Figuring out length of tube / size of bell for a 6" system

[Bram]

Hi there,

I would like to start off thanking all of the billiant innovation going on in these forums. Greatly inspiring!

I am currently in the design phase of a RMH. I'm kindof doubting between a 6" Batchbox (sidewinder or regular) and a Walker Riserless Core. If I go for the Batchbox it would most likely not have a regular barrel but a bell instead (topped off with a metal plate). Folks here don't like the idea of a barrel in the room, which is one more reason to go for a WRC. I could in theory go for an 8" system but because of the increased size I would rather not do that.

From what I have read, this kindof system should put out enough heat to heat a small 55m2 home in rural Greece during winter. There are doubts in the family about whether this is true, especially for the whole house to be comfortable. And to be reasonable, I have heard people say it won't heat a whole house, mainly just the room that it is in. It might be enlarged at some point to 100m2 so I'm aiming for a slightly oversized animal to compensate for later expansions.
I have been asked if we cannot do a kind of underfloor heating as well, which I have tried to incorporate into the design. We don't have too much space so a smaller bench and underfloor heating folds in nicely with our space issue.

Now the problem I'm running into is calculating the Internal Surface Area of this system (ISA from now on). I have read on batchrocket.eu that a 6" Batchbox will heat about 5.3m2 of ISA. I did some calculations and that comes down to about 11 meters of 6" ducts. A 15 cm circle x 100 cm length gives a surface area of 0.47 so basically half a square meter of ISA.  5.3 / 0.47 = 11.2.

This should just about fit what I had planned for this heater. There would be one pipe going north from the heater, then a 90 degree turn traveling east, make 180 degree turn to go west and then down into the ground. So three lengths of pipe in the bench, one at about 140cm and two at 160ish. The big red line represents the pipe underground and it would be insulated on the bottom with perlite. Perhaps all around in the area where it passes under the bench, dunno.

I measure the underfloor pipe to be somewhere around 650/670cm. Perhaps a third of the pipe is lying in the perlite. My thinking here is that the heat is of no use to us under the pipe and it will help retain some heat for the eventual flue. I have shown with pink the two options for flues. The other option would be to have a slightly bigger bench with the flue exiting close to the burner. In that case we could use a fan to drive convected heat into the underfloor part.

Couple of questions:

1) Am I overlooking something here?
2) I've heard people talk about turns creating drag but have no numbers...
3) Am I going too large scale for this heater?
4) Perhaps its better to go for a bell in the bench? As it would eliminate the zigzagging and thus some drag.
 
Cheers for your support!
Looking forward to actually getting to work!

Hugs,
Bram

[Bram]

ps. I got the model for the Walker Riserless Core from one of the threads here at the forums. Cheers for that to whomever posted it! I have probably read three books worth of threads now so I don't remember who posted it...

[Bram]

I just realized that if you were to build a bell over the 6" Batchbox it would probably top off at about 140cm. Thats using a distance of 30cm above riser and assuming a metal plate on top of the bell... I had thought it needed to be much higher.

I do still like the idea of being able to cook on a WRC, which would be slightly harder at 140cm...

[peterberg]

Some people here coupled a batch box to a piped bench. But: you need to realize every elbow will add friction. Even with a J-tube you need to distract 4' or about 1.22 m for every 90 degree elbow. Your planned bell also takes up heat so this ought to be included in the ISA calculation. In my view, I wouldn't use the combination of a piped bench and a batch box core of whatever construction. You'd do better with a bell bench, probably even a blind ended one in my opinion. Very low friction and no convoluted gas path. You could use Matt's half barrel system to build that bench, saves you on brickwork.

Donkey did a batch box rocket and two-sided blind benches last year... I've seen that one on facebook, a whole lot of pictures how he built it.
It's on the facebook page of the Sundog School for Natural Building.
www.facebook.com/pg/Sun-Dog-School-of-Natural-Building-184675818301910/photos/?tab=album&album_id=800693863366766

[matthewwalker]

Where is the chimney? I wouldn't do it if you don't have the chimney next to the core and a bypass. Particularly in a mild climate. It will be a bummer.

[Bram]

Cheers for the responses!

So if that's true Peter, the underfloor flue is also out? That big red line? I could make the whole bench one big masonary bell and then channel it into the underfloor flue perhaps..

And Matthew, I envisioned two spots for a possible chimney both marked in pink. The preffered method would be in the bedroom on the right, because then the mass is spread out through the house. Perhaps an even more slimmed down version is required without the double loop in the wide part of the bench.

Or simply give up on the idea of an underfloor heated by the RMH and use convection instead.

[satamax]

What do you mean by underfloor flue?

[matthewwalker]

Okay, I see now. I believe you could make either core/system concept work well if you skip the underfloor extension, keep the heated area to that seating area, and use the chimney location next to the core with a bypass. I believe you can use either flue runs or a bell for a bench that size. My big home heater is a batch into long flues and works great once primed with the bypass.

[Bram]

Fair enough, do you reckon that would heat the whole house though? 55m2 or 592 sq ft.

Especially my in-laws are scared that it won't heat the whole house and just the one room. Which to be fair, I've heard people talk about. "RMHs heat people, not houses." is a line that gets dropped pretty frequently.
But to counter that: "RMHs are super efficient woodstoves that heat your home on one eighth of the wood." Or E&E's The Annex: 6" RMH PDF reads: "Performance: Heated 900 SF cottage (with poor insulation, long wall surfaces, and no solar gain) in maritime climate (Portland, OR)."

The suggestion of my in-laws was a convection fireplace with tubes going under the floor but I am pretty certain that works until the fire goes out. I'm thinking now to make a slightly larger bench, probably a bell using barrels. And to use bricks to make a kind of zigzag pattern under the bench that I can take air from and pump throughout the house. Room temp air goes in, hot air comes out. Greeks have high opinions on what is the right temperature for comfort

[Bram]

Oh and Satamax, I was hoping to have the gasses run under the floor. Its path is marked out in red in the above pictures. But it looks like I have to give that idea up

Cheers for the quick responses guys, really worthwhile.

[satamax]

Well, in my opinion, running underfloor is possible. But only with an extremely good chimney. And lots of draft.

[matthewwalker]

Berry, my neighbor did a reasonable forced air system by cobbing in dryer exhaust tubing behind his barrel and bench. He then used a little computer fan to push through and sent the warm air into his bedroom. He seemed pleased with it.

As for heating the whole space, there's really only one way to find out. So many variables, climate, build, weather, season, fuel, schedule, expectations, and so on. I think a barrel will easily heat that space in most climates. A cook stove, maybe. It would keep everything warm once it was all there, but if you wake up cool and want quick heat, it won't do that.

[Bram]

Thanks for the responses anyone!
I have decided to go for the option of pumping the air around using small 12v fans and forgetting about running the exhaust under the floor. I have posted a new thread to highlight the sketchup model I made for the riserless core.

[Jura]

Feb 1, 2017 6:04:41 GMT -8 Bram said:

Thanks for the responses anyone!
I have decided to go for the option of pumping the air around using small 12v fans and forgetting about running the exhaust under the floor. I have posted a new thread to highlight the sketchup model I made for the riserless core.

Hi

 Any further developments to share ? 

[Bram]

Yes actually. I built the core in a friends house to practice how to do it and because they needed a good heater. The bench is not finished on that one.
Here's the pictures of that build:

www.facebook.com/profile.php?id=100011126505062&sk=photos&collection_token=100011126505062%3A2305272732%3A69&set=a.398419023872311.1073741835.100011126505062&type=3

Then we built the basis for the bench in our future home. I'll have to make an album with a selection of pictures for that. Hmmm. I'll get on that now.