Size requirement for 30x40 shop and mass dimensions

Hey I'm new here. I recently got sick of being in the city and moved my place as well as my small shop into the country. I moved the shop into a barn that was split in half and my side is 30L X 40W X 16H

I want to heat it with a rocket mass heater and want to know your opinions on my design. All I know so far is that for something that size to get nice and toasty, I need an 8" system using a 55 gallon drum and quite a bit of mass as well as something to refract the heat back into the room and move it around with the assistance of fans.

So the plan is to put it in the corner of the shop to utilize as little shop space as I possibly can. This will enable me to use an L shaped mass equal on both sides with the drum and exhaust stack in the center of the mass. The drum will be 2-3 feet from either wall.

The interior wall of the barn is going to be steel tin just like the exterior is. In the corner where the heater/drum is there will be a brick wall built up with insulation/an air gap between the interior wall and the brick. On the heater side of the brick will be a steel plate to refract the heat back towards the room.

I drew a preliminary sketch on a post it note to show you what I mean and I intend to design this in sketchup as soon as I'm ready to build it. Should I be doing anything different here?

How big would you guys make the mass for this size shop? Please keep on mind that on the opposite (south) wall I will be using solar air heaters to supplement my heating during the day. I only need the mass to hold about 24hrs worth of heat.

The style of heater will be a simple J tube with a separate air intake and a fuel magazine like the one pictured.

Hi Martin;  In my 20 x 30 poorly insulated shop . I started with an 8" J tube into a large brick bell.
I have since converted it to a 7" batch box with double barrels and a brick bell.
Here is a link to the conversion  https://permies.com/t/138779/Breaking-News-Shop-Dragon-admitted
And here is a link to the original build  https://permies.com/t/94980/Brick-Bell-Shop-Heater
A small footprint with a lot of heat!

Welcome to Permies Martin,

Being a shop, if you are not going to be using the benches for a place to sit but just looking for thermal mass, you may want to look into using bell towers instead. They have a smaller footprint for the same, if not more mass, depending on how high you want to go. Thomas has a shop heater with a tall bell and also a double barrel, so both instant and storage heat are both utilizing a lot more vertical space than horizontal.

I don't want to go too high with my mass nor do I have the bricks available to build a huge bell, also a bell takes a while to heat up. I'm more or less after fast heat instantly refracting off the heat wall I plan to build. This stuff is pretty expensive in Canada so I want to spend as little as I can on it. I just spent $100 on 60 used fire bricks. I guess that's not too bad.

I wouldn't mind having some benches to store wood on or to sit on though. Maybe I can make the benches  36" high and lined with brick to have a fairly large mass on either side of the drum. I still haven't decided on the final design.

Hi Martin;
Yes , for some unknown reason Canada has limited brick available.
Down here new firebrick are $1.50 each and home depot sells clay brick for .50.

The design rocket you show in your post is an outdated design.  It was found to not perform as well as a J tube. (all the air rushing down the feed tube keeping the fuel cool)

Before you build indoors permanently. I suggest you build both designs out in the yard and see which one performs better.
As I recall the "lid" over the feed tube overheats and eventually the entire load of wood will burst into flame , venting smoke and flames into the room.

Thanks. Good to know about that particular design not working as well. What is your opinion about a closed top and an intake over the fuel such as this design? I can see how this would solve the problems of the fuel getting too hot. It would allow a closed top while still getting air into the system like a traditional J tube.

I really want to play around with the J tube design due to its simplicity so I think I'll have to test this out for myself. I do have a power engineering background as well as aerospace. So I definitely understand basics of thermodynamics and how this is supposed to work in theory.

As for the cost of the bricks, yeah they are expensive here. It seems everything in Canada is two to three times the cost. New fire brick is $5-6 a brick. I managed to find 60 of them for $100. They are used but in very good shape. I don't think there is quite enough there to build a batch box though. Most are half size (see pic)

Hey Martin;  
Yes, I hear you about the cost and availability of things up there. Canada has so many good things going for it... but that's not one!
There is nothing wrong with a J tube. They will be around forever.
They are a good place to start your journey, of adding the title Rocket Scientist to your resume!

The new design you showed has one huge drawback.  There is not nearly enough air getting through that inlet to make a dragon roar!
The whole thing about rocket's in general, is they need every bit of air to create the super hot gasses to incinerate the particulates.  Choke it down much at all and they become no better than a box stove.  An 8" J is right around 19 cm x 19 cm  with a 25-30 cm long burn tunnel roof , Riser height is variable. Change those number's very much and you might not like the results.
Are you aware of five minute risers and how easy they are to make ?  
Are you aware that there are shipable J tube cores made with ceramic fiber board?

Martin,   dragon heaters is offering a design with a separate air inlet similar to what you are proposing but it only feeds a supply of secondary air to the fire, not the primary. As Thomas pointed out, these dragons need plenty of air. Choking them off is usually only needed when they are first lit or dying down.  
As their feed tube is made from steel which can get hot over time, this air supply helps to cool it keeping the wood touching it from igniting. As a bonus, by removing the heat from the metal, it preheats this secondary air to the fire which keeps the fire hot and burning cleanly.

I'm from Canada too and understand your firebrick woes. I recently found an ad for free bricks and got a pile of red clay and some firebricks too. Took a while to clean all the mortar off and cut the large ones into splits but to me was worth it.

EDIT: Just noticed the picture of your firebricks that some are splits and some are full width. I used a diamond blade to cut mine in half lengthwise. The reasoning for this is that you don't want to have too much mass in your firebox. Its great for abrasion in the feed tube but elsewhere can rob too much heat from the fire making it burn less efficient and cleanly - especially at startup. Much preferred to keep as much of it light with ceramic fiber board, insulated fire bricks, perlite/clay etc.
Also see Matt Walkers J tube design here: rocketstovecores.com



source

Thomas, I'm definitely aware that I could buy a core but they are $400 US. I'm in Canada so by the time it gets here plus the $50 shipping I'd be looking at $700. I may as well have fun with the fire brick and build something.

As for my design I'm definitely going to have a bigger air intake than what that picture shows.

Hi Martin;   Yes , that pesky exchange rate really changes things. I thought $450 was steep but turn it into $700 and NFW!

So if you try your second design, you'll want to match or exceed the air volume of 19 x 19 cm.
I say exceed, as I have been told recently that forcing air around hard corners lessens the volume.  
So size must be enlarged to compensate. I do not know how applicable that is.

Gerry mentioned splitting your firebricks.  Its not easy, but if you can it would double what you have to work with.

While you are experimenting, remember that an open rocket stove performs differently than one with a barrel over it.
Its a whole new ball game when the barrel goes on and also when your mass is still cold and wet

Can I ask why you are not building a "standard "design J tube?
Could it be the open feed tube feature has you concerned?
I have lived with an 8" J tube rmh in our greenhouse for the last seven years.
They can smoke back or even have flames creeping up the feed tube,  but its not very often at all.
Once a mass gets heated up, the problem becomes keeping a match lit because of the draft!

Building a rocket for the first time can be very overwhelming.
It's hard to understand all the features and how they all go together.
Sometimes the published information has been updated but not published.
We have several tricks to make things go right the first try.
Keep us posted with your plan and we will give you tips to try to make it easy for you.

I struggled with smoke back for a little while with my j tube rmh. All it took was cracking a window so it would draw enough air till the mass created the natural draw. I would not cover your feed tube because it will start to reverse burn. Give it plenty of air! I found out if the air didn't cool the feed tube enough, the fire thinks the feed tube is the riser. Maybe be able to a mesh screen like used for open fireplaces? Also I would think you might want to use the two barrel design or a really tall riser design if you are using it for instant heat since you have the height clearance. Good luck! Fyi, I had to buy most of my material new, so I feel the pain of cost, but dang, that is expensive in Canada! Wish I could just mail you the fire bricks from Michigan!