RMH run of exhaust to chimney questions

Hi, y'all. Haven't posted in a while, glad to see this place is still thriving. I'm working on a custom greenhouse build, a wraparound on a house, roughly 56' x 20' for the main area. I'm combining passive gain and climate battery methods with a rocket heater for the coldest nights here in Wisconsin. The running design theory is to place the RMH core at about 33' in, and run its 6" exhaust through the center of a stone retaining wall about 18" thick. Fill around the exhaust would be 3/4" gravel, and I anticipate about 2' of lift over the 30' run of the exhaust, after which it will meet an outside stovepipe chimney that rises around 8' to crest the top of the greenhouse.
My questions are; 1. Is this run too long, or too steep? 2. Will having too much rise in the stovepipe chimney cause back draft in cold weather? 3. Am I designing the retaining wall too thick to properly absorb and radiate heat out into the greenhouse? And 4. What glaringly obvious things am I missing that might cause this system to fail?
Thank you for your kind attention.

3/4 gravel is going to leave a lot of insulating air gaps. You may want a denser mix with less air spaces, something that makes a solid dense consistency.

Hey Ben;
That retaining wall needs to be entirely inside the greenhouse or be completely insulated from the outside.
If it has any outside exposure it will suck the heat right out of the mass.
If it has outside exposure, you can use an insulator (perlite, rockwool) and have an insulated stud wall between an outside wall and your mass.

My experience is that an 8" J-Tube is a better choice for northern greenhouse use.
3/4 gravel is not going to perform well at all in a greenhouse.
Solid dirt would be better.
Ask your county road crew about clay pockets on the gravel road system.
Talk to a contractor or three and ask if they have run into any clay while digging foundations.
As a heat-holding mass, it just needs to not have any insulating air pockets.

Hey, Thomas. Thanks for the reply!

thomas rubino wrote:Hey Ben;
That retaining wall needs to be entirely inside the greenhouse or be completely insulated from the outside.
If it has any outside exposure it will suck the heat right out of the mass.
If it has outside exposure, you can use an insulator (perlite, rockwool) and have an insulated stud wall between an outside wall and your mass.

The wall itself is entirely inside the greenhouse. However, the Eastern end wall is designed as a partial rock wall with a timber framed section on top going up to the ridge line. Would a few inches of rigid pink foam in between the retaining wall and the Eastern rock wall be enough of a barrier to prevent heat loss? Or would it be better to go with an insulated stick frame wall instead of of stone for the lower wall? (I could install a very convincing stone veneer to hide it)

thomas rubino wrote:My experience is that an 8" J-Tube is a better choice for northern greenhouse use.
3/4 gravel is not going to perform well at all in a greenhouse.
Solid dirt would be better.
Ask your county road crew about clay pockets on the gravel road system.
Talk to a contractor or three and ask if they have run into any clay while digging foundations.
As a heat-holding mass, it just needs to not have any insulating air pockets.

The local subsoil is a relatively sandy clay. If I'm setting the retaining wall on a 2' wide footer foundation, my thought process goes to a layer of packed gravel and sand at the base just above the cement, then a vapor barrier and a layer of pink foam, then a bed of local sandy clay to seat the 8" exhaust, which would then also be packed in the same stuff, . Do I need to worry about moisture in the soil causing corrosion of the exhaust pipe? Also, as far as the transition from buried stovepipe to the chimney goes, what kind of connection and insulation do you recommend?
I appreciate the advice, thank you very much.

Hi Ben;
A couple of inches of foam would work fine as long as the mass is isolated from the outside.
A 3.5" stud wall would insulate even more.

So, your local soil is a sandy clay?  Ahh, Cob is sand and clay...
You're overdoing your bottom layer.
No foam or moisture barrier is needed.
Several inches of sandy clay with straw mixed will work fine.   Sand/ straw is a great insulator.
When I buried my 8" pipes, I covered them with cob, and then I used supported flat rock over the pipes to keep them from slowly being crushed.
Solid rock is your best mass. You want no air pockets.
I put down a layer of cob, then a layer of rock, then covered that rock with more cob, and repeated. This is called a cob and rock lasagna.

No worries about moisture. After you heat the entire mass, there will be no moisture left. (This does take time to dry.)
An 8" J-Tube can push apx 50' of pipe. Any bend is a 5' deduction. A 6" can push 30'.
Your pipe does not need to rise; horizontal works fine.  Once you reach your chimney, the vertical pipe is not counted as part of the total length.
Nor is that last 90-degree bend counted against the length either.