Rocket Mass Heater Plans: Annex 6" L-shaped Bench by Ernie and Erica
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John Harrison

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since Jun 10, 2016
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Recent posts by John Harrison

Working on my BB RMH and wondered what the recommended distance is from the floor channel vertical secondary air tube and the heat riser port entrance?

Many thanks for any help.
2 years ago
Thanks Glenn.

As regards the concerns with the granite slab, I have an option to use a larger, thinner slab as the bell ceiling. I could protect this with 25mm Superwool and cut out a section that would line up with top of the heat riser. I could then use, say a 10 inch square, thin or insulating firebrick to withstand the intense heat from the riser. Would that work better than the granite slab protected by a layer of Superwool?
2 years ago
Many thanks Peter for your detailed reply and figures for my application.

I'm factoring in easy access to the top of the bell so that I can monitor how the 'Superwool protected' granite slab performs and also to keep a check on the Superwool liner of the 5 minute riser. I'm planning to rely on a friction fit of the Superwool in the riser - would that work or do I need to fix it in place with some sort of adhesive or hardware fixing?

Thanks again.
2 years ago
Many thanks for your reply Thomas.

I was going to roof the top of the bell with a 3" granite slab with a one inch (25mm) layer of Superwool between the riser and the granite.
The  'chimney' for the whole system is about  11 feet (3.5 metres) high of which over 7 feet of it is insulated. I'm hoping that this will provide a good draw so I could possibly compromise on the riser/bell clearance and the heat riser length without too much affect on performance.

Thanks again for your help.
2 years ago
I'm working on the core of my outdoor Batch Box RMH/heated bench project - it's a 6" (150mm) system.

On the Batchrocket resources site it lists the heat riser length for a 6" system as 43 3/16" (1080mm). I think this may be too high for my project if I allow enough clearance for the exhaust from the top of the riser to the ceiling of the bell. I'm proposing to build a '5 minute riser' using a 25mm  'Superwool' blanket inside an 8" ducting pipe.

I have two questions :-

1/ What is the minimum clearance between the top of the riser and the ceiling of the bell?

2/ Can I shorten the length of the riser and if so, by how much?

Many thanks as always for any help or advice.
2 years ago
Thanks for the replies Mark B and Thomas.

The bench top temps of around 22-44 C you mentioned are what I was hoping for. The lower section of my indoor BB RMH Bell get to around this figure after a decent firing.

The reason I want to heat the benches is so that on cold nights we can sit out and watch the stars in some comfort..

Thanks again for the help.
2 years ago
Thanks for the reply Mark.

I was thinking that the actual temperature at the top of the benches would be fairly low, hopefully much less than 100 degrees C. If it's going to be more than that, it's probably going to be too hot to sit on...

Thanks again.
2 years ago
I'm building an outdoor 6" Batch Box RMH with two dead end benches feeding off the main bell.

I've obtained some nice limestone slabs that are 20mm (0.7 of an inch) thick. I plan to use these as the bench tops, they're nice and smooth and feel quite comfortable to sit on

My question is: will they be able to take the heat without cracking or suffering any other damage? Does anyone know what sort of bench top temperature such a system would generate?

Thanks as always for any help.
2 years ago