Hi! Thanks for all the great info here! I'm new. I've been studying and sketching for a couple of months. Have a design I'd like input on. Planning to build this in a small, well-insulated house on the coast of Maine in the next month. The basic idea is to use clay chimney flue tile to build a rocket mass heater with two heat storage bells that will finally exhaust into an unlined existing clay chimney. I want to use clay chimney tile because I'd like relatively few pieces to put together, I need to have a small footprint, I want to tile the outside and I have the $ to spend on new material. I've studied both "the book" and the Dragon Heater site alot.
Here's my plan:
Here is my plan:
1. Construct lower J-tube of 8”(outer diameter) clay flue tiles, cut and then cemented together with refractory cement. The feed tube will be a 13” tall flue tile. The burn tunnel will be a 6” long flue tile. The lower heat riser will be a 13” tall flue tile. Connector holes will be cut using an angle grinder in the lengths of flue tile to connect burn tunnel to feed tube on the left and lower heat riser on the the right.
See the first attachment.
2. Using castable insulative refractory (Delta Creek http://www.heatstoprefractorymortar.com/insulcastdelat.pdf, insulate the area between the feed tube and heat riser above the burn tunnel. (The rest will be insulated in step 4.)
3. Using an angle grinder cut two holes on one side of a 16” clay flue tile to slightly more than 8” square for the feed tube and heat risers to come out of.
4. Insert this lower section of the J-tube into the 16” clay flue tile and cement the feed tube and lower heat riser into place aligned with the holes in the larger flue tile.
5. Using the castable insulating refractory insulate all around the lower J-tube inside the 16” clay flue tile. This will leave about 3” of insulative refractory at the bottom and about 2.75” all the way around the rest of the J-tube, except for the left side of the feed tube which will be flush with the left side of the larger flue tile and so insulated only on the sides. A clean out door can be installed in that end.
6. Upper heat riser: Cut a hole in another 16” clay flue tile that will connect to heat storage bells (as dragon heater castle build design http://www.dragonheaters.com/castle-build-masonry-heater-exhaust-flow/. Then build the upper heat riser by cementing an 8” flue tile over the lower heat riser and placing this 16” flue tile over that and again insulate all around the heat riser with castable insulative refractory.
7. Attach ceramic insulative blanket to inside top of this 16” heat riser flue tile to protect above the area where the castable covered 8”flue tile heat riser stop.
See the second attachment
8. Using the castable, cast a connector piece to fit in the 8X12" hole from the heat riser to the first heat storage bell. Cement that into place.
9. Build the first heat storage bell which will be a 7’ tower of 8X12 clay flue tiles inside a tower of 18” clay flue tiles. Cut upper exhaust pathway into sides of both the 8”X12” and 18” flue tile towers to align with the hole from heat riser. Also cut exhaust 8X12" hole low to connect with the second heat storage bell.
10. Cast a second connector piece to connect first and second heat storage bells.
11. Build the second heat storage bell of 18” clay flue tiles. Cut both 8X12" and 8” circular hole to exhaust into two story existing, unlined brick chimney.
See the third attachment.
I think I have the critical dimensions right in that the burn tunnel is not larger in square inches across than the heat riser or exhaust system. The height of the heat riser is 30". Not sure if that is sufficient. Could easily make that taller. Thoughts?
Could the cleanout door also be a source of extra oxygen during a burn? Would it improve things to have some other way for extra air to get into the feed tube?
I think the J tube is sufficiently insulated for functioning. I'll also be placing it on a further insulated slab besides the 3" of insulation at the base of the J tube.
I'm thinking of using an 8X12" stack of flue tile INSIDE of a stack of 18" square flue tile for the first bell. Looks like the Dragon Heater uses a 13" square stack inside an 18" square stack for the first bell. Mine might slow things down a bit. Not sure if that's good or not. I don't have access to 13" chimney tile and the 12" square flue tile are more expensive than the 8x12" flue tiles. Would love some wisdom on this.
I'm not concerned about getting every last BTU stored because the 2 story brick chimney will grab some of the heat on the way out and help distribute heat upstairs that way. Not sure if an unlined brick chimney is sufficient. It is old but is in good shape as it was servicing an oil hot air system in it's previous life. Does it need to be lined or sealed from the outside to prevent gas leakage?
I would like to feel pretty confident about building this in place because we don't have a great place to test it outside and seems like it would get heavy and cumbersome by the time the J-tube is completely cemented and insulated together. Is that a foolish plan?
Thanks so much for your thoughts and wisdom and experience. BTW, because I have taken so many ideas from the Dragon Heater people I'm planning to send them a donation if this all works out. Seems like they have done a lot of work on the bell system.
BTW, I attached three drawings and I have more. I would have liked them to show up in the text but couldn't figure out how to do that.
There's one serious flaw in your design, the castable refractory should be inside the clay flue liner instead of outside. I'm sorry, your plan to use flue liner for the feed, tunnel and riser won't work. Because this isn't able to cope with the heat which is produced in a properly working rocket combustion chamber.
Thanks for the reply Peter. Before I proceeded with this idea I talked to a tech guy at the flue tile manufacturing plant. He said that the flue tile was fired to 2000 F. He also said that it would crack IF it was exposed to the high heat on the inside and room temp on the outside but that if it was insulated from the room temp that it might crack a bit but still be fine. The castable refractory is used on ovens to insulate...2 inches or so...so I figured 2.5 to 3 inches would be sufficient to keep the flue tile intact. I'm wondering if anyone has actually tried this? It is the same flue tile that is used to line chimneys so it will take a fair amount of heat from a traditional wood stove and is designed to protect brick from chimney fires. Any experience or other thoughts?
Maybe then instead of using the 8" flue tile I could cast the equivalent of that 8" flue tile by using a casting form that would burn away on the first firing but keep the outer shell the 16" clay flue tile? I remember seeing a core that was cast...
Sally Erickson wrote:Before I proceeded with this idea I talked to a tech guy at the flue tile manufacturing plant. He said that the flue tile was fired to 2000 F.
Sally, I'm sorry but a good rocket heater is able to come up to that tempearture every time it is fired. Top temps that I have seen are around 2150 F. That's well over the firing temp of the flue liner itself. In broad terms, I would reckon any fired ceramic object should be able to withstand half of its firing level as a heat shock, not more.
A question: have you built any RMH by-the-book before? This way, you'll get the feel of what such a stove is capable of and where the possible snags in the building process are. If that's not the case, it would be wise to do so first before committing yourself to an own design inside the house.
I hope you don't mind me saying this: it's better to ask just a couple of questions at the time. When your first article is overly long with a lot of options plus several deviations from the known setup there won't be a lot of people be bothered to answer.
Thanks Cindy and Peter. I did more reading last night about thermal shock and can see that's the rub! And thanks for the feedback on the culture of the forum, keeping posts short with few questions. I'll keep that in mind as I proceed. I am so jazzed about the efficiency few emissions of rmheaters, especially here in the cold north! Thanks for being patient with me. I'll report back if I have any success or failures to add to the knowledge base.
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