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Peter Smith
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Location: NEPA
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I really want to build a rmh in my basement, but I see these pictures of glowing barrels. I am a volunteer firefighter, and wonder how hot these really are, and how much clearance is recommended top and sides.
 
Jerry McIntire
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Location: Oak savannah - Viroqua, Wisconsin - zone 4 - 34"/yr
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I've heard 4" from a combustible wall via Ernie and Erica. Remember the heat riser barrel is usually surrounded by considerable thickness of insulation, then another barrel/chamber, then the downdraft area, then masonry/cob. With 4" of clearance, that would be 12" or more from the hottest point to the wall. In a basement, you could leave even more space.
Jerry
 
Peter Smith
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Location: NEPA
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Maybe I'm confused then. Isn't there an exposed barrel directly above the heat riser? Isn't it very hot? I guess the glowing barrels I saw were pocket rockets, but I imagine the barrel in a mass heater is also crazy hot.
 
Jerry McIntire
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Here is one built fairly recently by Ernie and Erica. What you see is the outer barrel. It's much cooler than the inner barrel which is surrounded by high R-value insulation-- except for the open top. So the top of the outer barrel is the hottest part, and as you can see it's quite a distance from the wall. The wall is both protected and offering more thermal mass/heat storage with its brick behind the stove.


Here is a diagram showing the innards.



I hope that helps.
 
Peter Smith
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Location: NEPA
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Yes it does. So what materiel is commonly used to insulate the heat riser? Is it triple wall SS chimney pipe?
 
allen lumley
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Peter Smith, Todays Heat Riser is usually ether fire brick wrapped in rock wool, or a blend of Perlie and Clay Slip, the better perlite being a non-greenhouse grade,and
both heavier and slightly less absorbent than the agricultural/greenhouse/florist grade. Clay slip is simply a smooth mixture of clay and water, blended to a thick cake
batter like consistency ! Thick enough to coat a spoon, and when a gob is dropped back into its pail or bowl leaving a pronounced ring behind ! A. L.
 
Jerry McIntire
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Location: Oak savannah - Viroqua, Wisconsin - zone 4 - 34"/yr
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Peter, you'll find lots more answers in the forum or in the rocket mass heater book by Evans and Jackson, or in Ernie and Erica's new DVD (link at bottom of page). There are some youtube videos of rocket mass heaters, and Paul has done podcasts on them which you'll find here on permies.

Stainless triple wall is more expensive than most RMH builders want to use. Metal barrels, thick wall steel pipe (reused), or square risers of firebrick are all used. Insulation can be rock wool, kiln brick, perlite, perlite + clayslip, vermiculite, pumice (in order thinnest to thickest material needed).
 
Peter Smith
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Location: NEPA
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Thanks guys, so the heat riser does not need to be round? Fire brick would imply square. Most pictures and drawings I have seen are round.
 
allen lumley
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Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Peter Smith : Yes it can be square if you use soft fire brick ( weight ~28-30~ ounces ) Like Jerry suggested goto>rocketstoves.com to download Your PDF Copy
$15 of Evans' and Jackson's great book 'Rocket Mass Heaters' with ~100,000~ built word wide, most were built from 'The Book', and 95% of all the fist time builds
(that worked) were built following 'The Book !

This will allow you to speak the same language with your fellow members and have pictures of physical objects to look at when a new friend is carefully explaining why
his/her way of looking at a 3 diminutional object is different than the one in your head ! -not right or wrong, different ! For the good of the craft ! big AL
 
Jerry McIntire
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Location: Oak savannah - Viroqua, Wisconsin - zone 4 - 34"/yr
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Peter, here is the most complete page Paul Wheaton has put together on RMH's:
http://www.richsoil.com/rocket-stove-mass-heater.jsp
 
Erica Wisner
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Peter Smith wrote:Thanks guys, so the heat riser does not need to be round? Fire brick would imply square. Most pictures and drawings I have seen are round.


Hi Peter,
I'd definitely go with clearances like you would use for a woodstove around the barrel. Top third of the barrel can reach as hot as 600-800 degrees, depending on fuels. Normal operation is maybe 400-600 at the top edge.
So we use clearances like for a non-certified woodstove: about 36" to combustibles, which can be reduced to 18" or 12" with good heat shielding.

The lower part of the barrel tends to stay more like 200-300 F or lower. Below that, there is usually at least 4 to 5" of masonry around it (5" would be to code for masonry heaters), you can do the 4" air gap from the outside of the masonry in that area if you need to.

Glowing barrels in home heaters are rare - I think the most I've seen on purpose-tweaked stoves (for cooking) was a small red spot dead center on the top surface.

Heat risers can be round or square, as long as they are
- the right internal cross-sectional area (same as all other channels e.g. exit chimney)
- well insulated (1" rock wool, 2" perlite, 4" vermiculite, or similar)
- leave enough space between riser outside and barrel interior for good flow (minimum same CSA as other channels, can be 2 or 3 times larger. 1.5" to 2" is a practical minimum so it doesn't choke on fly ash.)

Brick risers are more durable, but sometimes it's worth it to use a cast refractory or cast-in-place riser to save space, so you can use a smaller barrel and reduce the footprint in the room.

Yours,
Erica W
 
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