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Hello all, I'm searching for advice on my new system being fitted into a very old stone house in France.

We have purchased a thermflow 300L thermal store with a built-in expansion tank to work with multiple heating inputs (solar, electric and wood burning stove boiler). We are running 7 rads and underfloor heater from a 14 KW wood boiler, along with underfloor heating and domestic hot water coil. There is an electric immersion back up (which will be supplemented by solar thermal next year). I've attached a diagram of the levels everything is on.

The tank company have told me it's fine to pump the water to the wood boiler but my understanding from local heating engineers (and from my extensive internet reading!) is that this is not advisable due to it being an uncontrolled heat source and the lack of gravity creating a risk of the WBS boiler exploding if the pumps fail (power cut, mechanical failure, etc).

We have an excellent journeyman plumber but he's not overly experienced in heating systems of this kind - and he is very uncomfortable with the installation suggested by the thermalstore company.

It will be extremely difficult to put this tank in our loft due to access issues (and will greatly decrease the efficiency of the system due to increasing the distance from all the inputs and outputs). However if it's the only way to make the system safe, we will have to do it.

If we continue with this installation as is, our plan is to use 2 pumps, one solar and one electric both with a battery back-up. So we would need both pumps to fail simultaneously to have a problem. We would also have a pressure relief valve for the wood boiler (any recommendations on brand here would be welcome!). There is also a quench coil which came with the thermal store, but I'm not sure that would be effective in the case of a power cut / pump failure, except to protect the thermalstore itself from boiling - the wood burning stove boiler located higher than the store wouldn't receive any of that cold water.

My understanding with the above plan is that we will risk boiling the wood burning stove boiler dry (and thereby probably destroying it) but that is a risk I would take given the unlikely set of circumstances required for it to occur, (i.e. a power cut occurring at the almost exact moment we have banked up the fire; left the house for hours and all our pumps fail too). And also the cost of a new stove versus the cost of siting the thermal store in the loft is actually about 1:5.

We are 100% committed to avoiding a catastrophic boiler failure i.e. an explosion. We have a young son and can't bear the thought of a dangerous installation. So are we taking a risk that this might occur with the system we have in mind?

If yes is there any way of removing this risk with additional fail-safes? Perhaps another pressure relief valve on the boiler? Or is the thermal store going in the loft the only reliable way?

Thanks in advance for any thoughts / experiences / recommendations here.

Yours, a worried Mum
 

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The tank company have told me it's fine to pump the water to the wood boiler but my understanding from local heating engineers (and from my extensive internet reading!) is that this is not advisable due to it being an uncontrolled heat source and the lack of gravity creating a risk of the WBS boiler exploding if the pumps fail (power cut, mechanical failure, etc).
The tank company will only be able to talk about what's okay for their tank. It'll be the boiler company's opinion you need...
 
The tank company will only be able to talk about what's okay for their tank. It'll be the boiler company's opinion you need...
Thank you. You're right of course but the tank co gave me a schematic of how the wood boiler would be connected so seemed to be providing advice on the overall system.
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Just an update that I have heard back again from the tank co and they are now saying nope it's not OK to put it lower, it's a gravity fed system. I had told them that a few time I was planning to put it downstairs and told them again yesterday when I called them to discuss my plumber's concerns but only when I put it in writing (again) this morning did I get the proper response. So yes this tank needs to be above the boiler and we are now working out how to manage this logistically...thanks for reading / responding.
 
You have to put a "soupape de sécurité thermique" next to the wood burner if the water overheats it will discharge the hot water into a waste pipe and refill the heating system with cold water from the mains.
 
The schematic that you have shown is far too simplistic for a multi source heating / hot water system.

In respect of loadings, a 300l store is quite small for a 14kw solid fuel appliance - particularly if you are adding renewables in the future.

Protection of the wood burning stove is unrelated to the thermal store. So you can place the thermal store wherever you like ( provided that the system is designed to suit the location) . On a personal basis I would not want to sleep beneath a 300l thermal store mounted in the loft.

If local regulations allow, the easiest way to protect the solid fuel stove is to use a gravity loop fed by a small header tank in the loft and controlled through a cooling loop comtroller, such as a Heat Hero ( circa £600) - it is probably the same type of device as described by the poster above.

in your case I would also consider using a heat exchanger and having the thermal store as a pumped system.

There are two key aspects to these type of systems (and we have a lot of experience with them). The first is safety and the second performance.

If they are not designed and installed properly, at worst the system will be unsafe, but also most likely to offer years of disappointing performance.


Apologies, if the posting appears to be blunt, but all to often we see expensive items of equipment cobbled together into a multi source system that just does not work.
 
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The schematic that you have shown is far too simplistic for a multi source heating / hot water system.

Thanks for your reply. The schematic is only a quick sketch intended to show the levels that things are on, not the system itself, which is a great deal more complicated than shown.

Also the heat store is a little small on paper, but the 14kw output is from a wood stove with quite a small firebox so I doubt it will be able to run at full capacity all the time - certainly not overnight - and, if the system is hot enough, the rads and underfloor will be letting off a great deal of heat and so we won't have the need for the 4kw of space heating the burner provides and will just run it lower. I think it's more likely the stove will be underpowered to run the store to be honest! But we will have our summer system as solar (both PV and thermal) which can supplement the winter supply if needed. I believe there is room to adjust how we use it as we get to know how it works best.

And yes, I'm not totally happy with the huge tank in the loft but we have some pretty hefty beams up there and will be reinforcing to the spread the load across a few, so it should be OK. Fingers crossed. But I will suggest the option of the other tanks to the plumber and see what he thinks too.

Thanks again!
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You have to put a "soupape de sécurité thermique" next to the wood burner if the water overheats it will discharge the hot water into a waste pipe and refill the heating system with cold water from the mains.
Thank you - I think this is the safety valve that my plumber was talking about. Great to know the French term for it - would never have found that in a million years :)
 
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Please don’t approach genuine structural issues with ‘fingers crossed’.
Yep, I'm really not. See also "We will be reinforcing it and spreading the load across our beams". Fingers crossed was more referring to that process working out smoothly.
 
Without seeing the full design schematic and calculations, it is probably wrong to comment. However, A 14kw solid fuel stove at 60% efficiency will charge a 300l thermal store in less than 20 minutes. The object the store is draw all the surplus energy over a long operating period
 
Without seeing the full design schematic and calculations, it is probably wrong to comment. However, A 14kw solid fuel stove at 60% efficiency will charge a 300l thermal store in less than 20 minutes. The object the store is draw all the surplus energy over a long operating period
Wow just 20 mins at 60% that is fast - I imagine that with the store of 'standing' water though not circulating? We have calculated we need 15-18kW of heating output to run the central heating rads + underfloor heating on full capacity + domestic hot water so likely to take a while longer to charge the tank when system circulating. The calcs have had to be a little imprecise as all our rads are reclaimed cast iron and so we've had to estimate their outputs rather than having factory specs to work with. Would you like to see the design schematic? I think the schematic is from our thermal study which is all written in French so I would need to re-label the drawings and translate the calcs. But I can do this pretty easily if you're keen to see it!
 
The issue you will probably have is that when you introduce renewable sources of heat into the system - as soon as the solid fuel stove is used the store will reach temperature very quickly - at which point the renewable heat sources cannot be deployed.
Renewables generally heat low and slow - Solid Fuel heat high and quick. Hence the system needs to de designed and controlled with renewable priority.

Otherwise in the coldest months of the year you will get little or no return on your investment in renewable sources
 
The issue you will probably have is that when you introduce renewable sources of heat into the system - as soon as the solid fuel stove is used the store will reach temperature very quickly - at which point the renewable heat sources cannot be deployed.
Renewables generally heat low and slow - Solid Fuel heat high and quick. Hence the system needs to de designed and controlled with renewable priority.

Otherwise in the coldest months of the year you will get little or no return on your investment in renewable sources
That's very good clear advice. We don't have a lot of room on the roof for solar so we were looking at a ground placement for a few hot water panels, as it will primarily be to feed our domestic hot in summer. The roof space will be for the PV panels which will direct to the electricity supply year round (but obviously a higher output in summer) and power the electric immersion heater in the thermal store. Won't be a lot of need for the solar hot water panels if the fire will be as effective as we expect it might be. We will have a season to assess that over winter, and can also take a bit longer to install that part if we need to (ie. over the summer too).
 
Just out of curiosity who designed this system? It seems strange that they couldn't calculate the output of radiators accurately and that the water is circulated between the tank and thermal store by gravity alone. I have been working as a plumber in France for years, everything here is unvented, no new installation uses gravity circulation, and calculating the output of old radiators is done in week 1 of plumbing school.
 
Just out of curiosity who designed this system? It seems strange that they couldn't calculate the output of radiators accurately and that the water is circulated between the tank and thermal store by gravity alone. I have been working as a plumber in France for years, everything here is unvented, no new installation uses gravity circulation, and calculating the output of old radiators is done in week 1 of plumbing school.
Well it was designed with input from a number of people but no expert heating engineer with experience in renewables. Sadly the local expert died 18 months ago and no one has replaced his skills in the area so it's been a bit hit and miss. I have tried to find people from out of area to consult but no one wants to travel for the job as they are too busy anyway. So we are stuck cobbling it together. Luckily we have a good plumber, 3 friendly engineers, an ecologically minded project manager and an electrician. But I have been trying to compile all these inputs into designing the system. It should work, but it will be muddling through to a certain extent by necessity. Due to the lack of local skills, it was either this way or not do it at all.

It was my father in law who installed and serviced central heating systems for 40 years who told us it's not possible to be very accurate with the outputs of old cast iron rads as the weight / thickness of the iron along with the design of the panels has an effect on it so you can give a range but not know exactly how they will perform.
Also we aren't planning on solely using gravity and plan to pump at points.

Can I ask you how systems with wood burners are managed safely in the event of a power outage or pump failure? I'm genuinely curious about it :)
 
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More like 150 minutes according to my back of the envelope calculation. (Assuming the thermal store mean rises by ca 65°C)
That seems a bit more like what we thought it would take. That's very good news as it will be easier to regulate and not overheat the system :) I was worried there for a bit!!
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Chuck - correct!! I appear to have used 40kw rather than 14kw!!

Still on the small though.

Thanks for picking up the mistake
Thanks for confirming the first calculation was in error. I really appreciate your pointing out the system parameters though, even if it shouldn't be as much of a problem as first indicated, as we will still have to keep it in mind when we are using it and adding the solar inputs later.

I would really appreciate it if you could tell me if your calculations to heat up are based on the 300L store "standing" or if would still heat up this quickly even if releasing heat via the 7 rads + underfloor heating system + domestic hot? We were expecting to have to have the fire on most of the day during winter but would be interested to hear if you think that will overheat the system :)
 
The calc is for the store standing - but without understanding the logic of your control system - it is likely that the store as the path of least resistance will heat up first.

I can only comment from the UK side, where we instal tri fuel ( gas, solid, solar thermal) with the solid fuel protected by a quenching system and the interface for fuel priority through a Heating Innovations interface.

If you have the space (and a source of cheap fuel for the stove) a buffer store will give you a lot more flexibility
 
I doubt it will be able to run at full capacity all the time - certainly not overnight - and, if the system is hot enough, the rads and underfloor will be letting off a great deal of heat and so we won't have the need for the 4kw of space heating the burner provides and will just run it lower.
This sentence is of concern. The majority of backboiler stoves have the water jacket inside the firebox (a few take the heat for the water from the smoke, after the combustion chamber). If yours is the majority type you may find that the fire either smoulders inefficently and creates a lot of pollution, or you run it flat out, with 'shutting down the fire' not really being a good option at all.

If you have a thermal store aka accumulator aka buffer tank instead of an unvented cylinder, this may not be a problem as you could keep the house warm from the thermal store for a considerable time after the fire has gone out. But bear in mind that just 'running it lower' when space heating is not required may not be a viable option.
 
This sentence is of concern. The majority of backboiler stoves have the water jacket inside the firebox (a few take the heat for the water from the smoke, after the combustion chamber). If yours is the majority type you may find that the fire either smoulders inefficently and creates a lot of pollution, or you run it flat out, with 'shutting down the fire' not really being a good option at all.

If you have a thermal store aka accumulator aka buffer tank instead of an unvented cylinder, this may not be a problem as you could keep the house warm from the thermal store for a considerable time after the fire has gone out. But bear in mind that just 'running it lower' when space heating is not required may not be a viable option.
Thanks for this information, it's helpful. It is the majority type of boiler so I guess then it will either be 'going full' or 'out' from what you've said. The point is to avoid pollution so we don't want inefficient combustion. We haven't had a wood boiler before so there will definitely be a learning curve in the best way to operate it!
 

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