John Patick Conrad

Just picked up some supplies and going back at it.  Pouring the slab over the baddie backer was the plan. I'm going to try a skin coat of Portlant cement to try to get it to seal. If that doesn't work I'm going to use a heat resistant caulk and just run a bead along the outside edge. I have it set up so there is an overhang to give the bench a bit more width along with ascetic appeal. My main concern was having gasses flow along the hardy backer.

As far as clean out goes I'm making my bell (which is essentially the whole build) easily removable for inspection/cleanings as I will be able to also look into the channels from this point.

For a better idea of what I'm talking about, the build is acctually a batch box model with a pyramid bell, I have a thread started showing its progress and 3D sketch here https://permies.com/forums/posts/list/60340#515032

Thanks for the help I'll keep yah guys updated!

P.S. The pinkys gonna make it!

Has anyone used Hardie backer as a cap to their mass? i had it all cut and set to go (i was simply going to use this as a base to then our a slab on top of). and then began to get second thoughts. My mass walls are made from cinderblock filled with cinders and when trying to attach the hardie backer to the block via mortar it did not stick. i then scraped that idea and made a mold and just poured myself a concrete slab to cap it off with. However just about an hour ago trying to lift said slab, i slipped and down goes me and slab, cracked now in half and my pinky finger nearly exploded........ so im thinking i might want to go back to the hardy backer pour in place slab idea. my question is do you guys think it will be ok to have my hot gasses flowing against the hardie backer board? and dos anyone have any ideas on how i can get a secure seal between hardie backer board and cinderblock?

Glenn Herbert wrote:You can set your account preferences to receive notifications or not. I haven't been online much this weekend so it wouldn't have helped to get e-mail notices anyway

I think the tee you show would work as is; no benefit to extending the ducts to the corners of the space, or ramping the whole back of the cavity. I would even think it would be better if you cut off the bottom half of the tee and suspended the remainder from the ceiling of the cavity, keeping the bottom of the partial tee 5 or 6 inches above the floor. This would tend to funnel the flow with minimum restriction.

sounds like a plan, i couldnt help myself from starting before i got a response so this is what i just made up. but ill just keep it handy in the event that what you have suggested doesnt work.

Glenn Herbert wrote:If your chimney exhaust is coming straight up out of the bench corner, you would want its entrance to be low in the bench cavity, not out of the top of the cavity. So the flared duct can be entirely within the bench, and all you will see coming out is a straight 8" duct.

From my HVAC design experience in a past job, you will get smoother flow if the 12" end of the duct is set smoothly in a flat sheet of metal instead of sharp-edged in the cavity. The sharp edge makes air flowing into the duct bend hard and concentrate toward the center, reducing the effective aperture. The flat sheet lets the air take a gentler bend and use the full diameter of the duct entrance. The 12" vs. 8" entrance already helps with this constriction aspect, so the flat plate is not as critical as it would be with a straight 8" entrance. The smoothest possible entrance would be flared like a trumpet.

first off do people who comment on my post get a notification when i someone responds haha? im quoting because i wanted you to take a look at my above post but am not sure if you will recive a notification that i have responded....

Peter van den Berg wrote:It isn't clear to me what you mean, 24" wide and 10" high for the exhaust opening? As long as you are able to keep the opening in the lower half of the bench it should work.

This is what i came up with for the funnel. I plan on using the left over hardybacker i have from the forms of the benches. they will feed into a 8" T connector on the floor of the stove base. On that base i will also have a layer of hardybacker to make for a smoother gas flow. i was going to extend the funnel out to the start of the rear cavity. I feel the need to ask of its approval haha.

Peter van den Berg wrote:

John Patick Conrad wrote:Thank you Peter! To have the benches as part of the bell was the idea behind leaving those blocks open. I should have at least a 3" Clarence between the top of the heat rider to the peak of the pyramid as long as my calculations are correct. I want it to be a perfect pyramid so to make it taller I would need to widen the base as well, which may not be as big of a problem as I originally suspected. You do not think 3" is enough?

As far as the exhaust I'm set up with an 8" as it is now. Does that mean that I will need to create a funnel type system or maybe even a small pyramid feeding into the exhaust that say have a 10" base? Would that be sufficient enough?

No, 3" isn't enough to avoid friction areas. The recommended minimum is 12" though some people used about half of that and reported good results in their specific situation. Or otherwise you have to lower the riser a bit when there're no other possibilities.

A funnel shape for the exhaust opening is perfect. Let's say 12" wide by 5" high, leading to the 8" dia chimney stack. Where gases  need to go around a corner, give it plenty of room. A corner of 90 degrees, 150% of system size at least and 180 degrees 200% of system size, preferably.

So the cavity in the rear that leads to the exit flue is only 10" so i do not have the room for a 12" into 8" set up. do you believe that 10" would be sufficient? i could also make it span down to 8 across say 24" at 10" opening stating at the corners?

Peter van den Berg wrote:Interesting! I viewed the drawing and I think you need to make the bell somewhat higher. Don't restrict the openings to the bench, leave all three blocks open on both sides. The exhaust opening to the chimney should be situated in the lower half of the bench and remember to make that opening much larger than the chimney cross section area. When done correctly, you would be able to have a row of open blocks on three sides of the bell, that way the bench is effectively part of the main bell.

Put this because I'm not sure if you get notifications to as if I replayed but I did further up there! Thank you for the help!

Matt Walker wrote:Wow.  How fun!

I cannot tell you how many time I have watched your videos haha thank you!

Thank you Peter! To have the benches as part of the bell was the idea behind leaving those blocks open. I should have at least a 3" Clarence between the top of the heat rider to the peak of the pyramid as long as my calculations are correct. I want it to be a perfect pyramid so to make it taller I would need to widen the base as well, which may not be as big of a problem as I originally suspected. You do not think 3" is enough?

As far as the exhaust I'm set up with an 8" as it is now. Does that mean that I will need to create a funnel type system or maybe even a small pyramid feeding into the exhaust that say have a 10" base? Would that be sufficient enough?

Decided to go with the brick fire box. This is what i am talking about however, excuse my mess. As you can see there are three blocks on each side that have been laid on their side. to close two on each or not to close?