Building My First Rocket Mass Heater (Tommy's Tea Dome)

Glenn Herbert wrote:Cross posting... Good for insulating under the RMH. Making an actual air separation so air can carry heat from all sides of the mass to the interior would be best, but insulation is good. Aluminum foil is effective as a radiant barrier, but that only works when exposed to an air layer. Touching other material, it does absolutely nothing.

I'm curious what prevents you from exiting the roof at the peak with your chimney. You will need to go to the same height wherever you locate it, and anywhere else makes more pipe exposed to the cold exterior, and needing to be supported against wind.

Yeah, I understand that ideally, my chimney should exit right out of the highest point of the dome. However, there will be a ceiling fan mounted directly in the center of the dome which will hang down a few feet. There would be no way for me to make a chimney exit in that location. I'm curious if you think the draft would be sufficient if I located the chimney roughly where the silver stove pipe is in this picture:


Which would look like this on the exterior:


Although I understand that I would probably have to lengthen the chimney higher than my drawing shows. But aesthetically speaking, I am hoping that a short-ish chimney will work. This dome is dug 2' down in the ground too if that matters any. Choosing where the chimney exits is proving to be quite difficult, can anyone else provide input? Thank you

By code, the chimney is supposed to be at least 2' higher than any roof/structure within 10' and at least 3' above the point of exit. This is not just bureaucratic mumbo-jumbo, there are good reasons for it. With the pointed dome, you could probably be safe with a bit less than the 2' rule as it will not generate the kind of turbulence and downdrafts that a gable would. You would need to make the chimney secure against wind, which means either making it very strong and connected to the dome structure, or braced to the roof above.

I doubt you need to have the fan so high that there could be no space next to it for the stovepipe. A small ceiling fan should work fine for a space the size of your dome. I would hang the fan with 8" to a foot of clearance from the dome sides, or possibly slightly off center, and run the stovepipe up to near the top. Exiting there would give a less obtrusive look overall.

If heat in the summer is a serious concern, I might make a removable section of stovepipe so hot air could escape the top of the dome.

Sonny,   Glenn's comments about the chimney exit are all tried and true methods which would provide the most trouble free installation however, as your building is round and streamlined to the wind, perhaps downdraft may not be an issue for you? There are so many factors that make for a good draft. You could skimp on a few and still be OK but best to not make them unless you really want to accept the possible consequences (aka have a backup plan).
Sorry I couldn't add any assurance for you, but perhaps someone else may have more experience with a structure like yours.

Glenn Herbert wrote:By code, the chimney is supposed to be at least 2' higher than any roof/structure within 10' and at least 3' above the point of exit. This is not just bureaucratic mumbo-jumbo, there are good reasons for it. With the pointed dome, you could probably be safe with a bit less than the 2' rule as it will not generate the kind of turbulence and downdrafts that a gable would. You would need to make the chimney secure against wind, which means either making it very strong and connected to the dome structure, or braced to the roof above.

I doubt you need to have the fan so high that there could be no space next to it for the stovepipe. A small ceiling fan should work fine for a space the size of your dome. I would hang the fan with 8" to a foot of clearance from the dome sides, or possibly slightly off center, and run the stovepipe up to near the top. Exiting there would give a less obtrusive look overall.

If heat in the summer is a serious concern, I might make a removable section of stovepipe so hot air could escape the top of the dome.

Glenn, I see what you are saying and not only will the ceiling fan be an obstacle but there are many other variables which would make exiting through the top of the dome a nightmare. It is sort of out of the question, unfortunately, and I am not disagreeing with you in the slightest bit. However, I have read Ianto's book which showcases many possible chimney designs, even a horizontal exit for example. I know that a lot of these chimney designs are far from ideal, but from what I have read they can work. With that being said, with my current idea of exiting next to the barrel, could my design work? I'm imagining on cold days, a cold start would be difficult and I might have to prime the system. Although once the system heats up, in theory, wouldn't my draft be sufficient enough? Could my barrel radiantly heat my chimney enough to provide a sufficient thermosiphon? I'm sure nobody can definitively answer this question, but I'd love to hear speculation only relating to the design that I've roughly outlined above.

Gerry Parent wrote:Sonny,   Glenn's comments about the chimney exit are all tried and true methods which would provide the most trouble free installation however, as your building is round and streamlined to the wind, perhaps downdraft may not be an issue for you? There are so many factors that make for a good draft. You could skimp on a few and still be OK but best to not make them unless you really want to accept the possible consequences (aka have a backup plan).
Sorry I couldn't add any assurance for you, but perhaps someone else may have more experience with a structure like yours.

Gerry, this is similar to how I am feeling too and I am not skimping on anything other than chimney design, as far as I know. I'm hoping that this will work, otherwise, I may have to really rethink this RMH's design. Worst-case scenario, I think that potentially it is possible for me to exit where Glenn is saying but it would be a huge challenge and I am not opposed to it at all especially if others feel that I would be screwing myself by not routing my chimney this way. As of right now, I haven't started this build and I won't until I've established where to install the chimney. Once again, I'd love some more input on this. Thank you all, I've learned many things from you all so far.

If you feel it would be too problematic to go to the top of the dome, what about going halfway up or so, not interfering with the fan? Even that would make a significant difference in draft and the ease of supporting the chimney above the roof.

Horizontal exhausts can work, but only where you can ensure that the outlet will always be on the lee side of the building. If the wind can come from more than one direction in the heating season, you would be asking for occasional smokeback with the space filling with smoke.

My temporary exhaust is an insulated stovepipe on a flat 3-story face, ending three feet below the roof edge. It is too risky for me to want to go up and install the last section of chimney without someone standing by and good weather. 99% of the heating season, it draws fine, but there are always one or two days in spring when there is a steady or gusty east wind, and I can't have a fire without strong risk of filling the house with smoke.

You might do fine with a short chimney low on the side of the dome, or you might have issues with draft and smokeback... nobody can say for sure without detailed knowledge of the climate and microclimate. A taller chimney will make it more likely that there will be no issues.

Sonny your building is so cool, would fit right in on Tatooine

Aaron Tusmith wrote:Sonny your building is so cool, would fit right in on Tatooine

Thank you, and yeah I get that a lot! But I have to agree.

Update 2/15/19 Basic Floor Plan & Duct Work Progress:

A few days ago, I drew up some rough plans for my rocket mass heater. This should explain my design, though only to a certain extent, but more importantly, it will help me to keep this build in line and I will probably update it as I go.



In terms of actual progress, I have decided to route the chimney in the original location I had planned but I tried to keep it inside as high as possible. I understand that will affect my draft, but I have decided that it is one of my only feasible options. I will, however, be making my chimney taller than I had previously said on the exterior, and this should hopefully allow for a good draft. We will find out in the coming months.

Once again, here is a picture of where I intended to exit with the chimney as well as the core of the system:



I successfully carved a hole into the structure to exit the chimney with and installed the interior portion of it:



I continued on with the ducting and have stopped at this point:



I will soon build the core of the system so that I can connect the duct work to it and begin building the thermal bench for this heater.

Note: Before the final assembly, I will be insulating with perlite underneath the ducting, the core of the system, and most likely against the wall.

Its been a month since my last update but things are now going great as of the past few days. I was able to accomplish a lot on the rocket mass heater and I even got to light it for the first time!


I took everyone's advice on insulating underneath the system to protect the floor and also to maximize the system's ability to heat the room.

I made a border out of clay and sand for the footprint of the whole system (including the shape of the thermal mass bench) and then added 2" of clay stabilized perlite and then added a layer of aluminum foil over the perlite.



Then I did a quick mock-up of the core of the system, to make sure the corbel of the dome wouldn't hit the barrel from the newly added height of 2" (from the perlite).



After that, I built the system for real and I also constructed a brick manifold in an octagonal shape and then connected the ducting to it. I then sealed the barrel with a clay-sand mixture.



After that, I installed the chimney on the exterior. It looks a little rough right now but I will repaint the surrounding areas as soon as the plaster fully cures:




Then it was finally time to test the system, indoors!

It must have taken me 15 minutes to get the system running normally, mainly due to my inexperience lighting rocket stoves but the draft was working pretty much right off the bat, even with the system being damp and cold. Once I got the system running nice and hot with some bigger pieces of wood, I noticed it was producing a lot of smoke but upon further observation, it was mostly water vapor coming from the wet mortar in the system. I ended up running the system for about 4 hours and it was working perfectly! I honestly cannot remember the last time where something I've built worked exactly as it was intended to, and what a nice feeling that is. I owe it to all of the wonderful resources on Permies as well as of Erica and Ernie Wisner. Towards the end of my burn session, I noticed it wasn't producing much of anything out of the chimney. I climbed up on the dome to feel the temperature of the exhaust, and I was easily able to put my hand on the exhaust without burning my hand. Now it is time to make this thing beautiful to match the rest of the dome. I will update in the coming weeks/month. Thanks for reading.

Here is another update, although not a whole lot has been done since the previous one. I started giving the rocket mass heater some layers of cob as well as stuffing in some bricks that I had no use for:




I also properly mounted the chimney on the exterior and sealed the exit:




Pretty much all that is left to do on the rocket mass heater is shape the thermal bench and we are good to go. I have lit it another few times since the last post and it works amazingly and I can't wait to use it next winter.

Daniel Ray wrote:Wow, great build. Love Calearth and what they do to teach.

I'm going to reiterate Thomas and mention the importance of insulating under the burn chamber. My first build was similar and it eventually burnt away the cement beneath the burn chamber.

Dillon, i think you were referring to Thomas' mention of the stratification chamber rather than the double barrel initial bell technique show in my photo. Thomas prefers using bricks for the transition area.

The half barrel stratification method is using barrels cut in half lengthwise and used for the second bell of the system as demonstrated here http://donkey32.proboards.com/thread/716


Sonny, if you do stick with the piped bench, remember to calculate the total distance with +5 feet for every 90 turn. If you use a lot of elbows you'll need to figure how many 90s you end up doing before the chimney which looks like it will be in a good spot near the barrel. You should clear the top of the structure by several feet.

If you are unsure about cutting your bricks for the octagon riser, you could do a square riser with corner pieces for the first half of the riser which is successful at giving my stove the "rams horns effect". I've included another photo of this.


Good luck! and post some photos.

Daniel, when you've added the corner pieces to your riser, do you take those into consideration in the CSA or ignore the CSA?
Do you feel it's a good addition?
Why do you only do it for half the riser?
Thanks
Dan