Not getting the heat I expected

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"

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.

Never got caught up in the math ???   You did follow The Wisners dimensions in the core , riser  and manifold correct ?  If so all is good !

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.

Hi Damon,

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.

Best wishes

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?

Hi Damon;  

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.
 

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.

How well is your building insulated?  

I've got R-19 in the walls and R-30 in the ceiling.  I cut and taped sheets of 1.5 inch foam to the doors at both ends (2-car door at one end, 1-car door at the other).

I do think that for your climate, the 8" heater would have given more insurance on those really cold nights. But you can probably still improve the situation with more insulation (we did R30 walls, R40 ceiling and crawl spaces ), maybe weather sealing, and by firing more often.  Try four hours morning, four to six hours evening, if it's not too inconvenient.

The 6 inch systems were originally developed as comfort heating for small huts and single rooms in milder, coastal climates. They do not typically burn enough wood to heat a larger space in more severe climates.

The choice to include Portland cement (which we don't typically recommend) may reduce your options to recycle the cob into a larger replacement system.

I also want to confirm that you insulated around the heat riser with some kind of refractory insulation. If it's just bare fire brick, I am not clear why it would be working at all, so I hope you already did this and it just doesn't show in the pictures.

If it really isn't working, one other option might be to look at removing and replacing the fire box with a rocket batch box. It is more costly to build, and takes more special tools and parts, but can burn more than double the wood, dramatically increasing your heat output. Look for discussion of these by Peter van den Berg and associates, on these forums here at Permies.com, at proboards, or at batchrocket.eu.

Before investing in big changes, you might check your heat loss situation to ensure that double the heat would do the job, or if you need o go even bigger.

You can check your heating situation on paper by using a home heat calculator like the ones at builditsolar.com, but a better reality check is to compare interior temps with desired temps, or to get ahold of an IR camera  and do a home energy audit. Or see if your county offers energy audits - some do, for free.
You want the IR camera to check for warmth escaping the building, which may reveal areas of preventable heat loss around windows, doors, rafters, ceiling and vents, etc. A better heat seal on the building could offer long term benefits, both for comfort and to reduce the total fuel burden for years to come.

Hope this helps. When you do find a good solution, please keep us posted!

Hi Erica,
Thanks for the response.  
I made a mix of clay, sand, cement and vermiculite for the base.
Then I built the rocket portion with firebrick on top of that base, so it is well-insulated from the cement floor.
Then I used bricks to form the ring for the barrel and the outer wall to contain the runs of exhaust pipe.
The exhaust pipe comes back behind the barrel and then goes up to the ceiling, then it follows the ceiling to almost the peak, and then penetrates the ceiling.
I used a sand, clay and cement mix to create my mass around the firebox and encase the exhaust pipe until it went vertical.  There is a clean-out right after the ring for the barrel, at the start of the exhaust run, and another at the bend in the exhaust run.
The pictures don't show it, but I built a wall of bricks up to the level of the window, where you see cement board in the pictures.  There is also more mass (several inches) on top of the exhaust run.

The rocket ran fine last winter.  I saw barrel temps over 600 degrees, consistently.  I put one heat-activated fan on top the barrel.  A second one blowing in a different direction would probably be a good idea.  I only ran the heater for a couple hours at a time (after work).

I just ran it again, last week, twice.  The first time, it ran fine.  The second time, it actually burned backward.  I had a fire coming out of the feedbox.  I put a couple wads of newspaper in the first cleanout and set it on fire, to create a draft through the barrel and it kinda worked to pull the flame back into the burn channel, but then the flow reversed and the fire came back out of the feedbox.  So that was kinda weird.  We've had a really weird winter so far (up to 50 degrees), and I've been busy with other stuff, so I haven't tried to run it again.  After the second burn I opened the barrel from the top and everything looked fine.  I did vacuum out the feedbox and burn channel.

Glad to hear an update. With a full winter's worth of use, how has your system performed? The barrel temp sounds good; did it keep your shop warm? How long did you burn per day in cold weather?


If your weather recently has been warm, the outside temperature might even have been warmer than inside after the shop sat overnight, and air would want to drop through the building and flow out the lowest cracks. You would need a serious draft established up the chimney before it would want to flow correctly. Air currents could affect the draft in this kind of edge situation. My chimney draws fine in most situations, but in warm weather in spring I occasionally get easterly winds which put back pressure on my chimney top. I haven't been able to get an assistant here to stand by while I go on the roof to mount the last 5' of chimney above a three-story drop.

Hi,

do you remember the ratios of the mixture for both the "mix of clay, sand, cement and vermiculite for the base... [and] a sand, clay and cement mix to create my mass around the firebox and encase the exhaust pipe until it went vertical".   I dont have the experience of mixing cement and its properties to see what might be the best combination of items.  I am assuming that the vermiculite was to keep it lighter?

Michele

J tubes with bench are finicky at the start of the season, due to a cold plug https://www.ironandwood.co.uk/cold-plug/#:~:text=This%20occurs%20when%20temperatures%20drop,overnight%20if%20temperatures%20drop%20rapidly.

My fix in the past, fitting a metal refrigerator's fan on top of the chimney (i did it permanently)  and running it until the cold plug is gone for good. Insulating that vertical pipe chimney would be clever too.

Michele, cement is not recommended at all for cob making. It would make the mixture harder, but that does not matter unless the clay is very poor; ordinary cob is quite hard and strong by itself. Adding cement may make it difficult or impossible to break up, rewet and reuse the cob if modifications are desired later - one of the good properties of typical cob.

The vermiculite would have been for insulating qualities under the mass, to keep from losing heat to the ground which will soak up an infinite amount of heat without warming up if the soil is conductive or has groundwater.