Dear Pinhead,
We calculated that the benches (pre-cap) are 5 to 5.5" higher than the outlet to the chimney (maybe not relevant but 8" if you include the thick cap we used), therefore we do have some desired stratification, about 5" worth, so don't jump the gun on us! We had a pretty good team. Im a newb but we had 3 experienced builders, and I must say that we are not having a major fundamental misunderstanding, (that sounds catastrophic my goodness) rather we just have maybe slightly "half choked" benches - or to use Peter's word "guenched". Or we could say there is some dead space in the benches to deal with.
In truth, we wanted to build this "BBBBB" (big badass batch box bell)
as close to the existing chimney as possible, and as a result we are up on the hearth 7.5" and include a firebrick base and we are building on 10" worth of base total. I thought this would be safer and a better height to build on, and try to utilize the existing masonry in some way! also we did not continue the "base" all the way under the benches as we did not see it necessary.
In a way, this was more of a "chimney conversion" to the more advanced BBB RMH - batch box bell rocket mass heater. I like to think maybe I'm not the only one with a rather wasteful open fireplace of the modern ignorant kind sitting in my place haha, maybe its just me...?
But if anyone chooses to convert their wasteful smoky fireplaces I hope this can help them. All of that masonry is just sitting in place, it would be a crazy endeavor to take it all apart, but maybe the next person wants to cut off his mantle
AND hearth? It might take that much, we chose to cut the mantle alone, and the hearth might require a jack hammer or rotary hammer (and of course precise angle grinders with diamond blades) or a bit of multiple tools to cleanly bust apart, but it can be done. Hard dusty work, dust masks recommended !
In the end, might as well have a safe strong exit flu for your long lasting solid brick BBB RMH, I certainly prefer it no matter what, over the wasteful fire place !
- - -
Also, Dear everyone,
I am uploading new photos shortly, but Brian and I had a chance to do some math today, so above and below are the result and I hope this is helpful in some way.
Here's what I believe to be the relevant math numbers for this situation we have been discussing:
The hearth is 7.5" tall and there is a sideways firebrick laid flat for base: 2.5", that makes our base height 10"
the benches are 20" tall from the cement slab up. There is a 10" differential.
the gap between the base firebrick (at 10") and the top of the bench which is 20", a simple: 10"
a gap for exit gases is (i believe) a half firebrick on end which is 4.5". (Joseph crafted this section and so he may know more accurately the outlet size, but I believe a half brick was used.)
Therefore I believe we have a gap of 5"-5.5" for heat to move in stratification.
This is what Brian and I determined and we are convinced that although we have gentle warmth in the bench - some stratification is happening.
Now, here's some speculation: to try to encourage cool gases to flow back out of the bell perhaps if we backfill with perlite in the lower 10" then maybe cold air won't get "trapped" in there? Its a weird consideration. Another thought I had was bricking in the lower 10" to prevent any dead space where air might stagnate. Lastly, could there possibly be any benefit to their being a "slight" dip below exit level? Like what if it was only 1" vs 10" that it is now, is it possible that it would encourage cold air to "roll out and up" the chimney?
Here's some stats:
After 4 full batch burns today, the benches got up to 105 degrees F or so, at least near the bell (the first seat), at the further end as low as 85-75 F. Still a warm seat and certainly better than cold! My wife Joy was happy with it and the height it came out to sit on. If we went a little higher it could be a little awkward but it wouldn't be a super hard tweak, we just might consider adding a step at the bottom like Peter suggested.
We are so fortunate we finished this bell heater right before the first snow here (today) ! I feel incredibly grateful for this info and our east coast team that rocked this out - and we will do our best to tweak this thing as needed to make it even better.
The best part for us is that its burning clean (love that) !!! and we have warmth!
We were pretty satisfied with the heat it was giving off the bell, still with its only firebrick skin on the bell, was over 300 degrees F and was "too hot to touch". The benches did at least hover warm... 80-100. Thats what 5" of stratification will do here in our situation and environment. To give further example the high today where we are was 35 F, we had rain, slush, snow. We were warm inside! and the bell operated at the above given numbers after 3-4 batches.
For your info, our barn house is practically un-insulated, except for one wall really in the downstairs, so we were hanging out around the bell and feeling enough warmth to feel comfortable. Once we close off a bit of the barn and insulate then we will be too toasty I think!
It took 2 good batch burns and when the top of the bell read 250 F with the laser, it was sufficient for the bypass to be closed and the chimney was drawing smooth and roaring making a great rocket sound through the sidewinder style.
I have asked Brian's help with sharing our math, but the build is slightly different than what was on paper also, as is natural, and so we will try to take measurements tomorrow and post our actuals of the overall build. That I think will be the most help for others who might try to learn from this one. Thanks folks.
alex