New Build - Need some help

Hello all,

I'm looking for a bit of input on this build; here is the history.

Initially, I was impressed with Paul's pebble rocket mass heater series which was supposed to be "temporary" in nature such it could be torn down and rebuilt elsewhere.   I scoured the forums and purchased the DVD series and looked at Ernie and Erica's book for details. I probably spent close to 6 months reading up on the how to's on rocket mass heaters before attempting to actually build one.
The nature of the experiment demanded a six inch system and since weight was a factor I purchased a set of plans from Matt Walker for his ceramic RMH core build. I figured no sense in redesigning the wheel. Below are the pictures (see attachments)  I've taken of the build.  The ceramic board was one inch thick and rated at 2300 degrees per board . This design call for two inches of ceramic board everywhere except the feed tube where 1/2 split bricks were used to avoid wear and tear from the wood.
see 1-MW-build 1
       2-MW-build 2
       3-MW-biuld 3


When I dry stacked the core, at this stage the riser was 24 inches tall. A this point I tested the core with a first burn.

The burn tunnel temp reached approximately 1400 degree in this build.  I thought I could do better, and after reading that the higher the riser the better the draft, I took the rise to 39 inches and ran another test burn.  (the only change made was the riser height)

This time the burn tunnel temp hit 1900 degrees and I had flame coming out of the riser. Not wanting to exceed the insulating factor of the ceramic board I figured this would be the design stop point. Inspecting the plywood underneath both layers of ceramic board revealed charring but the charring did not extend to the other side of the plywood. That was cause for concern as apparently the temp further up the burn tunnel exceeded the 1900 I measured at the tunnel opening.  To compensate for this issue I laid a 1.25 inch granite pad immediately under the second layer of ceramic board.  This seems to have resolved this issue.

At this point I deemed the core design a success and proceeded to begin the inside build.

As a small aside, it should be stated that Matt Walker in one of his videos said he used aluminum foil to protect the ceramic board of the riser. I opted for this approach in my initial build inside.
The core foundation consisted of 1. 3/4 in plywood lined with aluminum tin foil, next 1.25 inch granite pad, next 1 inch ceramic fiber board rated at 2300 degrees.
see 4-core-foundation

Then the core build out. Version 1 of the firebox was built using 1" ceramic fiber board, with the feed tube using 1/2 brick splits.  the feed tube was 4.5 x4.5, the riser internal dimension was 5"x5" for a CSA of 25.  Six inch duct has a CSA of 30.  So based on the math I figured the system was sound.
see 5-core-build

The riser was 39" tall made out of 1 inch fiber board. I reinforced the base with another round of fiber board and surrounded the base with firebrick. I wrapped the riser in three layers of tin foil per matt walker.
see 6-riser build v1
     7-riser v1 final

After reading that moving the firebox a bit off center in the direction you wanted the heat to radiate, I moved the firebox 3 inches off center in the direction of the feed tube.  Its true, you will get more heat radiated out in a specific direction by doing that.

Since this experiment is not necessarily predicated on permies low tech preferences, my first attempt at sealing  things up was to use refractory cement.  This proved a dismal failure, as cracks appeared even before it dried completely.  My next option and much better solution was to use high heat silicon, rated at 600 degrees. Since the exhaust gas temp is not supposed to be that high at this stage of the drum I went for it.  It works wonderfully well.
See 8-high temp silicon

For the mass ducting, I used 6 inch single wall piping and referenced Ernie and Erica's book for rise over distance and went a bit more using 1/2 inch over 10 feet for version 1. The design called for 2 ten feet runs which were tied into the chimney which consists of 3,  4 foot runs , 1, 90 degree elbow and 2, 45 degree elbows to get outside.  I saw this solution at Paul's place when he built the pebble mass heater in his home. While he had an 8 inch system, I copied it for my 6 inch system.  The Jury is still out on whether that was sound or not.
See 9-mass ducting v1

With the ducting cabled up and sealed I ran my first indoor burn.  At this point, I thought I had solved the riddle.  The top of the drum reached 1200 degrees and had a cherry red spot just above where you'd expect the heat riser to be located. A check outside revealed steam exiting the chimney.  

The only down side and I knew this would happen going in, is, you really have to tend the wood feed on a RMH.  Every 20 minutes I had to add wood to the feed.  This heater really ran through it.  

Sadly this is where the story takes a dismal turn.   This was the only time I achieved these results.  Every subsequent burn thereafter was less and less effective until finally on about the 6-8th burn it started back blowing smoke and fire out the feed tube.  For a system with effectively no moving parts, initially I was not overly concerned.  Annoyed yes, to be sure, but I was pretty certain I could figure it out.  (*dare to dream lol)

So, i popped the top drum off and discovered that the riser had pretty much fallen apart.  The aluminum foil had disintegrated and the ceramic board had warped enough to break the refractory cement seal around the board borders.  Another thing I noticed was that the ceramic board that made the fire box tunnel was not holding up as well as I'd hoped.

So, I rebuilt the firebox using #2 refractory tile cut to the size and specs of the ceramic board parts, and rebuilt the riser using 1/2 split brick cut to size, and insulated the brick with the ceramic board.
see 10-core v2
       11-core v3 pt2
       12-riser v2
       13-riser v2 insulatred

I rebuilt the riser out of 1/2 split firebrick per Ernie and Erica's recommendation in their book (except I cut the bricks to size) and put everything back together.

At this point I mention the CSA measurements in inches
The feed tube is 4.5x4.5 = 20.25
the riser internal is 5x5  = 25
the 6 inch duct has a csa of 30

I figured that made the system solid.

Additionally before placing the drum back on, I put 2x4s over the riser and measured the gap between the two drum sections.  I got 1.25 inches.   Since the 2x4s are 1.5x3.5 that would give me 2.25 inches of space between the riser and the top of the drum.  

This time during the burn I had to close off 50% of the feed tube to get any draft and when the drum hit 400 degrees it flamed out and I got smoke blow back and fire out the feed tube.  

I sealed off the feed tube with bricks and let it die out.

About this time, it occurred to me that maybe my mass duct really didn't have enough rise, so I tore that apart and reinstalled it with a 3 inch rise in each 10 foot section with a 1 inch rise between the end elbows for a total of 7 inches of rise before it hits the chimney.  And I had the same issue as before: at 350-400 smoke blow back made its appearance.

In the meantime I met a young gentleman at one of the local fireplace shops who had attended one of Paul's RMH Seminars, and explained to him the problems I was having and showed him my photos.  He offered his opinion that the CSA of the exit duct going to the Mass was probably not enough.  So, back to the books and sure enough there is a 3 inch CSA requirement there I had totally overlooked (much to my chagrin).  

I tore out the back of the support bricks and again cut them to the size I needed and also considered since the surfaces of the ceramic board, the tiles and the firebricks that support the riser were not perfectly smooth, I would put a thin (1/2 inch) layer of cob between the riser and the core to make sure everything was sealed up tight.

This having been about the fourth time I had torn things apart, another thing became apparent.  While the silicone is wonderfully easy to apply and does seal things up, its a real pain in the rear to get off and clean up.  So, this time since I was cobbing things up, I decided to use cob for the resealing of the drum.
see 15-sealed by cob
       16-exit duct csa

With the new duct gap in place and everything rebuilt, the final burn was more dismal than any of the others such that even a small fire to start came back through the feed tube.

So this is where I currently sit.  There has to be a choke point somewhere,  and having just read the post on how to make your RMH rockier, the only other think I can think of is to lower the riser, or move the whole thing back the the three inches I moved it forward.  

While I'm not freezing, as the propane back up is in place, my pocket book is hemorrhaging to keep the propane flowing. So the sooner I can fix this the sooner my pocketbook will love me again lol.

Thanks in advance for your posts and constructive comments.

Jim

PS I did attempt to upload the movies of the test burns but the forum wasn't having any of that.  Sorry.  Is there a solution?

Hi Jim; Welcome to permies!

Let me start with the only real answer I have for you.  Upload your videos to youtube.  Then when making a new post on permies, and you want to add your video ,hit attachments like your posting photo's then hit the tab that says youtube at the top of the post. a screen will pop open asking for the youtube link. That should post your videos.

On to your build. To start with you did a superb job with your build (even though it's currently mucking up) Very nice work!

From all your photos and description that stove should be roaring. It did at first. Something is stopping the flow, I assume that the chimney cap and pipe have been checked, Is it possible you have ash build up in your horizontal run?  A leaf blower can make short work of that problem. You need to visually confirm a free path from the tunnel, up the riser , into your manifold.And from there into your flat run.  The manifold is the most common place for a system to back up.
I'm sure your using very dry wood.  Cold wet bricks and cob always resist flow in the beginning, but yours roared & then choked up... Has to be plugged  exhaust somewhere.

If you are not aware there is another forum strictly on rmh's . The guys there are very technical , they may have better insight into your problem.  Here is their  site    http://donkey32.proboards.com/

I'm suspect of your transition area being too small. The pipe leaving the barrel is 6" diameter and really should be more of a bell shape--think about replacing this with a reducer that narrows from 8" to 6". The problem with the current dimensions is that the transition is probably not using all 6" of pipe diameter--the gases are moving down the barrel and not up from the floor so they are only entering the run from the top half of your transition. This makes your csa cut to 3/4.

Peter has stated this as a common problem in stoves and recommends making the  manifold area have some depth as well. I couldn't find the permies page where he discussed it so if someone here could link it that would be great, but I believe he suggest raising the transition 4" from the bottom of the barrel to allow the gases to move into your horizontal run from all directions and not just from the top. The goal is to make the gases have a much easier time changing direction so rapidly and not slowing down too much.

I like the way you built the core.

Just curious to how you joined the 4 pieces of CFB together in the riser.  Did you use steel pins? Or were they all first sealed with fire cement?

I'm thinking that the heat caused the boards to warp allowing the fire to break the joins and then melt the tin foil.  Maybe using rigidiser on the boards first could have prevented their deformation.

Is the draft is restored if you bypass the horizontal pipes and go straight to the chimney?

First I'd like to thank you all up to this point for your insightful comments.  I'll get something going on utube and post the links here.

The bell issue and troubleshooting by bypassing the mass ducting will mean a complete tear down of the mass as things currently sit, and I'm in the middle of winter, and the rock alone is a complete days work to get out of the wooden case its in.  However both items deserve a thorough check out.  I'm currently buried in snow, so when I get those checks done I'll report back and let you know how things sit.   I suspect I'll need a new barrel to get the 8 inch bell going and my supplier wont get those in until next month (first week). All the stuff you hear about a 1 hour build is bunk.  If you have 12-20 people maybe that happens, but its just me and every time I tear into this thing is 2-3 days to bring it back up.  (and hopefully you'll understand if I really hate working in sub freezing temperatures lol)

As for the ceramic riser, I used stainless steel pins to hold them in place, and added the refractory cement and let it cure for a couple of days before I installed the riser.  I had it in mind the refractory cement would hold things in place.  On the final wrap I use stainless steel wired wrappred top middle and bottom to hold the insulation boards in place now.

Thanks again for  your insights.

Jim

Additionally before placing the drum back on, I put 2x4s over the riser and measured the gap between the two drum sections.  I got 1.25 inches.   Since the 2x4s are 1.5x3.5 that would give me 2.25 inches of space between the riser and the top of the drum.  

This time during the burn I had to close off 50% of the feed tube to get any draft and when the drum hit 400 degrees it flamed out and I got smoke blow back and fire out the feed tube.  

I sealed off the feed tube with bricks and let it die out.

About this time, it occurred to me that maybe my mass duct really didn't have enough rise, so I tore that apart and reinstalled it with a 3 inch rise in each 10 foot section with a 1 inch rise between the end elbows for a total of 7 inches of rise before it hits the chimney.  And I had the same issue as before: at 350-400 smoke blow back made its appearance.

In the meantime I met a young gentleman at one of the local fireplace shops who had attended one of Paul's RMH Seminars, and explained to him the problems I was having and showed him my photos.  He offered his opinion that the CSA of the exit duct going to the Mass was probably not enough.  So, back to the books and sure enough there is a 3 inch CSA requirement there I had totally overlooked (much to my chagrin).  

I tore out the back of the support bricks and again cut them to the size I needed and also considered since the surfaces of the ceramic board, the tiles and the firebricks that support the riser were not perfectly smooth, I would put a thin (1/2 inch) layer of cob between the riser and the core to make sure everything was sealed up tight.

This having been about the fourth time I had torn things apart, another thing became apparent.  While the silicone is wonderfully easy to apply and does seal things up, its a real pain in the rear to get off and clean up.  So, this time since I was cobbing things up, I decided to use cob for the resealing of the drum.
see 15-sealed by cob
      16-exit duct csa

With the new duct gap in place and everything rebuilt, the final burn was more dismal than any of the others such that even a small fire to start came back through the feed tube.

So this is where I currently sit.  There has to be a choke point somewhere,  and having just read the post on how to make your RMH rockier, the only other think I can think of is to lower the riser, or move the whole thing back the the three inches I moved it forward.  

While I'm not freezing, as the propane back up is in place, my pocket book is hemorrhaging to keep the propane flowing. So the sooner I can fix this the sooner my pocketbook will love me again lol.

Thanks in advance for your posts and constructive comments.  

Well Jim.

This sounds like a typical case of condensation plug.

I have been fighting with this before.

First of all. I would be happy if you could increase a bit your top gap. 3 or 4 inches is better, imho.   Even if this is a square riser. Re checking the numbers, you have a theoretical gap surface of 45, where the absolute minimum is 37 sqin. I see you have a red spot, and that's no good. I mean, the metal expands, and buckles down, chocking your gap, and hence the burn.

the chimney which consists of 3,  4 foot runs , 1, 90 degree elbow and 2, 45 degree elbows to get outside.  

If your chimney going through the roof?  Outside part insulated?  It is not going through a wall?   I think you need to insulate that chimney  outside. And may be inside too. Down to the barrel.

When you do a burn, can you hold your hand on the chimney for any length of time, say 4 feet above the barrel? If yes, this is no good, this means your exhaust gases are too cold. Best time to check would be when the stove fires back.

To me it all seems sound. Except that the burn seems bad, by the look of the soot inside the barrel. But that's normal if it is stalling.



What did you use for mass?  Pebble or gravel is mildly insulating due to the air gaps, so it shouldn't be a problem. But the first layer where all the air gaps touch the pipe, let it radiate faster than a solid mass. So this could be a potential problem.

We have heard here, of several semi failures, where people burned, with bare pipes. And the heat extraction was too much, so the stove stalled. I think you are in the same kind of case.

When i say condensation plug.  The steam in the exhaust (usually in a vertical part) condensate into droplets, this releases heat above, re heating whatever moisture is above, so the system still functions for a while. It works as an ensemble. Since the gases, and the steam or fog are all gases, they tend to move in the same direction. But the more water condensation in that column of gases, the heavier the column of gases becomes. Until a complete stall. And a reversal. Shooting flames inside, then putting the fire out, due to the lack of oxygen in those gases.

Do i make sense there?  But with your black pipe stuck to your barrel, this shouldn't happen.

You say you are buried in snow. What kind of outside temps, since this can effect your build too. If it is really really cold, this could effect your system, if it is really on the verge of condensation. Also, do you have a sort of outside air "intake"


Anyway i stop babbling away.

The solution i see. Insulate the vertical chimney above the barrel. If that doesn't work. Reduce the length of your tubes in your bench, say a foot at first. If that doesn't work, cob the pipe, may be on two or three inches. If that doesn't work, insulate the whole pipe in the bench. And if this doesn't work, burn the whole house down, and move south!

Hey Jim, nice looking build. H'm, how is your draft when the stove is cold without a fire going? A really good natural draft will pull the flame off a lit match held at the entrance of feed tube.

Max mentioned the chimney and that reminded me of my system. Living in a hilly area with shifting wind patterns, I've found that my RMH performs best with a tall chimney. Currently that consists of the following: single wall stove pipe exits the RMH rising to the ceiling (Selkirk ceiling box) where it transitions to double-wall insulated Class-A and passes through the attic and up through the roof to a height of 4 feet above the roof's peak. Total length from RMH to chimney hat is between 15 and 16 feet. My RMH initially had problematic draft issues on a shorter chimney, and extending the chimney made a world of difference.

Byron and Max,

This RMH is housed in a 20x48 foot tent.  The tent thing is a self imposed preparedness exercise on my part to see if everything got cut off (for whatever reason) if I could make a go of the situation.  It's something I've wanted to do for a long time, and since at this point in my life there isn't anyone around to tell me I can.t, I am.  

Given what max said, I think he might be on to something.  When I've run the heater I get a ton of condensation, so much so that I have to put a bucket under the last 45 degree elbow (which exits the roof of the tent) to catch all the run off (and its not raining either) what condensation doesn't fall into the bucket runs down the horizontal portion of the chimney and into the final vertical run of the chimney. I took a second look at the top of the drum, and where the hot point was, it has buckled down a bit. Even when the RMH ran at 1200 at the top of the barrel I could hold my hand on the chimney closest to the drum with no discomfort.  I thought I was loosing the heat through the mass ducting and gave myself two thumbs up. Apparently that was wrong thing on my part.

The chimney is single wall pipe all the way and is not insulated at all. Even with this cold weather it would not be too much effort to take the top layer of brick of the heat riser which is about three inches, trim up the ceramic boards and put that back together and conduct another test run.  I can get flexible aluminum insulation from home depot and wrap up the chimney piping at the same time. Given the situation, these troubleshooting steps would be the least intrusive as far as tear down, and could be completed next weekend.

I used 3/4 inch gravel and some 4-6 inch river rock as the mass, since I was going for a "temp" given the test scenario. However, I have considered next time around, with mixing it all up with layers of cob to keep it more "mass" oriented so it retained the heat better.  At least that's my current thinking.  The downside to that is the whole system becomes less temporary and a bit more permanent when it comes to moving the heater somewhere else.

Winter has made its appearance in a big way, and the temperatures have been around 54 in the day and 36 or so in the evening.  Last night it got down to 19 degrees and I had 35 mph wind to add to my entertainment factor. lol If being cold affects the RMH  I'm probably a prime candidate for that issue.  Right now there is no natural draft on the heater at the feed tube.  I've always had to prime the riser to get a significant draft going.  The only intake for this system is the feed tube.

I initially wondered about extending the chimney.  I read where a taller chimney helped with draft.  Given the tent arrangement short of installing guide wires around the cap and tying them off to the tent roof supports, I'm not sure just how much higher I could safely go.

Jim

OK. I think your mass is too long. Reduce your heat riser, for sure, you will extract less heat at first, so you will have more for the mass. Explaination, less température differential means less heat exchange. So,  more heat going to the mass. And, i don't know what is aluminium  insulation. Just get batt insulation. Aluminium is usually a heat conductor. Rockwool is good stuff.

Hth.

Satamax Antone wrote:We have heard here, of several semi failures, where people burned, with bare pipes. And the heat extraction was too much, so the stove stalled. I think you are in the same kind of case.  

LuAnne,   Your pictures make it some much clearer to what is happening. I agree with Satamax. Because your chimney is all uninsulated and in a tent with no insulation, it no wonder the condensation is forming and your not getting a draft.
I had similar problems years back until I insulated mine.... then the rocket took off!
I just double wrapped mine with that foil bubble wrap stuff and it works pretty good - Heat retained to produce a draft and next to no condensation.

EDIT... But please read Matthew's post later on about it not being the best choice for every situation...

I have heard of people insulating their own pipes by surrounding the existing pipe with one the next size up and then filling the gap with rock wool (or other). Makes for a nicer look than just bare insulation and also protects it from weather or critters.
In your case I probably would insulate as close as you can safely get to the top of the barrel.
Also, as been said, I'd cover up the horizontal pipe even temporarily so there is enough heat to get to the insulated pipes.
Stay warm!

Hey Jim, I like that tent setup, nice. The condensate issue will improve with improved (hotter, better drafting) stove performance, by pushing the condensate (as steam) up and out the chimney. Guess that's pretty obvious. Nice photos, and the one of the truck head-on with tent behind clearly shows the draft problem -- all those close in surrounding trees. Want to know the optimum height of the chimney?o) Make it a little taller than the close in trees. I'm thinking erect one of those telescoping radio/TV mast, guyed off of course, insulated chimney pipe strapped to the mast, if you want to experience the best an RMH has to offer. Using insulated double-wall stainless steel Class-A chimney pipe sections, this top notch setup shouldn't set you back much more than the cost of the tent itself!o)

Okay, maybe that's extreme, on to the second plan: add insulated chimney sections one at a time, inching the height up in stages (about 4 feet at a time) and testing for draft with each new addition, until you get the desired RMH performance. You'll know when it's right, firing the stove will result in all that blue barrel paint flashing off in a cloud of noxious stinking smoke, if it's not 1200° F. rated paint of course. After burning off my barrel, plus a bit of sanding, it was repainted with high temp 1200° F. woodstove paint. The high temperature paint is holding up very well, with exception for looking gray'ish at the very top of the barrel, only the horizontal portion. That hottest area easily hits 800° F. to  900° F.

Anyway, with the right chimney setup you'll get that mass warmed up in its current configuration.

Ok guys,  

I took the advice you all posted and I removed three inches off the top of the heat riser, and insulated the chimney with flexible aluminum insulation (I gave the pipe a complete double wrap - see picture); buttoned everything back up and had high hopes for success.  They were soundly dashed.  

I have taken the time to post the videos I've taken to utube (there might be some token entertainment value there lol) and here are the links to them. At this point, I'm dead in the middle of winter, and any further tear down of the mass will have to wait for warmer weather.  That being said, give that I've followed instructions this was not the results for a RMH I ever expected to experience.

Jim


ceramic 6 inch core with 24 inch riser test burn



24 inch riser peek
https://youtu.be/_i7mRGO3pBs

36 inch riser peak
https://www.youtube.com/watch?v=41IeEP4_aX4


6 inch core with 36 inch riser burn
https://www.youtube.com/watch?v=J-3ZEMHutvM


Rocket Mass Heater failed Burn 1
https://youtu.be/uN5k_PcoKgA


The failed burn continues
https://youtu.be/v4RFF-MbKn4

No improvement whatsoever ?

You haven't read well what i wrote before. I Said insulate above the barrel. Here you have modified something good you had done, which is sticking the chimney to the barrel.

First of all, take your sharpest knife, cut near the barrel, may be one inch each side. And remove the insulation against the barrel.

Do you have a cleanout at the base of the vertical chimney?

If yes,  do a little burn in there,  to warm the chimney.

If not. Try to start the stove on alcohol. ( well, in France, we call it burning alcohol)
It doesn't smoke. And you blow all you know on the feed tube, to get the draft going.

I guess your condensation plug either doesn't happen anymore and you have no draft whatsoever, or, the draft stopping occurs earlier no?

If nothing of this works, we'll talk fans.

I'll mirror what Max said about the chimney pipe insulation. Exterior stove pipe insulation is mandatory between the ceiling up to the chimney cap. Inside (living space) stove pipe insulation is optional. Insulating the stove pipe between the thermal mass exit to top of barrel is counterproductive, that's where the bare stove pipe is strapped to the barrel specifically to facilitate the draft.

RMH rule number one: avoid like the plague having firewood sticking up out of feed-tube opening, it's a recipe for smoke back, to say the least.

Starting a stone cold stove, especially in really cold weather, in a cold dwelling, is often problematic and frustrating. Break the cold starting procedure rules, attempt to rush it, and the RMH will bite you in the butt, rewarding the living space with smoke-back. Experienced RMH folks have their individual ways of dealing with cold start scenarios. Here's my tried and true method:

1) Remove the cold air plug in the chimney, using a propane torch to preheat the chimney's interior air. This is done at the exhaust Tee that's located right above the thermal mass bench exit's 90-degree upward turned elbow. The torch flame is directed upward into the open Tee until the stove pipe is warmed up and a good natural draft established.

2) Cap the exhaust Tee and light the J-tube kindling fire at the base of the heat riser. As the fire grows continue feeding it and work it back towards the base of the J-tube vertical wood feed. Use only small kindling wood. Continue feeding the fire until the barrel and J-tube warms up and a hot bed of coals form.

3) If number-2 fails, repeat starting again at number-1.

4) Once a well burning hot kindling fire is raging, then and only then, begin stepping up the size of the wood to larger pieces. 2-inch diameter and up firewood is added after about 30 minutes into the burning of fine splits and or small sticks, only when the stove is warmed up to operating temperature.