Post by wrekinwanderer on Oct 2, 2020 2:30:30 GMT -8
I'm just about to embark on a new bench for a new stove and would be really interested to find out what works great for you guys. I'm aiming for a beautiful sculpted clay rendered bench to be proud of!
Our previous system was a mattwalker half barrel bench, with earth packed into a wooden outer frame. This worked well, but not much space for 6" entry and exit in a dead end bench...
The new bench will be a dead end bench with a 90 degree bend half way down. I'm contemplating running a pipe down to the elbow to minimise flue gases short circuiting...
So, for the sides I mostly see brick chambers? Is this the best option? Anybody done a rammed earth one?
I have heard some put concrete paving slabs on the top? Other materials? How much earth to put on top?
Would be really interested in finding out how you guys are doing it,
Post by belgiangulch on Oct 6, 2020 3:37:50 GMT -8
Hi Mike; My mass is contained by a brick wall. It is 8" piped system. 29" tall x 24" wide x 12' long. It is filled with large flat rock and cob. Sounds like your wanting to use a half barrel setup again? Running your pipe to the elbow is a good plan. And you want to make it a sculpted cob bench?
Yes I've done a number of benches with heated backrest. I like to run gasses through the bench to the far end first, then back through a narrow bell chamber backrest. Otherwise the top of the bell closest to the fire box can get pretty hot and there is not much you can do to tweak it. The bell can be built as narrow as 4" and still achieve the desired free-stratification. This works well when using 4.5" x 9" x 2.5" firebrick shiners.
For bench materials check out what Max Edelson did a few years back at MHA gathering:
The trick is partially insulating above the flue liner closest to the firebox. If you use concrete slabs to cap you can do it initially with 1/2" ceramic felt as temporary gasket so you can remove the slabs at will for tweaking.
Post by wrekinwanderer on Nov 14, 2020 13:33:14 GMT -8
Thanks for your input Pat, and those great photos in the links. Always gives confidence when see so many different ways of doing things..
I have been cracking on, taking advantage of the family heading back to the UK for a week, so I could make an even bigger mess than usual!
At the moment am going for a completely open bell bench, capped with concrete slabs. They have rebar in them, which hopefully won't compromise them..
At the end of the bench bell, the hot gases will go up into the backrest before joiningthe exit flue. The backrest is made by a 50cms wide concrete panel resting on blocks and against a vermiculite concrete wall.
I like your idea of using a ceramic fibre blanket as a gasket while fiddling around with things....
What depth of clay mix do you tend to use above paving slabs, where you have used them, on your benches Pat?
No worries Above paving slabs i just apply brown coat (with fibre mesh) and then finish plaster. To get a desired flywheel/lag time for heat delivery, i add straw clay material under the slab, in the space between the flue liner and slab. The further from the fire box, the less straw in the mix to decrease insulative effect.
Concrete holds up okay so long as it does not experience a rapid change in temperature. I vaguely remember Karl saying 1c per 5 minutes. And of course there can be the usual stressors like part of the slab heating more than another area of it...
So, somewhere in any heater there is the point at which you can transition to cheaper materials. I would not use anything other than firebrick or castable refractory an areas above 500c. General rule of thumb with bigger (more powerful) fireboxes is the first 4 or 5 feet downstream from the firebox. Below 500c most any clay brick and flue liner work okay. I'm not sure at what point one can go with straight concrete in contact with the hot flue gasses. But i tend to make sure I am at least 8 feet down a flue run first.
Last Edit: Nov 14, 2020 18:35:28 GMT -8 by patamos
Post by wrekinwanderer on Nov 16, 2020 15:44:00 GMT -8
Wow Pat, super useful having that precision input. Thanks...
Ive been following Peter's old advice suggesting that concrete was okay in the bench... Perhaps I need to consider shedding a bit more heat from trev's core followed by a hotplate before I get down to the bench...
Or perhaps arm, as Karl was saying, the first bit of the bench to protect the concrete... Hmmmm
Ya I think a lot depends on how much heat you have shed upstream. I like to think of worst case extended firing scenarios. Case in point being the Quebec ice storms a few winters back. Durning a cold multi-day power outage people were firing their wood stoves 24-7. Quite a few houses burned down due to chimney overheating nearby combustibles.
Perhaps you could test with a concrete slab floating on CFblanket gasket. Otherwise, as mentioned prior, I think brick outer walls with flue liner core is simple enough: www.mha-net.org/docs/v8n2/wildac16f.htm Maybe even running 26guage (or thicker) metal ducting to the far end of the bench will cool things down enough.
Post by wrekinwanderer on Nov 17, 2020 13:56:50 GMT -8
Blimey, you guys do get some cold out there...
Oh, I do miss that feeling of stepping outside and your nasal hairs all freezing up. Yeah. Love that. Always miss Colorado winters at this time of year here in warm wet normandy... want to take the kids!
That's great advice though, to perhaps shed a bit more heat. I think I'll do our downer, so to speak, to the bench bell from the cooktop, in dense rather than insulative brick....
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deadstarsstillburn: The people over there recommended either a 6" batchbox or an 8" J-tube. I don't know what those are but am going to try to figure that out. What I need is a blueprint that I can scale to fit the need for my house. I have something likne 5000 square feet
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deadstarsstillburn: but I do not need to heat all of it by any means. probably only need to heat half of that, maybe less.
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deadstarsstillburn: moreover, the house has 3 storeys (large attic) so I assume if I get very efficient heating on the ground floor, that will go a long way toward heating the upstairs as well, no?
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