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Good day all,
Yes I am here following the youtube videos, where you talk about this stuff... all the time

My flatmate and I have built a rocket mass heater, starting protypes outside, following 'the book' (Ionto and Leselie), bu found that we couldnt get enough draw with thier dimensions, so we copied as best we could the proportions of the portable RMH from the RMH/Cycling you tube video.
Unfortunatly after getting a prototype that worked well outside, when inside it smokes back way to much, has not enough draw, and as a consequence produces lots of ash (and we assume results in incomplete combustion)
It starts well but gets worse as it heats up.

What we have done:
6" inner tube
8" outer
Filled with clay slip+polite.
Cut similar to that on the video, just without the false floor, so straight onto the bricks
5.4" square
with the small barrel as per youtube vid.
The horizontal section is one brick on wide + outer heatriser thickness. (Pretty much exactly the proportions of v1.2)
We built it off the floor with bricks supporting some galv. sheets, then brick base on top.
The exhaust can escape the barrel on all sides, leading to an enlarged opening (under the barrel) on the exhaust pipe side.
The exhaust pipe rises at a slight incline, and at this stage goes straight outside.
The fire is constructed of one layer of bricks sealed with clay. no extra material around the outside.
Placing a brick half over the enterance reduces smoking, but does not stop it.
The barrel gets nice and hot. I believe there is a 1.5" gap between the heat riser and the barrel top.
I remade the exhaust gas -> pipe bit to try and increase flow... it seems unlikely that there is a bottle neck there.
We have made sure that the horizontal burn chamber is isolate from the barrel exit.
Following the 'troubleshooting' section of 'the book' the only things I can think of are that the wood enterance is too wide or too hgh.
I wonder also if my burn chamber is too wide generally.

Anyway, thougths, comments, suggestions would be greatly appreciated! The house really stinks now... but its warm!


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Two major things are possible, (as well as many smaller ones). First is the connection of the barrel to the exhaust pipe. It needs to be LARGE!!! Look at the 2nd post here http://donkey32.proboards.com/index.cgi?board=experiment&action=display&thread=124

The other is the inlet. It needs to be small enough to allow the burning gases to hang around long enough to combust completely and generate the heat.

However, as it worked outside, I suppose it might be that the barrel was getting cooled more, and was pumping the gases better.

You have made a valuable point where you say you get loads of ash with the poor combustion.

(My apologies to permies.com for so many references to donkey32. It's just that I can find the info easier there as the forum is split into sections).

Posts: 3615
Location: Kansas Zone 6a
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Try opening a window on the upwind side of the flat. You might not be drafting enough to overcome a negative pressure inside. It is really easy for a horizontal exhaust to scoop air IN, adding a T style raincap can help.
Jasper Aorangi
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Thanks guys,
I have been going through the post on donkey32, and am having trouble relating this infromation to what I have. I think I am missing some info to piece it togethere. If you understand how to apply it, could you illustrate.
I will give my dimensions here: (I work in mm... so converting everything to ")

Opening size (We have to have a brick half over it...) 5.1x4.7
Burn chamber cross section: 5.1x5.1
Burn chamber length: 5.1
Heat riser inner diam: 6
Heat riser outer diam: 10
Heat riser height: 30.5d
Barrel Diam: 15
Top of heat riser to the barrel:2

And as I said, the exhaust gasses cn escape from the bottom of the barrel, all around, except for where the burn chhamber goes throuhg, then into a larger chamber where the exhaust pipe is.
We are concerned also that there are gaps between the burn chamber and the exhaust, so will refill/cover all this stuff. Also wondering if we should make it so the exhaust gasses leave the barrel at only one place right above the exhaust pipe.

.... and we will open a window on the draughty side
With the T cap, does this need to be horizontal, and perpendicular to the wind?


Jasper Aorangi
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We resealed all around the heat riser to ensure that there was no possibility of pre heat riser gasses getting into the exhaust and bypassing the heatrise/barrerl. Also opened a window, and it works much better. Really good draw to start with but tapers off as it heats up.
The wood does not burn down completely, and needed to be constantly cleared from the burn horizontal burn chamber.
So again, still not complete combustion?
We still need to 1/3 cover the enterance with a brick.

So improving, but not quite there I think.
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