Rocket Mass Heater Questions

Hi all! I'm looking to build a rocket mass heater in my 1300 sq ft well-insulated house in western Massachusetts. I have a great deal of thermal mass already existing around a fireplace which is in the center of the house. Ideally I'd like to incorporate much of this thermal mass but I have questions about retrofitting it into a bell. The ledge around the fireplace is approximately 11 inches tall, 16 inches wide, and 8 ft long. Would this work as a bell given that the entrance piping would not be much higher than the exit piping? In theory I could expand the ledge to make a better proportioned bell but I'd love to avoid the expense if possible.

The second question I have is about Dragon Heaters. I've read some permies threads that appear to speak well of their six inch Rocket Masonry Heater Castle Build Kit but I'd like to get some more reviews. I'm especially considering their kits because I want avoid a steel drum for esthetic reasons really. Does anyone have personal experience with their kits or have opinions about their specs?

Thank you very much!  

Hi Gary;  Welcome to Permies!
Let me start with your second question.  I have not heard any bad reports on the Dragon heaters. They appear to be all they claim to be.  
Next let me say that you absolutely do not have to use a barrel with a RMH.  All brick bells are becoming very popular for that very reason.
Between your fireplace itself and the ledger I think you have plenty of room to sculpt a bell.
Your big decision, is whether to build a traditional J tube rocket (like a dragon heater) Or build a batchbox design RMH...??? Decisions Decisions...    

Garry, that's the kink of project i love, and may be i can be of some help.

>For the type of rocket, go batch box.

In your case it's better.

The first thing i will ask.

How is your fireplace and  it's surrounding built?

Can you make a sketch of the outside and inside with dilmensions?


How good are masonry skills? Welding? Fabrication/metalworking?

Thank you for your responses. Initially I was thinking of purchasing the Dragon Heater and putting it along side the vertical chimney mass and retrofitting the ledge as a bell, but this isn't really a retrofit of the existing mass and seems to have a number of downsides. I've been reading permies threads talking about RMH inserts into existing chimney spaces, whether J tubes or batchbox. The fireplace itself has a relatively large volume, 3 ft long, 2 ft wide, and 28" tall. Are those dimensions feasible for a RMH insert of some sort that would take advantage of the existing thermal mass? If so, do you have any pointers or resources you would point me toward?

Also, it sounds like the batchbox is the second generation of RMH that are much improved, but I really don't have welding or fabrication experience at all. I've assumed this really disqualifies me for for the time being from creating a batchbox system. Is that your perspective?

Thank you very much for your help.

Gary Loomis wrote:Thank you for your responses. Initially I was thinking of purchasing the Dragon Heater and putting it along side the vertical chimney mass and retrofitting the ledge as a bell, but this isn't really a retrofit of the existing mass and seems to have a number of downsides. I've been reading permies threads talking about RMH inserts into existing chimney spaces, whether J tubes or batchbox. The fireplace itself has a relatively large volume, 3 ft long, 2 ft wide, and 28" tall. Are those dimensions feasible for a RMH insert of some sort that would take advantage of the existing thermal mass? If so, do you have any pointers or resources you would point me toward?

Also, it sounds like the batchbox is the second generation of RMH that are much improved, but I really don't have welding or fabrication experience at all. I've assumed this really disqualifies me for for the time being from creating a batchbox system. Is that your perspective?

Thank you very much for your help.

Well, with a little dedication, anybody can do a batch rocket. I know i did.

Batch rocket "insert" is most certainly feasible.

I don't have any pointers, per se.

What i would like to kn,ow, is the construction and size of the mass around the chimney. That could be used as a bell, and if it's all empty with no wood anywhere.   It could may be, be used as a bell.

If it's just plastrerboard with facing, you would have to rebuild a bell in this location.

http://batchrocket.eu/en/

https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven

Gary,

For what it’s worth, you’ve come to the right place and you’re talking to the right people for a project like this. I’m not a pro by any means, but I will say that if you have some building skills and time to do it, building your own custom heater with the advice of the crew here will be an extremely educational and rewarding process.

I’ll keep following.

Best of luck and may the force be with you.

Peter Martin 🏔🙏🏼🏔

Hi Gary;
Yes , I think a batch box would be best for you. Although I haven't built one yet myself , they really are not that technical.  Size specifications are the most important and need to be followed exactly, after that is just a bell and or mass.
As Max asked, is your chimney all stone/ mortar? Or is it faced with wood behind ?   I'm guessing its solid masonry.
The welding and or fabrication skills required are minimal. Really the only metal part is the incoming air feed.  I'm going to guess that no more than $50 could get one built at a local fab shop... really it should be $20.00 there's not much to it.
Have you been to Peter Bergs batchbox site?  Peter is a master builder who innovated and developed the batchbox design,  he freely shares all his results and is happy to offer guidance during a build.
Here is a link to his site  http://batchrocket.eu/en/
Another good site is our friend Kirk Mobert (Donkey) here is his site http://donkey32.proboards.com/  They go into much details on their builds and again are happy to offer advice.
Max is our staff expert on your kind of build. I'm sure he will be of great assistance with your design. The rest of us will be here offering advice as we can.
You can do this. Imagine using 1/2 or less of the wood you burn now!  No matter what style RMH you choose you will be VERY happy with the results.
Welcome to the wonderful world of Rocket science... Its a rare experience!

"Happy Rocket scientists eagerly waiting to help you".....looks more like:

"Hey Pa!....is that one of em wanna be permie types lookin' for some free advice on how to make one of dose der rocket ships? I think we should riddle him full of holes, skin em, then eat em up!....just hope he's not as tough as the last one.....Spit"

Why Gerry; That's practically profiling! :)
Those happy scientists are eagerly awaiting our questions... so they can run off to the celebration party and help prepare the spirits...  

Thanks again for your responses. I've been looking into the batchbox RMH more and it sounds great. It's really encouraging that the p-channel is the only metal fabrication needed. So my entire chimney structure is masonry, and as has been suggested, I would like to retrofit it to be the bell. About half the chimney structure (the fireplace in the picture) is connected to one chimney flue and the back half has two other flues running through, connected to what used to be the furnace and to a fireplace in the basement. I'm uploading another picture with the rough dimensions of the fireplace currently.

So my first question about the batchbox RMH: It sounds like the limiting factor for me might be the available space for the bell. Am I understanding correctly that a 6" batchbox RMH requires 57 sq ft Internal Surface Area for the bell? If so, I need to make some retrofitting adjustments to get to 57 sq ft ISA. Do you think it'd be better to use the fire place space as one large bell or use a smaller bell in the fireplace space and a second bell through the ledge around the chimney structure? The top of the fireplace narrows as it approaches 4 ft so I'd have to work on expanding that if it were to become one single bell.

Thanks again for all your time and wisdom!

Hi Gary;
I'll give you some answers as best I can.
A 6" BB does not require 57' isa but can utilize that much space. So a bell can be smaller if need be.
I would suggest two bells. A taller one on the ledge and then the firebox as a smaller one.
Your existing chimney is 6" ? I suspect Max will suggest adding a pipe inside your firebox to lower the chimney down near floor level.
Have you done any brick laying before?  I had not when I built my bell...  as long as you can do straight and level ... its easy... famous last words right ?  :)

Max lives in the French Alps and generally responds while the rest of us are asleep. I'm sure he will have more ideas.

Thanks for your suggestions. I've realized I may have gotten overly excited about this project and considered biting off more than I can chew. I figure that a less efficient, completed RMH is always better than an incomplete, more efficient one. So I'm thinking of going back to the 6" J-tube RMH designed by Dragon Heaters for now with the intention of possibly upgrading after a couple seasons of experience using it.

With that in mind, Thomas I like your idea of having the Dragon Heater heat riser and first bell situated on the ledge of my fireplace and then hopefully building a smaller bell within the fireplace. I spoke briefly with a woman from Dragon Heaters about the dimensions but ended up a bit confused. She mentioned the benefits of taller bells to better stratify the gases, which makes sense. But I can only build a small bell in the fireplace without major structural changes to the chimney. The secondary bell could be approximately 27" tall, 36" long, and 24" wide. I could have the exhaust pipe exit this bell at floor level. Is this too short to be of any real use as a secondary bell? Thanks again for all your help!

Hey Gary;
Yes the dragon heater can sit on the ledge and yes the original fireplace could be a second bell.
Not important that its lower. As the dragon heater folks talked about, the heat will rise to the highest point (top of dragon heater) then as more hot hot gasses arrive the cooler gasses sink.they then can enter your fireplace and again rise to the top as the currently hottest gasses. As more arrives the cooler air sinks and finds your exit pipe.

I assume your fireplace has a rectangular jog as it leaves the open space?  You will need to fab up a way to seal that and bring a pipe down towards the floor of the fireplace.
How close to the floor should it be ???  well its trial and error.   To high and your loosing heat, to low and it won't want to draft.  

It is a big job to build a RMH, especially a batchbox .  December is not a good time to start...
If you need it soon, then a dragon heater will fit the bill. You could always sell it to someone later when you build your batchbox!