Batchbox - some newbie questions

Hey guys, I just started to learn about building RMH so that I can build one next year.
I've red everything on Peter's website and figured I'd need 200mm (8"?) version to keep my butt cozy during freezing European winters. One thing I don't get is the meaning of "Calculated Base" term? What does it mean, which part of the batchbox is it?
For example, in the 200mm version the base is 144mm which is too small to be the actual base of the whole thing. Hence I'm lost on that.

A side question I have is about air channels. Peter says in regard floor channel;

This secondary air supply duct is an alternative to the p-channel, not to be used in the same heater.

So that means there's only one air intake on a batchbox, whichever is more convenient to fin in?

Hi Lucas;  Big Welcome to Permies! And a big Welcome to the wonderful world of rocket science!

Yes, the calculated base number... Pay no attention to that unless you are a math wizz! I sure didn't. It has no effect on your bell size or box dimensions.

An 8" batch is a beast , very big. In my auto shop 20 x30 poorly insulated Peter talked me out of an 8"... said they are monsters only for a huge area.
I scaled back and built a 7" instead, with double barrels for super instant heat and a large brick bell to store that heat for many hours!

Air channels)  Peter's original batch design used a top mounted P channel for secondary air .   It worked great but was hard to replace when it burned out.
Matt Walker developed a bottom mounted secondary air . Easy easy to replace.  It works every bit as good as the P channel. So much so that Peter is now building using the lower secondary air himself!
They have a slight difference with the stub design but both work exceptionally well!
So Peters comment referred to not using a P channel AND a secondary air tube on the same stove.

So a batch can have a separate secondary air inlet OR the primary air inlet can be enlarged to feed both secondary and primary air needs.
This is accomplished by shortening the secondary and placing a "blocking plate" to direct air to the lowest point in the batch.   Which happens to be exactly where the secondary air tube is!
Both methods work. On my batch I chose to leave them separate. It is purely a matter of choice , neither is superior.

I've enclosed a couple of pictures of my batch.  In the one you can clearly see my secondary air tubes below the door and the primary intake in the door.

Hi Lukas,
Thomas answered your questions already, bar the one about the calculated base number. That base number is used for to scale the combustion core and its parts without steps as in the provided table. To determine it, multiply the chimney diameter by 72.34%, the resulting figure is the calculated base number. All the other dimensions of the core are derived from it, that's why it is called the base number to begin with.
I have chosen this method so it would work all over the planet, irrespective of whatever measurement system one is using.

Having said that, all the above information and more is already on the website, namely above and below the dimension table you've already seen. I'd recommend you read that thoroughly.
Please see http://batchrocket.eu/en/building#dimension

thomas rubino wrote:Hi Lucas;  Big Welcome to Permies! And a big Welcome to the wonderful world of rocket science!

Thank you Thomas!

thomas rubino wrote:Pay no attention to that unless you are a math wizz! I sure didn't. It has no effect on your bell size or box dimensions.

Quite the contrary, the less math I need to do the better :D Good to know that, one confusion less.

thomas rubino wrote:An 8" batch is a beast , very big.

Yeah, tru that. Good that you've pointed out. I was a bit overenthusiastic with the "always oversize" recommendation. The heat loss according to that sizing sheet would be about 5.6Kw so apparently 180mm would do as it's rated output is 6.4Kw. Had to guesstimate a bit as that cabin isn't even built yet but I don't think I'm off a lot.

thomas rubino wrote:Air channels)...So Peters comment referred to not using a P channel AND a secondary air tube on the same stove.

Ok, got it now.

thomas rubino wrote:So a batch can have a separate secondary air inlet OR the primary air inlet can be enlarged to feed both secondary and primary air needs. This is accomplished by shortening the secondary and placing a "blocking plate" to direct air to the lowest point in the batch.

 

Great info, thanks! I just need to make it as simple and foolproof as possible, when it comes to installation/building.
Which of the air intake options would be the easiest one to install?
Not that I'm trying to cut corners but I am fairly inexperienced in stuff like that. Enthusiastic, yes and not a dummy for DIY kind of work, yes, but that doesn't compensate for the lack of experience. Hence I'll have to keep it as simple as possible.


Hi Peter, thanks for your input! Good to hear from you.

Peter van den Berg wrote:Hi Lukas,
Thomas answered your questions already, bar the one about the calculated base number. That base number is used for to scale the combustion core and its parts without steps as in the provided table.

The table is fantastic for those like me who are math dummies, great to have it. If I had to do the math my self, the house would burn or I'd have a nice masonry monument in my house :D

Peter van den Berg wrote:...the chimney diameter..

Talking about chimney diameter, those chimney metal pipes I've seen selling online in my country are anywhere from 120mm to 200mm. So does the pipe's diameter has to be 180mm if the heat raiser's diameter is also 180mm or there's no connections whatsoever between any of heater's dimensions and the pipe diameter?

Peter van den Berg wrote:..above information and more is already on the website

Thank you for the website and giving it all away for free. It's a great resource and I went thru everything yesterday, before asking any questions, but there's a lot of info so I might have a gap here and there in what I've learned.

One more question. Peter says regarding the relation of batchbox and bell size;

17.5 cm  (7")   ISA 7.2 m²    (77.5 sq ft)

That would be, probably, one that would suite my needs. So ISA of 7.2m2 - that should be divided by 4(sides of the bell)? Or by 5 where the fifth surface would be the cover/"roof" of the bell?

Lukas Muller wrote:

Peter van den Berg wrote:...the chimney diameter..

Talking about chimney diameter, those chimney metal pipes I've seen selling online in my country are anywhere from 120mm to 200mm. So does the pipe's diameter has to be 180mm if the heat raiser's diameter is also 180mm or there's no connections whatsoever between any of heater's dimensions and the pipe diameter?

Chimney (pipe) diameter need to be the same or at least very close to the riser diameter. All the other dimensions are derived from the riser diameter, that's how the spreadsheet works and consequently the table as well. In case you want to use an intermediate figure and are wary about using the spreadsheet I can do the calculations for you.

Lukas Muller wrote:One more question. Peter says regarding the relation of batchbox and bell size;

17.5 cm  (7")   ISA 7.2 m²    (77.5 sq ft)

That would be, probably, one that would suite my needs. So ISA of 7.2m2 - that should be divided by 4(sides of the bell)? Or by 5 where the fifth surface would be the cover/"roof" of the bell?

In case you want to implement the heater as a true cube, the ISA should be divided by 5. For determinating ISA all the surfaces count except the floor. In practise, the heater would be higher than wide and deep because of the required minimum riser length. Don't worry about the walls of the firebox and riser, those should be insulated with superwool anyway. Don't forget the distance between riser top and bell ceiling (known as the top gap) should be equal to the riser's diameter at the very least

Peter van den Berg wrote:
Chimney (pipe) diameter need to be the same or at least very close to the riser diameter.

Great! Then it shouldn't be a problem. If I go with 180mm version there are 180mm pipes available at most of retailers.

Peter van den Berg wrote:In case you want to use an intermediate figure and are wary about using the spreadsheet I can do the calculations for you.

Thank you Peter, but as those pipes are available and as it seems 180mm version would be enough for a log cabin of 40-45M2 I think I'll be fine.(not 100% sure on m3 but my guesstimate is less than 200m3. I might build it as open space, without room ceiling, so I'm not yet sure about exact dimensions)

Listen, I like how you're giving it for free and how you're willing to help out, so I'd like to offer something in return. I'm a web designer by trade so if you'd like I could build a website for you, free of charge of course. That's if you're not happy with your current one. Or if you maybe need any help in that area, let me know.Feel free to PM me if you like!

Peter van den Berg wrote:In case you want to implement the heater as a true cube, the ISA should be divided by 5. For determinating ISA all the surfaces count except the floor. In practise, the heater would be higher than wide and deep because of the required minimum riser length. Don't worry about the walls of the firebox and riser, those should be insulated with superwool anyway. Don't forget the distance between riser top and bell ceiling (known as the top gap) should be equal to the riser's diameter at the very least

No, doesn't have to be a cube. I did some math but I assumed that it's ok to divide it by 4. Hence in this case....

17.5 cm  (7")   ISA 7.2 m²    (77.5 sq ft)

180mm version requires height of the riser of 1296mm

Hence...

7.2m2/4=1.8m2

130cm(raiser)+50cm(the top gap)=180cm of bell height

Therefor the dimension of a single wall could be 1m length x 1.8m height=1.8m2

Is this correct?

Hi Lukas;  
The only real difference with a combined air versus separate air is, the length of the secondary air tube.
As you have seen in my photo. A separate  secondary air tube is longer and protrudes outside of the box.
To build a combined air system the secondary tube would stop inside the box.
The primary air inlet size is increased to supply air for both . I believe that size would be 20% of your riser dia.
A simple light plate is placed inside the box to keep ashes from falling down to the secondary intake.
As the outside air rushes in the primary, being cooler than the air inside the box. It sinks to the lowest point. That is where the secondary air tube is waiting to receive it.
10% or so will go up the secondary directly to the port. The other 10% remains as primary air and feeds the fire from the door side.

Neither is "better" they both work well.  Your choice of door might have a bearing on which system you go with.
For his larger builds Matt Walker suggests a "Pisla" #426 door as having the correct size air inlet to supply both air systems.
As I built my own door I found it easier to leave mine separate.

Thanks for the offer Lukas, I'll think about it, seriously!
Your calculation of a bell is not correct, you are ignoring the ceiling here...
Assume the ceiling is internally 85x85 cm, that's 0.72 m². Now 7.2 m² minus 0.72 m² is 6.48 m², divided by 4 is 1.62 m². This last one divided by the assumed 0.85 m means the walls are 1.9 m high internally. Externally the bell would be much wider, deeper and higher, of course. Lift the firebox 40 cm from the floor for convenience and bob's your uncle.

In case you dont want the bell to be higher than 1.8m internally then the ceiling should be 0.89x0.89 m. Walls are then 1.8x0.89 m.

Thanks everyone for your answers!
I think I have enough info for this initial phase. Will bookmark the thread and revisit it with more questions next year, when I build the cabin and draw some plans for the heater.
Take care and all the best guys!
P.S.
Peter, if anything regarding your website, just drop me a PM here at the forum.