thomas rubino wrote:Hi Joanie; Another big Welcome to Permies!
I have built a RMH in a green house. We are in zone 5 and it is only 12 x 20 with 16' peak. 12 x 12 pitch on the framing to shed snow rapidly.
No fire all night long. Less than 5 cord of wood. We have had it 7 years now. We love our RMH.
So lets start with your location, zone 3 is COLD ! You would want no less than an 8" RMH with a J design. A batch design might be even better.
A 8" J tube can push apx 50' horizontal pipe, but each 90 degree corner is a 5' reduction. That makes your thought of multiple turns less likely.
For your question of the minimum mass over your pipes. Well, the less mass you put on your pipes the less effect the rmh will have on your room.
For it to retain heat thru a long cold night, it needs mass. That is what the rocket stove is heating the mass. Less mass and it cools off quickly, defeating the purpose.
In our green house the ,mass is apx 14' long x 27" wide and 40" tall. Faced with red brick and filled with slabs of slate and cob. It holds heat well.
We started our babies on top. Kept their little roots warm all night.
About oval pipe. No reason it won't work provided the (isa) internal surface area is the same as an 8" round pipe.
You will find much information about batch box rmh at this site http://batchrocket.eu/en/
thomas rubino wrote:Hi Joanie;
You are correct in that water is a better heat sink than most other materials.
Here is what usually happens when water barrels are used in a severe winter conditions.
They soak up more heat than you can produce. Leaving them a little colder each day.And that much harder to heat back up.
That is not a cut and dry rule but it happens often enough that I wanted to make you aware of it.
Your question on how much mass to protect your barrels from melting. I would say 4" minimum.
Make sure you have a 4"+ base of perlite clay or straw clay under your horizontal pipes to ensure your heat is traveling up and not trying to heat the earth.