wish to connect it to a 6 bar max steel 100 litre cylinder fitted with one coil, situated two floors directly above the stove. This will be the primary buffering circuit, and from there will run the central heating system. I'm intending to use the coil for the heat input to minimise thermal inertia and the water jacket to run the sealed heating system.

The first question is vented or sealed for this primary circuit? Code, in Spain, it seems, calls for sealed. I'm prepared to ignore this if I cannot see a way to guarantee safety without a vented system.

The stove (Flamma 14CS, 15kW, of which 10kW to water) is designed for either. It has a cold water flush in case of overheat. I have no documentation on how this works and frankly, I don't hold much faith in it. Where does it flush to, and if the flush is internal, what drives the flush through the existing pressure in a sealed system??

If I vent it, I need to choose gravity or pumped. If gravity, protecting against power loss is a null issue. I have a 5m vertical run of flow and return, but the cylinder I already have (Kospel SWK 100) is top-feed to both sides of the coil, which acts a little against gravity flow. I'm guessing if I insulate the vertical flow pipe run and leave the return unlagged copper, it will help maintain the flow.

Either way I understand I need an anti-condensation valve in the primary circuit, and in a sealed system would fit a magnetic filter / charge point and an expansion vessel. Or I could buy a ready-made unit which has these components in it. Plus pressure relief valve, auto-bleed valve and drain point.

Could anyone advise / discuss / guide me please? I need to get this right, I don't want any accidents or a system which doesn't work well.

Background: I was a professional engineer and have a fair bit of plumbing experience, but it doesn't extend to uncontrolled heat sources, hence my wariness and questions.

Edited: there was a typo in the make of stove, here is a link to the relevant pdf::

https://piecepolska.pl/data/UserFiles/Image/Flamma_instrukcja.pdf

I can't find anything about a company called Fiamma making back-boilered wood burners. Fiamma appears to make awnings and related products for caravan camping. There are also some outdoor ovens branded by Fontana with the model name Fiamma that I can see. I suggest placing more effort into finding some documentation. If the stove is new it surely has some!?

Hi Louise, y encantado.

living off grid (nearly) in N Spain - I suspect there'll be more than a few people on here very jealous

My starting point would be to clarify what you're hoping the system will deliver for you. By that I mean;

- what is the heat loss for the building (energy demand form the system)
- how much heat energy you want to store for utilisation when the stove is not running
- how frequently and the duration you want to fire the boiler stove
- how much DHW do you wish to generate from it, if indeed DHW is heated from this, and how/where is it held
- is electrical power consumption, for pumps etc and issue (off-grid, or uncertain grid supply etc) though a small stand alone UPS can resolve some of that

You've listed various components which may or may not be appropriate depending on what route you choose to take, and so slightly cloud the view from this side of the canal ingles.

I personally never design these systems as sealed. Having been involved with pressure relief systems in a previous life, without regular maintenance/testing of SV's, I would never have a potential over pressure set-up in my lounge. So I always design open vented. However, if your local installers don't buy into that, you've got a problem.

If you do go for pressurised (sealed) the quench valve basically opens in the case of over-heat, to let cold mains water flow through a quench coil in the back of the boiler stove, to remove heat, running to open drain. Should never happen, but there are lots of modes of failure for that part of it (ie. cold water being available, under pressure, and actually able to get to and through the quench coil) that put me off sealed systems. Theoretically they work fine.

If you go for open vented, then decide if gravity feed or pumped is preferred. I most often use pumped for the boiler stove circuit, but also gravity feed if that's appropriate. There are pro's and con's for both, but your specific circumstances will determine which wins out.

Sealed/open aside, how your energy 'in' versus energy 'out' balance would work best for you, needs to be determined. I don't initially quite see the 100 L cylinder functioning that well as part of the whole set up, though I get what your reasoning is (buffering). Presumably you'll pump from that to the CH and the DHW? That brings back in the elect dependency, but maybe creates a potential bottle neck of energy flow from source to outputs.

There's an answer of course for your needs, but I'm not sure the bits of the jigsaw on view at the moment quite fit together. If I've patronised/man-splained over your engineering knowledge, desculpe me, I'm just trying to put some thoughts on the table to kick around.

Posted By: djh

Posted By: Dominic CooneyCould this be it, or similar from the same company?
https://www.stoveitaly.com/en/portfolio-articoli/fiamma/

Could well be. Well spotted!

I had a look at the web site and from a quick dip in - I couldn't see anything with a back boiler, lots of pellet stoves though.

Posted By: LouiseSJPPThe stove (Fiamma 14CS, 15kW, of which 10kW to water)

Another thought to add to the mix
If the stove is 15kW with 10kW to water that implies 5kW to air. If the 10kW is needed to heat the rest of the house you will only get that with the stove on full chat and how long will the room in which the stove sits be able to cope with 5 kW of heat before you are driven out by the rising temperature?

It is a common problem with back boilers in that you can't split the heat to air and heat to water and this leads to uncomfortable heat in the location of the stove if the heat to water is needed elsewhere.

Do you have experience of wood burning stoves? At 15 kW you will be using 3 - 4 kg of wood / hour and that is a lot of dust/ash/dirt. If it is for continual use rather than occasional use on cold evenings to back up the primary space heating then that is more than I would want in my lounge.

So for a Flamma stove there seems to be documentation on Edilkamin's website for one.

Posted By: LouiseSJPPThe buffer tank is meant to buffer between log loadings only, I'm not looking for heating when the stove is not running. The 100 litre tank is already there. If it proves inadequate, it could be dualled-up.

The buffer tank will give some heat between loading but not too much, however when you reload the stove then you will need time and energy from the stove to reheat the buffer before you get useful heat out into the rads, so IMO if the stove more or less matches the rads then all the buffer will do is shift (delay) the time when the radiator temps drop. (If the stove is oversized for the rads then careful use will be needed to avoid boiling the system as the 100 ltr tank isn't big enough to manage the full output of the stove for more than a short time).

Posted By: LouiseSJPPMy worry is how often it may need loading.

In practical terms you won't want to let the stove go too low on fuel otherwise you will be relighting it frequently - and that is a pain. Probably expect to reload every 40 - 60 mins. (stove data say 3.5kg/hour)

If you have a pumped system then you will need a laddomat or similar to avoid over cooling the stove and chimney but this also means that the start and end of a burn will be recycling the stove water to keep up the stove temp rather than going to the buffer tank.
If you have a gravity system then it is self regulating so when the heat from the stove drops the circulation slows and the reverse when the stove increases in temp. so a laddomat is not needed and you tend to get (lower) heat to the rads for longer.

Posted By: LouiseSJPPGravity would be failure-proof, I guess, but will be effective in shifting the 10kW??

I have run a 20kW stove and a 40 kW boiler on gravity without problems - the key is getting the design and pipe sizes right. Todays plumbers don't like gravity systems because attention needs to be paid to both the design and the installation for it to work properly. With a pumped system failures in design or installation can usually be overcome by turning up the pump speed.

IMO the simple system would be to install the stove on gravity system with the rads balanced to the stove output and the heat output is controlled by the amount of fuel you put onto the stove. - No tank,pumps, valves or electricity needed.

A pumped system will still have the heat output controlled by the amount of wood you put on the stove because you have no other control - but you will have a more complex system.

I would strongly recommend putting in an external air feed for the stove unless you have a very leaky house with old ill fitting windows - and even then put in an external air feed because when you get fed up with the draughts and fix them then you will need one.

Did you do the calculations to show that the stove will heat the house sufficiently?

Edit to add
I have a 40kW wood burning boiler with a quench coil which I never connected because I am off the water (and gas) grid so water comes from a bore hole. if the electricity failed the boiler gets v. hot but the quench coil won't work because without electricity I have no water !!

We've got a 17kw WBS heating our DHW tank via an open vented gravity loop that works fine. The tank is the other side of the wall at the back of the stove so about 1.5 metres of inclined 28mm copper connecting the two. Our CH is pumped out of a second set ot tappings on the back of the stove so.no buffer or thermal store for us. Control of the CH pump is via pipe stats on the gravity flow and return. Return temp has to be above 50c for the pump to run and when the flow hits 80c the pump starts.

All works well but CH is direct from the stove rather than from a buffer so a touch different to yours. You'd need to check your input coil to the buffer an dissipate 10kw when the buffer is up to CH operating temp.

Posted By: djh

Posted By: LouiseSJPPit does expect uncontrolled heat source plumbing expertise

That seems like a sensible prerequisite for installing an uncontrolled heat source. If you don't have it, then engage somebody who does would be my suggestion.

Yes, in my last house I had a qualified 'heating engineer', part of a large and accredited organisation, fit an air source heat pump.

He coupled the heat pump (high flow, low pressure) to the existing CH system (low flow, high pressure) using a low loss header, an baffle-less cylinder of twelve litres capacity on a 12kW pump. Of course, it short-circuited the system meaning the heat pump was running effectively at heating system flow temperature instead of return temperature. Those who understand their T to the power four will see that this would have hugely impacted COP, putting it way below the quoted figure in all the nicely presented and utterly fictional heat loss calcs.

The short circuit also short-cycled the heat pump leading to excessive stop-starts and effectively reducing the real power from 12kW to perhaps 6.

Because I was not a heating 'engineer' (but I was, before stopping the subs) IMechE C.Eng, I found myself unable to complain the plumber, the installer or the accreditation board. Maybe a man would have had better luck.

So, no, I don't need a professional to screw my system up, I can do that myself without paying thousands for the priviledge.

Sorry if this sounds harsh, I'm still a bit angry about the whole pro scene and renewable energies, as well as the whole disempowerment of the common people.

Posted By: philedgeWe've got a 17kw WBS heating our DHW tank via an open vented gravity loop that works fine. The tank is the other side of the wall at the back of the stove so about 1.5 metres of inclined 28mm copper connecting the two. Our CH is pumped out of a second set ot tappings on the back of the stove so.no buffer or thermal store for us. Control of the CH pump is via pipe stats on the gravity flow and return. Return temp has to be above 50c for the pump to run and when the flow hits 80c the pump starts.

All works well but CH is direct from the stove rather than from a buffer so a touch different to yours. You'd need to check your input coil to the buffer an dissipate 10kw when the buffer is up to CH operating temp.

Thank you for this, good information.

It would seem for my 10kW that 28mm would be more than sufficient. Primary circuit pump stats between 50°C and 80°C as you haven and then secondary circuit stats on the buffer to start heating at 50°C and alarm at 80°C. As you point out, the danger lies if the return feed temperature to the buffer goes up so it can no longer absorb heat. So maybe also an alarm on the buffer return at 60°C, giving us time to react.