! 8"J-tube with Bell to heat living room and kitchen

Hello rocket people,

After spending 2 winters (in our new house) burning a lot of wood and still not being very warm i have decided to put the rocket mass heater project higher on the prio list. So i'm working on a design at the moment with the intention to build it this summer.

I have an open living room with high sloped ceilings and a loft that takes all the heat. At the moment we have 2 ceiling fans blowing the warm air down. Total volume of the salon is about 150m3, with heighest point of the ceiling almost 5m. The heater will be placed in this living room, but the bell will be attached to a divinding wall of the kitchen.  My plan is to heat both the living room and the kitchen. The kitchen has a way lower ceiling (it is below the loft), so that helps. Ideally the kitchen would get so toasty warm that we can open the door to our bedroom and bathroom and also heat those a little.

In the picture you have the layout (not the entire living room or kitchen depicted in the floor surface, i just drew up the corner of interest), with the heater in the middle of the livingroom, the wooden frame is the opening to the kitchen, with above the black beam which is where the opening of the loft starts (the ceiling fans are in front of this opening).  The big brick column is not the end of the living room, but it is the end of the seating area. Before the brick column is a desk/workspace. I have chimey on the roof which is about 1,5m height of bricks and 2,5 meters of black steel pipe. I want to have the exit pipe of the bell more or less straight underneath it giving me about 9m of straight pipe. We live in a mountain house with 80cm thick stone walls, so we have quite some mass to warm up

2nd picture is the current setup. We have a huge open fire place that will only warm you because of all the tree trunks you have to haul into the house. So we put a simple stove/fire box in front of it. The entire open fire place including the chimey up will be torn down. We do not really like it, and it serves very little purpose and takes up a lot of space. Somewhere in that chimey is the tubing, which is 25cm diameter.

After some iterations I think a 8" J tube with a bell is easiest and gives us freedom to arrange the seating area with sofas etc, instead of having fixed benches. Although a batchbox looks nice as well, i would like to start with a more easy build. Also I like that the J tube has a higher woodfeed, so my 1 year old is less tempted to have a look at/meddle with the fire.

I will document my design and progress here.

I have some questions so far, if anybody would be so kind to chime in, that would be much appreciated:
- Having the cold air exhaust and heat riser close together: Would that cause any issues regarding draft? Only thing that I can think of is that the exit would be warmed immediatly upon firing and therefore draft would increase.
- I heard Glenn talk about his system which has already a very strong draft with a 8" system and a 6" exit. I'm thinking that in my case, because of my straight chimey, I might also have good draft with a 6" pipe. I have space for a 8" also, so reducing would only save me some money. I wouldn't want to get a too strong draft either, that is why i'm thinking of 6".  
- I'm taking the ISA of a 6" batch rocket system, I believe Glenn's build had the same and that worked nicely. Does anybody have any experience with 8" J tubes and corresponding ISA? I could not find a lot of details for this in the forums.
- With the current measurement of my bell the ISA is a bit to large: 5.77m2. I would like to insulate the bell at least on one side (which is an outer wall of 80cm stone, that feels a bit like an infinite heat sink), and maybe even on the top. Could I disregard these insulated surfaces in the calculation of the ISA, because they will radiate a lot less heat?

Thanks so far for reading. I'm really looking forward to this build, I have read about the these heaters and everything makes sense on paper. So i'm really curious to see what they can do, they sound too good to be true

I have made a more detailed layout of the core. I'm shooting for 35cm, 70cm and 105cm for the woodfeed, burn tunnel and riser. The layout I have now is pretty close (37cm, 70cm, 114cm)

The surface area of the burn tunnel is a bit less than needed. The height is 15cm, the width 18cm, which is 270cm2 instead of 314cm2 for the 8" system. Would this be a big problem? I have ordered the book of the Wisner's (assuming they will discus this topic), but Amazon says it is only going to be here in May...... i do not want to wait with the design untill then. I could add another layer of bricks at the bottom, but that would make the dimensions of the whole core a bit bigger, which would become tricky with the space I have inside the bell.

The measurements of the bell are 165x155x50cms. The purple areas I want to insulate to reduce heat transfer. Heat going out the top will not really warm the room and the heat against the outside wall will just heat up a very thick stone wall (~infinite heat sink). The ISA of the bell will be ~5,4m2 (not counting the insulated areas) which is a bit over the recommend ISA for a 8" J-tube (5,3m2). The ISA might change a little depending on the thickness of the brick I will be able to find here. Would adding a column of bricks for extra mass inside the bell change anything about the ISA calculation? Since the the column would not radiate heat outside of the bell.

I have also added a clean out in the left lower corner, right underneath the exhaust. Although I have read many times on these forums that a clean-out is not really needed with a bell system, because blockage is very unlikely. Eventhough ash will build up somewhere in the system, so over many many years there might be a big layer of ash on the bottom of the bell right?

I'm thinking about also adding a 45degree downwards T just above the bell on the exhaust pipe for chimney sweeping. But i feel that that should never be needed in this type of system, so i'm not sure i want to spend money on it.

Anybody has any thougths on overall weight? With the regular bricks i'm finding here the weight of the bell is about 600 kg. The core about 100 kgs. The weight would be a thermal mass, but the prolonged heat would be more because of the hot air in the bell. Are there any general guidelines about this? Or is the weight not really import, but the material (and how fast it radiates the heat) is?

Any inputs are welcome

Hi Mark;
Welcome to the wonderful world of rocket science.
I have not had time to study your plans in detail, but here are my initial observations.
I had to start by converting your numbers to inches, since we weren't taught the metric system (like the rest of the world).
Your feed tube is 37cm (14 1/2"). This should be taller. 41 cm (16") is the recommended depth. You have not mentioned the dimensions; they should be 20cm x 20cm
Your burn tunnel is shown to be 15 cm tall (6"). The width would need to be 26cm (10")   to achieve the correct proportions. The recommended size would be 20cm x 20cm
It is more common to have your burn tunnel taller than wider, as specified, to allow for ash buildup.
The burn tunnel length you show as 70cm (27.5"), provided the feed tube and riser are not larger than 20cm across, is perfect.
The riser at 114cm (45") tall is perfect.

The exhaust chimney, if located inside the bell, must be insulated.
Adding a cleanout door is always a good idea, but be aware that all of mine will leak condensation when the system is cold. (Black drips)






 

Hey Mark;
I took time to read more, and here are a few more observations.
I found the width of your burn tunnel.
This is not going to work properly at all. It must be the correct area; it cannot be smaller, or your feed tube will become a chimney.
Your riser is perfect at 114cm (45"), but it can be made shorter, 100cm (39")
Any wall or roof that is insulated is not counted towards the ISA.
Correct an 8" J-Tube uses the ISA numbers for a 6" Batchbox. Perhaps your bell can be enlarged
The exhaust chimney should be 8" inside the bell, with a funnel-shaped bottom.
I suggest using 8" indoors until you reach the ceiling and switch to 6" to exit the building.
Skip the extra 45 pipe; you will not be chimney sweeping anytime soon, if ever.

Hey Mark,
It's great to have another Rocket enthusiast in the neighborhood! We live in the Valle de Lecrín, maybe 45 min from you.
I'm happy to drop by on Saturday (if the roads are clear) and bring the Wiesner book, I'll be in Cenes de la Vega. Or you could pick it up there and we can chat over a coffee.

Trying to answer some of your questions:

-6" or 8" exhaust: if you have the space going for 8" has no downsides. Draft can always be controlled with a brick on the feed tube. Will you replace the 250mm exhaust or try to fit the 200 mm (8") exhaust in there?

-I've also read in more than one post that the 8" J-tube is comparable to the 6" BBR. So orienting on the ISA for the 6" BBR should be good. Worst case (too much mass, too low exhaust temp) you could always raise the exhaust pipe, making the ISA smaller.

-Definitely insulate the bell against the outer wall. Maybe even build a double wall filled with insultaion. I like to use Arlita (expanded clay), but you might need 15-20 cm as it's not very insulative. That would probably bring your ISA down already.
Insulated surface as well as the floor of the bell don't count towards ISA.

-Core design: You could have another row (course) of flat laying fire brick as the first course to raise the height of the burn tunnel. Which size firebrick are you using/planning with? Do you have a height restriction with your bell?

-I would add both the T and the cleanout. They're easy to integrate now and in case anything should not work as planned they are ways to look inside, measure, clean, etc. The T could also help you prime the chimney on bad days or with a very cold mass.