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

[KylieS]

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.

 

[Brambles]

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

 

[KylieS]

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!

 

[Brambles]

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

 

[KylieS]

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).

 

[mattfrance]

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.

 

[KylieS]

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

 

[Chuck]

A 14kw solid fuel stove at 60% efficiency will charge a 300l thermal store in less than 20 minutes.

More like 150 minutes according to my back of the envelope calculation. (Assuming the thermal store mean rises by ca 65°C)

 

[Brambles]

Chuck - correct!! I appear to have used 40kw rather than 14kw!!

Still on the small though.

Thanks for picking up the mistake