Wood stove above with thermal store below - can this work safely?

[KylieS]

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

 

[Chuck]

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...

 

[KylieS]

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.

 

[mattfrance]

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.

 

[Brambles]

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.

 

[KylieS]

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

 

[Ben-gee]

Please don’t approach genuine structural issues with ‘fingers crossed’.