How To Calculate Sizing of a Rocket Mass Heater

Hi Guys,

I hope you are doing fine and you have Merry Christmas.
I really would appreciate help if anyone can.

My Name is Flo and I am going to build a big Greenhouse for my private use the next couple of month (hopefully).

I am trying to figure out the perfect Rocket Mass Heater for heating my greenhouse. The Greenhouse has 100' by 28' and is 8' tall with open ceiling with 4/12 pitch.
This gives a total room, which has to be heated, of 851.4m³. The Greenhouse runs all year around and has some tropical plants inside.
We are getting -20 C in Winter usually.  It should have minimum +20 C inside. This gives us 40 Celcius in difference.
The Greenhouse will be build out of ICF Blocks as foundation with 4ft Frostwall and it will have Standard 2" by 6" stick frame between the doors.
The doors are insulated Bi-Fold Doors with Hollow Alluminum Frame and Thempered Glass. The roof and Gable ends will be covered with 4 layer, 3 Hollow spaces, total of 20mm Thick Polycarbonate Sheets with a very good insulation results.

I am looking a long time for some Calculations already and how they get done. Is there always one fix 8" Plan and one 6" Plan which has to be used or how does it work?
If anyone could give me some heads up with some Math Formulas, that I can calculate the dimentions from the beginning, I would be very happy and Thankful.

Please enjoy some couple of days off and some time with your loved ones.

Greetings Flo

Hi Florian,

Using a basic formula from the batchrocket.eu website:

Volume * Temperature Difference * Building Type Factor we get:

850 * 40 * 1.4 (a house insulated with 4 cm of polystyrene) = 47.6 kW
Let's multiply it by the factor of 0.7 and w get 33 kW. It's a lot.
Two firings per day in 250 mm batch box will be an equivalent of 8.6 kW.
In theory you would need at least 3 of them or you could go into unknown territory of building some monster with 400 mm diameter:

(33kW/8.6kW)^(1/3)=1.565
1.565*250mm=391mm

It would be cheaper to build one big one than three smaller ones.

Thank you very much for your fast response.

I definitely know it will be a big stove.

If I burn more often a day do I can stay with a smaller Diameter or doesn't make it any difference?

If I go with a bigger diameter:
Is the length of the J tube Always the same or does it change by the size of the riser and pipe diameter?

What would be the length of the cob I have to build?
Also what is the maximum exhaust length chimney height etc?

I know lots of questions but I would like to understand the ways of building and changing things.

Thank you beforehand.

Florian Bertele wrote:If I burn more often a day do I can stay with a smaller Diameter or doesn't make it any difference?

It already assumes that you fire 2 times per day and the factor of 0.7 assumes that when it's really cold you fire more than that - I would say 4 times per day.

Florian Bertele wrote:Is the length of the J tube Always the same or does it change by the size of the riser and pipe diameter?

Every dimensions scales up proportionally. The quick calculation I have presented was for the batch-box not the J-tube.
In batch-box it's easier to calculate the power output. You load a specific volume of wood that is determined by the firebox size, the wood has specific caloric value, it burns for some time and the power can be calculated after taking into account losses.

Florian Bertele wrote:What would be the length of the cob I have to build?

For batch-box of this size, the ISA (area of heat receiving walls) would be around 38 m2. So in theory the heater could be positioned in the middle of the greenhouse. It would have a bell and heating benches going symmetrically left and right and then returning to the chimney that would be also located in the center.

Florian Bertele wrote:Also what is the maximum exhaust length chimney height etc?

To answer this question I would have to dig into some formulas on my other laptop that I don't have by me right now.

Thanks again Sir.

I definitely understand. Is there a formula book which includes those formulas?

Maybe I can purchase it somewhere?

I am sorry for only short answers but I am answering via mobile phone.

Florian,

I would recommend this website:
https://donkey32.proboards.com/

And this small one as a quick reference:
https://batchrocket.eu/en/

Those a very good websites, thank you I will study them and will update my solutions when I am done.

Thank you for all your help

Hi Florian;
A 100' x 28' greenhouse is rather large.
A J-tube RMH would be a full-time job as they (8") need feeding every 45 -60 minutes. I do not recommend one, you would not be happy.
A Batchbox stove will retain coals well past 2 hours and produce significant heat.
With low expected temps at or below zero and tropical plants growing, you may consider having two batchboxes burning, one near each end.
Are you using lighting to supplement for short cloudy winter days?

We have a much smaller insulated greenhouse here in northern Montana.  20' x 12' with a 12 x12 pitch snow shedding roof.
Our 6" batchbox keeps it toasty warm with no fire all night.
This morning it is 17F outdoors and it is 51F indoors, no wood was added since 10 pm last night.
We have a Guatamolen coffee "tree" bush that we have been growing for 4-5 years,
It thrives all summer out on our deck, we move it into the greenhouse before temps start dropping.
It manages to survive each winter but with a lack of sun and an abundance of clouds, it is a sad-looking tree until the sun starts rising higher.

Hi Thomas,

Yes thats very true, the greenhouse is very big. It has lots of Water Resavoir, this will take care of temperature fluctuations.
I said one thing to myself couple of years ago which will change thoughts in building a lot: "If you want to build something, double the size of it, because space is always too less."

Once the sun hits the greenhouse we have to be more carefull getting the temperature low.

I was just wondering of J Tubes always have the same sizes and Diameters (6" and 8"), since I never saw someone changing something except the Length of the Cob or Chimney height.

The plan is to build one very big RMH (it might be one of the biggest ones) right in the middle of the greenhouse or a little offset, then running all the Pipes and cob into the other direction I also thought about installing a Fan, with temperatur sensor, into the Chimney to produce more draft to get the cob and Exhaust longer but I assume this will bring more trouble in Cleaning.

The snow on the roof will also help as insulation during cold seasons. (if it will stay there, otherwise it will get blown away by nature because we have lots of wind here)

I have to take several calculations to see which way I go.
There will be power in the greenhouse but no sepparat Heat source other than the RMH.