8" brick design help

Hi! Im going to be building a 8" RMH in a greenhouse this spring. My idea is to use 8" pipe as I have a friend who has the ability to make stainless steel courgated (spiral crimped .....what they use in coal mines so super heavy duty) for free. Im planning on a 20' run in my mass which will be the sandy soil locally available (no clay ive found I can easily harvest ) and mixing that with concrete or something yet to figure out the mass entirely.

but I need help designing my j tube and heat riser. I will be using a 55 gallon drum and brick. Ive bought the RMH pdf book but idk im just still "lost" lol. anyways I am looking for some sort of plans I can easily read...for some reason words just dont work I need a picture and i havent found anything that seems to be for an 8" system.

im in zone 4 and only need to lengnthen the grow period by a bout 2 months each side of the growing cycle.

the math in this is idk im no good with math so anyhelp i can get with a 8" system design wpuld be gratly appricated!!

also is a 20' run too long? it will need to be a single tube ive seen pictures with 2 tubes in the mass.

my greenhouse is 18'w x 40'l x10'h on the north wall and 8'h on the south wall. the north wall is covered with osb and maybe just maybe ill be able to insulate it somehow before the cold of fall will be opon us but the end walls, roof and south wall are all only covered with 6mil greenhouse film...single layer.

I would like to be able to keep my temps no lower than 50f at night im sure it will be between 10f and 40f during the times i need to use the RMH to heat my plants.

I am on a very limited budget and my greenhouse is my livlyhood so a failure is not an option!

thank you!!!

Hi Sam! Relax... you have come to the right place for advice on RMHs.

There is a separate thread for RMH in greenhouses. You should look over the entire thread as there are people with similar set ups. In fact my RMH is also in a greenhouse and the pipe is buried directly in the ground.

A 20' run is not too small. Are there any bends or 20' straight out to exhaust? As long as you dont have an unusual amount of bends an 8" system will be more than enough. Maybe too much? Hopefully someone with more experience can chime in...

Basically you want you burn tunnel, riser, and exhaust pipe to have the same cross sectional area. Formulas are found online. Since you already have an 8" pipe to work with, figure out that CSA. Now make your burn tunnel and heat riser the same CSA as the pipe (It will be roughly 7"x7" square).

For example: My burn tunnel is a 7"x7" square tube that is 21" long. The heat riser is 7"x7" square tube that is 63" high.

The moderators of this forum have plans available for sale. They are at webpage Scroll down and you will see them. I have not personally used their plans but many people have and only have good things to say. Might be an option to consider if you are having issues with "the book".

Most importantly is be patient. It is worth your time to do your homework especially if you are on a budget. The forums here at Permies are a perfect place for homework. There is a search function that is helpful for specific topics. Make many mock ups of your RMH with the bricks and do test burns (safely) before cementing things into place. This helps see things first hand before going all the way.

Hi Amos i have attached a crude drawing of my idea. the run will be straight with 2 90's at the end of the mass one to turn to exit the greenhouse and one to turn the exhaust stack upwards to a run to get above the greenhouse. so i m guessing in addition to the 20' long mass, i will have 10-12' of uninsulated exhaust piping.

my drawing shows a ash pit and a way to pull the air from below not sure if this is such a good idea or not however.
im still looking for bricks as i cannot afford more than 10-15 firebricks which i want to use as much as possible in the burn tunnel. i will more than likly be using stove pipe and perlight for the heat riser.

also has anyone ever tried soaking a piece of wood in waste oil and using it to burn once everything is up to temps? I have alot of waste oil i was planning on trying to get rid of by doing this, I figure it would help quite a bit when used sparingly with other plain dry wood.

thank you

Sam: The drawing and description sound good to me. Still waiting for someone with more experience than myself to confirm that for you..... What do you plan to make your mass thermal mass out of?

You should plan to make an area to catch fly ash before the exhaust gasses enter the pipe in the mass. The ash pit that you have now will serve to catch ashes right in the fire box but anything that makes it up the chimney will end up in the mass; eventually bogging the system down.

People including myself have ash pits with secondary air feeds. They can be problematic so make sure you search around the forums enough to see what other people have done and what mistakes you can avoid so you dont waste your time and $.

Do you plan to use metal in the heat riser? If you are please reconsider this as the metal will surely burn out long before you may think. If you search you tube "web4deb" you will see his RMH videos. Look for the RHM autopsy and see exactly what happens to a metal riser.

See if you cant find some old, orange bricks maybe from a clean fill site, old chimneys, old buildings, local contractors, etc etc. In fact if you search permies for "fake fire bricks" you will happen upon a thread where different bricks are discussed and will be helpful for making a decision. We would always like to use the best materials for optimum performance but we can only do what our resources allow. That being said, spending a little more time now gathering the right materials vs whats immediately on hand will ultimately save you time and money in the future.

People have used waste oil for that specific reason. I am not familiar with the practice enough to comment beyond that. You should make that into a separate thread or use the search to find a relevant thread and leave this thread dedicated to questions regarding the build of your RMH.