Oval shaped pipe for rocket mass heater?

Hello all! I'm a newbie here, happy to be talking to this knowledgeable community!

Now to my question. I would like to build a RMH in our 21x36 greenhouse. We live in zone 3. I ordered the book "Rocket Mass Heaters, the Builder's Guide" but it is not here yet and I would like to work on some other things in the greenhouse in the meantime but need to know a little more about the Rmh specs before I am able. We have plastic barrels along most of the back wall. I plan to fill in the small bit of space behind them with broken brick and earth for insolation... In front of the barrels we are putting a trellis with woven wire and posts set in front of the barrels. I was thinking I could have more woven wire attached to the backside of the post and fill that space between the wire on the back of the post and the barrels with the earthen mass and run the pipe for the Rmh in front of it. I could wind it back and forth probably 4times before going out through the roof. I do not know much about rmh as of yet. I've only watched a handful of videos... I got this idea and want to know if it is feasible and if I can use oval pipe in order to save space. Will the oval pipe draw the same? It is available in different sizes. I believe all of it is only a few inches wide. The sizes change in the length of the oval. Also, what is the minimum amount of mass you need on either side of the pipe in order for it to be safe along side plastic barrels, wood posts?  I hope this makes sense! Thank you so much to all who try and help!!!

Hi Joanie and welcome to permies!    

I must say that I don't have any experience with greenhouses so I'll leave that for others to help you out with.
I do know that the Builders Guide you ordered is an excellent resource to help you build and understand RMH's but only have some information on greenhouses starting on page 55.
The Wisner's also have much more detailed plans and information on greenhouse builds that you may want to consider getting : Greenhouse-Rocket-Mass-Heater-Plans

Also, if you do a search on this forum for "rocket mass heater greenhouses" you should get lots of information from others who have shared their experiences with their builds.

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;  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,

Thank you so much for the suggestions! They are very helpful and answer multiple questions. In regards to the minimum earth mass around the pipe, I was thinking the barrels would be a part of the mass. I would have the bench and then run the pipe in front of the barrels within a mass. My concern is how much earth mass do I need to have between the pipe and my posts that support my trellis. And the same for the barrels. I was thinking that my bottom barrel and maybe my top row as well would be enclosed in the earth mass. It is my understanding that water is the best natural material for storing heat. I believe the rating I saw for it was 64 as compared to bricks that were at 24. So the barrels would be part of my mass. My question is the same with the posts, how much earth mass do I need to have between the barrels and the pipe? All of it will be within my earthen mass. I don't want my plastic barrels to melt or be compromised from too much heat. Would 3-4" of earth mass seperating my pipe from the plastic barrel be enough? It seems like people only have a few inches between their pipe and wall so I'm thinking a few inches would be ok. I could be very wrong!

P.S. Thank you for the nice welcome!😁

Garry Parent-

Thank you for the suggestions and links. I will certainly check them out!!😁

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.

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.

Thomas Rubino
Thank you again for sharing your wisdom! Please give me your opinion on the following.

We want a RMH in the green house to extend our season. It is not reasonable for us to be out there everyday through the winter. Besides extending the season I just think that RMH are awesome!😁 That being said do you think it would be best to leave the rmh separate from the barrels? I don't have to run the pipe in front of them. Last time we were out there working I was wondering if it would be best to leave more of the barrels exposed so that they could still be heated by the sun when we aren't using the rmh. Maybe just have pipe pass in front if them once, enclosed in the earthmass and leaving the top third of the barrel as well as the top row of barrels exposed.

It does get too cold here for the barrels to work properly in the dead of winter. However I do think they are still beneficial in early spring and late fall/early winter. And they still help regulate heat a but in the summer too.

I know just enough too get myself in trouble i think. That book seems to be taking forever to get here!

Your rmh looks awesome! Do you have more pic posted of it anywhere? I'd really enjoy seeing more.

One more thing, am I replying to responses on the post properly? I thought there would be a reply option for responses so that whoever you are responding to would know that you commented.

Thank you again!!

Hi Joanie;  Thank you for the compliment!  There are literally hundreds of my RMH photo's right here at Permies.
Both my original build and several rebuilds, then there is my shop stove build as an 8" J tube into a brick bell and new posts converted it into a 7" batch box style.
To view more than you probably want , click on my name, that takes you to my profile, then click on my posts... there is a lot!

So on to your greenhouse and impending rmh build.
So it sounds like your still thinking a low cob covered mass?  For your location even though you are not planning on year round operation,I highly recommend a large thick ,rock / cob mass similar to mine.
You really need heat holding material or your greenhouse will cool to unhappy temperatures. Remember a J tube burns 45-60 minutes before reloading. If you really want it to rocket you will want to be there shaking chunks of wood, adding extra as soon as room appears. That is how you get a rocket to go to mars. Loading it and walking away will generally only get you to the moon!
Myself, I would empty those barrels for winter and store them elsewhere. Come spring when the sun comes back I would fill and return inside.
You could even set them on your mass , to keep them out of the way.

Yes, you are properly responding. I receive an email when anyone responds to a post that I also have responded too.
Your book is coming it will get there! The mail is way slow with the virus about.
Read bunches right here at Permies. If you want more info try here at our friend Donkeys site )https://donkey32.proboards.com/
If you are interested in the batch box then try this site (it is very technical) http://batchrocket.eu/en/  That is the site of Master builder Peter Burg. He has fine tuned the batch rockets to amazing outputs.