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Discuss Wood stove above with thermal store below - can this work safely? in the Renewable Energy area at PlumbersForums.net

OP
K

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!
 
B

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
 
OP
K

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).
 
M

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.
 
OP
K

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 :)
 
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C

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)
 
B

Brambles

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

Still on the small though.

Thanks for picking up the mistake
 
OP
K

KylieS

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!!
Post automatically merged:

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 :)
 
B

Brambles

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
 
R

Ric2013

Plumber
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.
 
OP
K

KylieS

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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