I built a 6 inch version of Ernie Wisner's RMH. I installed one inch foam and cement board on the bottom and sides of the site. I have about 15 feet of ducting through a sand/clay/portland cement mass before venting up and out of the roof. I have a temperature gauge at the top of the barrel (not on top). I've fired it about ten days, so far. I get up to 500 degrees at the top of the barrel, and I can keep my hand on the vertical exhaust ducting, above the mass, so the heat is staying in the system, and not getting ducted to the outside. But, I can still see my breathe a foot away from the barrel, and the mass does not feel warm. I have to work to get the temp to 500, using finely split, really dry wood. It gets over 300 pretty easily, and seems to generally run around 400. I'm heating a 20x30 shop. The RMH is centered in the room, along one wall (the barrel is just over 18 inches away from the wall. I only have a couple inches of mass on top of the horizontal ducting, so far. I have materials for a couple more layers which would equate to a couple more inches of mass.
Anyway, I'm not getting the room as warm as I expected, and like I said the mass isn't getting warm, like it is talked about in the videos I have watched. Lots of people have said their mass bench is too hot to sit on directly. But, the heat is all getting stored somewhere. I'm considering getting a thermal fan to put on top if the barrel to move the heated air around the shop more. Does it take a number of burns to get the mass heated up? Should the barrel be getting hotter? I built the riser out of thin firebrick and clay slip. I did not wrap the riser in ceramic cloth or anything like that. Any ideas why I am not getting the heat I expected?
Hi Damon; Welcome to Permies!
Your OK , It takes Weeks of burning to dry out and heat up a mass . Fire bricks take time to heat in the beginning and it seems to take forever to get up temp.
Your riser would work better if it had kayo wool wrapped around it.
500 F at the lip of the barrel is not bad. How large is the top gap between the riser and barrel ?
With only 15' of duct it shouldn't take too long to dry. However, each load of cob and rock you place on top has loads of water that starts the drying process over again.
As I told another poster with similar issues. Keep burning and keep cobbing. It will not work as expected until dry and it can't finish drying until cobbing is done.
I will say that in a 20 x 30 shop I would have recommended an 8" over a 6"
Not all who wander are lost... J.R.R. Tolkien
posted 5 months ago
OK, thanks! I don't remember my exact thought process to arrive at the decision to go with a six inch system. I think I was at the hardware store and saw the six inch ducting, and went with that. I watched a lot of videos and bought Ernie and Erica's book, but never got too caught up in the math.
How long ago did you add the cob? It is common for systems to have difficulty heating up for the first while, as all the energy is going to drying out the mass. A 6" J-tube for a 20 x 30 shop might be on the skimpy side, too, depending on your climate. If you are seeing your breath inside near the heater, I would guess it's pretty cold outside. If you're in Canada, I would venture to say your RMH might not be sized large enough for the conditions.
I'm just going to parrot what the others said about the mass. My 8" system took a good number of burns before it dried out the mass and got nice and toasty, temps on the barrel seem good so don't lose heart yet and just dry that thing out.
Welcome to Permies. First I will add yet another vote to the "you might want to have picked 8" instead of 6" thought. Next can you tell me what the dimensions are on your feed opening. I can see it is square and an ideal 6" system will be 28.3 si in CSA (cross sectional area). This means that your feed opening would be the square root of 28.3si or 5.3". I am thinking from the photos that you are a good bit larger than this on both the feed tube and the great riser, like 6"x6" or perhaps 7-3/4" x7-3/4"?
Erica and Ernie use full bricks in the feed tube and burn tunnel and half bricks in the riser. The writer up is a little cryptic in the book on doing building the riser, but they tell you to line up they inside corners. It looks like you may have lined up the outside corners. They show the bricks overhanging around the outside, but yours line up perfectly and that would only work of you cut all your bricks on a tile saw. The ratios from the photo do not appear that the bricks have been cut. If I am wrong please forgive me, but if I am right you have an unbalanced build.
posted 5 months ago
I live in southern Wisconsin. This week has been warm (highs in the 40s). It is usually in the 30s this time of year, and can get below zero in Jan and Feb.
I didn't measure, but the riser is within a couple inches of the barrel lid. The bricks just happened to work out well. I used regular bricks to create the base for the barrel, and it came flush with the top of the firebricks that formed the burn chamber. I added a little cob to create a seal, so the barrel is just a little higher than the riser.
I used Vogelzang fire bricks, which are 4.25 x 9 x 1.25. The riser is 6.5 x 6.5. The feed tube is slightly smaller. I don't remember why it happened, but I did cut bout 1.5 inches off the bricks that go across the firebox on the two courses that make the feed chamber, just so they didn't stick out. One of the images I attached to this reply shows the stick-out before I cut the brick down.
I think I've mixed six batches of cob so far, maybe seven. The ducting in the mass runs from right to left, makes a u-turn, and comes back past the barrel and then goes up. I shimmed with bricks at each turn in the ducting, so each run ends a brick higher than the run before it. The run closest to the wall is actually two bricks higher, because it is longer than the first run. Currently the cob covers all the ducting up to the elbow that changes the ducting from horizontal to vertical. I have enough materials to make two more batches of cob. I think I am going to make a wall of bricks behind the vertical ducting, to give the wall more mass. I had several days between the first couple layers of cob. Layers 3, 4 and 5 were added within couple days of each other. I just added a layer tonight.
I stayed with the fire as I worked the cob tonight and added wood as I went, for a longer burn. It actually went up to 600 during tonight's burn. It's only burning about the equivalent of a 6 foot 2x4 each time I fire it up, so I am happy about that. I guess it is just a matter of firing it every day for a while. I am getting some water oozing out onto the floor in a couple places. Claudia at Cabin Talk on youtube showed that happening during her build.
Is there any reason NOT to put a stove fan on top of the barrel to move heated air around the space? I feel like that would help a lot to warm the space more.
Hi Damon, no doubt that moisture play a huge part in restricting the overall performance and temps of a rocket stove!
I know you have quoted a few measurements but i think it would help us no end if we can confirm the dimensions.
Your riser is 6.5” x 6.5” and you mention your feed chamber is slightly smaller?
What about the burn tunnel?
It looks low in the pictures?
Yes , use a fan to blow the heat around. A ceiling fan really works best.
I'm getting the feeling that your measurement's are larger than stock for a 6"
This could be a problem if your core is larger than your mass pipes.
Rocket mass heaters are very accommodating about their mass or bell...
BUT they must be built to specification's, if you expect to have hot fires with no smoke and very little ash.
They will function with wrong dimension's but will ultimately fail, or perform so badly that you will become disillusioned with RMH and go back to a standard wood gobbler.
Not all who wander are lost... J.R.R. Tolkien
Location: Upstate NY, zone 5
posted 5 months ago
Looking at the pictures, unless all of the bricks have been cut shorter than the original 9", your riser would be 7 3/4" square inside, and the burn tunnel would be 7 3/4" wide x 5 3/4" high (there is a flat course of split firebrick below the ones visible from the side). The feed looks to be about 7 3/4" x 7". I'm not sure how those visual proportions match with the dimensions you gave. The burn tunnel being the smallest cross section in the core is alright, though making it an inch or so taller would allow for an inch of ash on the floor without choking the flow.
Building the riser from full firebricks with outside edges aligned as the photos show would give a 6 1/2" square internal dimension, but those look like splits, not full firebrick.
You mentioned that you have a riser top gap of around 2"... this is a critical area, you want to be certain that it is not less than 2", or you may have the flow choked.