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Help!! New Build - Rocket Mass Heater not Rocketing!

 
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OK, So we finally got all of our RMH components assembled and cobbed and/ or mortared in place and have done our first test firings. Results are underwhelming, and we could use some help diagnosing the situation.  Photos of system below, and here's all the specs:  It's a 6" system.  Combustion unit: We installed a Matt Walker style ceramic fiber board J-tube core w 32" riser (measured from base of unit).  30 gallon barrel, riser is offset from center of barrel.  Manifold - We cut another 30 gallon barrel for the manifold.  We used a short piece of 8" pipe connect manifold to the bench.  Bench/ Bell/  Mass - We're using 2 split barrels for form a bell/ stratification chamber under what will be a cob bench.  

First firing was last night, around dark.  It's been mild here, exterior temps were high 60's or low 70's.  At first there was a lot of smoke from the cobbed & mortared seams, which we patched and sealed.  The fire never really "rocketed", there was some draw, but fire regularly back-drafted a small puff of smoke.  There was no draft to speak of - exhaust exiting top of chimney was a lightly drifting smoke with the odor of dank clay.  Temperatures:  The barrel top reached 450 - 500, top of sides 450, lower manifold sides 180, bench inlet 200, end of barrels closest to fire 140, far end of barrel 110, exhaust pipe 90 - 100.  We called it a night after some puzzling, pondering and hypothesizing.  The morning I sealed the union between the manifold and barrel, something we'd skipped the night before.  When I fired it up it it behaved a little better, much less back smoking, there was a slightly vertical column of smoke coming our the stack on the roof, more than the night before , but temps were still not impressive, and exhaust was cool.  We then added 6 more inches to the CFB riser bringing it to 38" and 2" from the barrel lid, and added a few more lengths of insulated pipe to the top of the exhaust pipe on the roof, and fired it up again.  Results were even poorer, the extra bit of CFB riser had not previously been fired and let off some noxious fumes, the back-smoking was worse, exhaust still cold.  Very disappointing.  The system seems to perform a little better with the 32 inch riser, though we have not yet tried something in between.  Definitely not rocketing!  We're trying to figure out what's wrong, and sure would appreciate any input.  Here are possibilities we're considering:

Is Matt Walker's 6" J-tube CFB core is too small to drive this system?  Is there not enough capacity or "push" to drive the heat through?  Can this system be tweaked, or is it a tear down and start over?
Are there still too many leaks in system? We've cobbed and mortared a lot, but?  Barrel has a removable lid and we have not yet installed a gasket.  
The riser is not centered in the barrel.  Our understanding is that this is OK, but is it?
We used an 8" pipe to connect the manifold to the barrels, we have read that the manifold is a possible pinch point and you want to avoid any constriction at all costs, and that it's fine to go just a little bigger here.  Is this small stretch if wider CSA/ pipe an issue?  
Is the 30 gallon barrel a problem?  All advice has been that a 30 gallon barrel heaves differently than a 55 gallon, but it still provides the same function as a downdraft bell.
Maybe the system would work better with pipe in the mass rather than a split barrel bell?  Right now the heat enters the barrels high and we draw off the exhaust low, as per the bell / stratification concept.  We're considering raising the exhaust outlet from the barrel to see if that improves the draft.  

Any and all ideas most welcome!!







RMH-Test-Run-001.jpg
Approximate temps in our first trial runs.
Approximate temps in our first trial runs.
 
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Hi Mark;
Promise it will get better as things dry out &  heat up.
New construction rockets need to dry out and it takes time.
Also having all bare metal lets to much heat escape into the room rather than up the chimney.
For start up purposes being able to move your exhaust from the floor to the roof of the bell will help as well.
Once your chimney starts drawing, all your little smokes and the back smoking will go away!
 
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Some questions that may help:

What size/shape/type of wood and is it perfectly dry?

Is there moisture in the system that just needs to be driven off?

Is the building well-sealed? Have you tried with windows open?

I am sure there are others people will add. I would leave the wider gap between the riser and barrel top. The early 2" figure from Ianto seems to cause lots of people problems. Many people have issues getting one to draw when the temperature outside is as high as you report. The 6" cfb should not be the problem. It's best for a draft to suck air through rather than the fire push it through if possible, otherwise smoke and CO are a concern. Definitely seal any leaks. Uncentered riser should not be an issue unless it restricts flow near manifold. 30 gallon barrel should be ok as long as it does not restrict the flow, it will just run hotter. You could temporarily raise the exhaust pipe to try to dry out the system and work out some kinks, but in the long run you will lose a lot of efficiency if you leave it that way.

I suspect your problem is likely to do somehow with the chimney or building/location itself. Is the chimney insulated? Are there structures around it anywhere near? Was there wind? Lots of possibilities, but don't tear it apart yet. You may be able to get it right without too much work.
 
Mark Dumont
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Wood used for test fire is finely split and very dry softwood (doug fir and cedar) as kindling, and then dry madrone (which has a high heat value) split to about 2-3" thick.
Yes, there is some moist cob and mortar, but since the combustion is a prefab CFB core, not a lot. We have to cob up the manifold inlet and exhaust, and try to seal the base of the splits w/ mortar, sealed the union between manifold and barrel w/ mortar.  It's located in a new addition, which still has plastic on two windows, all windows and doors are open.  

It's best for a draft to suck air through rather than the fire push it through if possible, otherwise smoke and CO are a concern.



Not sure I totally get your drift here...  How does one create enough draft to suck the air through, rather than pushing it through?   Is it often necessary to prime the draft as is explained ion the RMH books?  Ianto Evans refers to the rocket as pushing heat through the system.  Though I freely admit that with this test firing I'm trying to wrap my head around new concepts of draft and air movement.  I was expecting my dragon to roar, imagining a rocketing sound in the combustion unit, and expecting much more heat moving through the system than I am seeing, trying to figure out how much I need to change my understanding and how much is a problem I need to diagnose and solve...

 
Mark Dumont
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And here's a photo of the chimney (sheets on front of the house are protecting a new lime plaster from drying out to quickly, it is warm today, 80's!)  It's 16' of double insulated pipe past after the 10 feet or so of single wall pipe in the house.
IMG_4309.JPG
[Thumbnail for IMG_4309.JPG]
 
Mark Dumont
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correction, make that 12" of double insulated pipe, not 16"
 
Jordan Holland
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Your place looks nice! Those trees may cause issues with draft when the wind blows. A rain cap may help with that. At least for now, I would split the wood finer, 1" or less to get the most heat possible while you troubleshoot. You may end up needing to insulate the inside pipe as well. Hopefully Thomas is right and when it dries out and is covered up more, it will help. I was fortunate when I got mine sealed up to stick my hand in the intake and feel a good, strong draft with no added heat. Some structures want to draft and some don't. I was in a house once with a conventional wood stove that simply would not draft with a north wind unless the stove was running wide open.

What I meant by the draft sucking is that the vertical chimney should create an upward current of air on it's own. If the RMH is generating the drive and forcing it out the chimney, the positive pressure can push exhaust gasses out into the house if there are any cracks in the system. If the chimney is sucking the exhaust out, any crack would simply allow it to suck fresh air from the building out the chimney. You can try heating the chimney pipe with a torch or heat gun to test it to see if a warmer chimney helps the draft. Some people do add a bypass on the mass to prime the chimney when they get the fire started. You might leave the manifold area uncovered until last in case you might need to do that.
 
Mark Dumont
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Thanks for the reply Jordan.  Yeah, all that about the chimney and vertical draft is pretty straight forward wood burning stuff that I get (I've got 5 woodstoves on my property and have heated w/ wood for 40 yrs).  What I was referring to is the nature of low temp exhaust systems in mild climates.  Ernie & Erica Wisner delve into it on page 146 of their book.  Namely that low temps exhaust systems have a higher efficiency (more heat is exchanged in the living area but may not be hot enough for reliable draft in warm or humid climates.  They mention California specifically, I'm in the far NW corner, almost Pacific Northwest really, but still relatively mild compared to many who post on this forum.  They point out that masonry heaters are built to exhaust at 200-300 degrees to avoid the dew point of water in all conditions, and a RMH can be built to these specs if the horizontal heat channels are limited to 20 feet or less, yielding a reliable draft in all conditions, but a potential loss in efficiency up to 15%.  My point about the push is that one logical way to increase exhaust temp is to increase heat output.  Perhaps my 6" J-style CFB core is undersized for what i want it to do?

One possibility is to replace the split barrels w/ pipes, but this would be really short, like 12 ft of pipe plus a bend (a bend = 5 feet in E & E's boot, so that would be 17 ft, close to the 20 ft they cite).  But how much heat can I expect from so little pipe in mass?  
 
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Hi Mark,  Sorry to hear about your dragon woes! Really sucks when your dreams of a fire breathing dragon turn out to be a smoke belching couch potato.

I'll strongly second the motions on 2 of the biggest things that have been mentioned that I would say are causing your problems:

1) Your very low chimney stack temp of 95 and an outside temp of 80's is a huge obstacle to overcome. No Delta T means very little temperature differential to produce enough draw.
The colder it is outside and warmer it is inside will get things going in the right direction much better.

2) Those half barrels not being covered with cob are sucking all the heat out of the gases and leaving hardly anything for the draft to start. Don't know what you would have on hand that could be used temporarily insulate them with to prove this but I think it would make a large improvement.

Do you have a priming port on your vertical chimney you could light a small fire in or stick a propane torch into to bring up the stack temp? You could also just blast a torch directly on the exhaust pipe or a heat gun might work too.   Good luck!
 
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I would add. More pictures are needed. Of the bottom of the vertical chimney area, manifold etc.

Are you sure of your gaps? Really sure? Have you ever heard of ring projection?

https://donkey32.proboards.com/thread/1406/calculating-ring-circumference-projection-gap

New builds can be temperamental, even more with hot temps outside.
 
Mark Dumont
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Satamax, would this photo give you the info you need?

It's a 6" system.  We're using a 30 gallon barrel for riser and for manifold, with an 8" pipe as an inlet to the bench.  The exhaust sits on the floor just behind the inlet.  We assumed the 8" inlet would be enough to prevent constriction, is this correct?

Another consideration - how critical is the profile of the floor of the split barrel bench?  We lined the bottom of the right barrel with brick, as you can see in the photo.  We left them out of the next barrel.  Since this is on a wooden floor I've been extremely concerned about maximum operating temps on the wood, so I'm trialing different amounts of mass and insulation.  I now think this could be causing drag in the gas flow in the stratification chamber, perhaps blocking a pug of exhaust at the far barrel.  
IMG_4273-001.JPG
[Thumbnail for IMG_4273-001.JPG]
 
Jordan Holland
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I believe your manifold is ok from the pic. So, is there some cob or something on the bricks on the bottom? I don't think the bottom is causing drag if that's what you are thinking, but if it is not sealed some way, I would bet you may be getting a lot of air leaks.
 
Jordan Holland
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Mark Dumont wrote:  My point about the push is that one logical way to increase exhaust temp is to increase heat output.  Perhaps my 6" J-style CFB core is undersized for what i want it to do?

One possibility is to replace the split barrels w/ pipes, but this would be really short, like 12 ft of pipe plus a bend (a bend = 5 feet in E & E's boot, so that would be 17 ft, close to the 20 ft they cite).  But how much heat can I expect from so little pipe in mass?  



Your core should be fine. Pushing the gasses through the system can definitely work, but it is dangerous and less effective than the draft sucking them through. Remember, in a J-tube there are two places where the draft is going down against the natural way hot gasses want to rise. You are relying entirely on the powerful draft created by the riser to overcome this, plus push all the gasses through the system. I would not switch to pipes. I believe you are correct that it is too short. I would troubleshoot the current system a little more, especially in cooler temps, before any drastic changes.
 
Mark Dumont
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Well, here is my latest working hypothesis on this situation:  

What I have right now is a low temp exhaust system, which is going to be finicky in my relatively mild climate.  Once our winters kick in, we keep the stove going all the time, but we have a substantial shoulder season in spring and fall when 50 degree temperature swings are not unusual and we range from warm, balmy days to cold, clear nights.  Based on my reading of E &E Wisner’s discussion on page 146 of the Builders Guide –

“an exhaust chimney temperature of 100 to 150 F… is hot enough to rise under cold, dry, winter conditions…  but may not be hot enough for reliable draft in warm or humid climates (California, Gulf States, Israel, Australia).  

Lower Temperature exhausts may be more fuel efficient but have less draft and are more cantankerous to operate in warm or windy weather.  When this disadvantage combines with cold-start conditions such as a cold chimney, or when the mass is below optimal temperatures, you can have a very balky stove that wants to run backwards 9r not at all).  Especially for milder climates or second homes, we favor a shorter bench for a more reliable draft.

European masonry heaters typically exhaust at 200-300 F, to avoid the dew point of water in all conditions.  They rarely run more than 20 feet of horizontal heat exchange channels.  A rocket mass heater built to this convention, in the short to mid-range for our parameters, will enjoy ore reliable draft in all conditions, but may lose up to 15% of its potential efficiency.  



Hard to believe that I’m finally grokking all this this late in the game…  

I now see very clearly that I need a bypass, so I’ll work on researching and designing that.  My challenge is that I need to bypass my 8” inlet to my 6” exhaust.  All thoughts, suggestions or links welcome.

In addition to a bypass my new objective is to have a mid-range exhaust temp in the 200 – 300 F range.  It’s late October and I need to get this project done before winter sets in, to preserve my sanity and my marriage.   I can’t afford too many time-consuming detours, tweaks and experiments.  If I can’t do this without a huge amount of effort, I’m afraid I may be looking at a tear down.  I’m also going to design and install a bypass, I see now that this is crucial for my conditions.

Correct me if I’m wrong, but it seems the two ways to increase exhaust temps are to increase the output, i.e. larger/ more efficient combustion unit; or reduce drag on the system.  Replacing the combustion unit is beyond what I can take on now, so I’m going to focus on reducing drag.  

The drag points that I can identify include 1) the split barrel bell? (see discussion below),  2) sealing the miscellaneous cracks and covering the system with cob masonry to improve draft 3) Floor of the combustion unit is 6” higher than the floor of the bench.

1) Split barrel bench bell – My guess is that a bell creates drag/ friction on a system, as compared to a duct, correct?  If I were to replace the split barrels with pipe, I think I would greatly increase the draft and elevate exhaust temps.  This will of course come at he expense of efficiency, but how much?  And would the pipe be sufficient to heat the mass?  I estimate my 6 ft x 3 ft x 18 inch high bench would accommodate about 6’ of horizontal run and I could install 2 or 2 ½ runs (note that the exhaust chimney in the ceiling is actually near the  middle of the bench).  2 runs with 1 180 d bend = about 13 ft,; 2 ½ runs with 2 180 d bends = about 16ft.
 
Or, maybe I need to tweak/ improve the split barrel bell – how critical is the floor surface profile to heat flow?  As noted above, we were experimenting with different amounts of mass in the barrel floor to see how it might affect the amount of heat radiating downward to the wooden floor below, so we put bricks in the barrel closest to the inlet and left them out on the other barrel.  This must be creating some drag as well, having the floor on the far barrel be 2” lower than the other.  I imagine this disrupting the laminar flow/ stratification of gasses and creating a plug of smoke in the far barrel.  The bricks in the right hand barrel are just placed there, with gaps between the bricks and the edge of the barrel, our plan is to cob all of the voids for the final install, but right now the floor is pretty irregular, and I’m not sure how much drag this could be creating on the system.  

2) Finish sealing the miscellaneous cracks and cover the bench. I realize the system is going to perform better once all of the cracks are thoroughly sealed and the cob is on the barrels,. I plan to do a bit more sealing and find something to cover the benches with (I have several water heater blankets that might do the trick).  This is easy enough so I’m definitely going to do it, but I question whether this alone is enough to push the exhaust temps up to 200 F or more.

3) Floor of the combustion unit is 6” higher than the floor of the bench.  It’s a long story, but basically I was really concerned with the heat over the wooden floor so I put a lot of insulation under the combustion chamber in the form of perlite concrete and CFB board, probably went overboard (my overall design also changed along the way, I was going to use a batch box, and the extra output from that would probably handle the extra downdraft better.  (I bailed on the batch box because of the extra design challenges it presented – door, insulation etc).  I’ve read in several places that it’s possible to downdraft these systems p to 12” or more, but it is a drag on the draft, so I’m guessing it has an impact.  I see in hindsight now that it was a regrettable error, and one that is not easily fixed.    Lowering the combustion  now would be equivalent to a tear down.  I’d have to remove the barrel and manifold and by then I’d probably just keep going…

So that’s my story.  I’m alternating between despair and obsession.  I want to understand and solve this fascinating puzzle in a creative fashion that yields a functional RMH for all the reasons we well know, but I can only afford so much more time, energy, materials and lost sleep before I have to run up the white flag of surrender, tear everything out, and put in a woodstove instead.  I’m so thankful to have this forum to bounce these ideas around, thanks for all of your input and support!
 
Gerry Parent
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Mark,  
You've got the theory of operation backwards. Bells pose very little drag whereas a pipe system has the potential to put much more, especially when many bends are included in the run.
A good comparison can be read here: wood-heat-storage-flues-vs-bells

Floor of the bell should not matter how bumpy it is as it won't put drag on the system. Its called the Free Gas Movement because the exhaust is like water and seeks its own level, stratifying into all these nooks and crannies, not scraping by them.  

Again, cover your exposed half barrel bench, wait for a cold enough morning and let her rip!
 
Satamax Antone
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Well, you seem to have plenty of gap. 4 inches may from the bricks to the  manifold tube? Have you added insulation around the "core"?

If your top gap is good too, i would say use at least 3 inches. It is then that your heater is wet, or shedding too much heat for the moment.  

About your bypass, you don't need full pipe size. A four inch pipe can be sufficient. From your eight inch pipe or half barrel to the six inch vertical. With a flap in the 4" pipe.

I think i would also add 4' of six inch insulated pipe above your actual stack. I see the dormer thingy at the back, and keep thinking the chimney is rather low.

But before you do any of this, try to cover your barrels temporarily with batt insulation. To see how it reacts.  
 
Mark Dumont
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OK, thanks for talking me through this guys.  I realize I'm probably over reacting, but I also have a property with a long history of various natural building/ permaculture experiments, some mine, some others; some of which have have done well, others which have failed spectacularly.  I've developed a phobia of having one the the latter examples as a long term installation in my living room...

I plan to continue cobbing/ covering the system and test firing.  Finding a window to test fire is a bit tricky as any escaping smoke fills our house, and right now we're in the midst of a 2-day wind event that may disrupt a fragile draft, but you and others have given be more confidence in the existing system.  

Gerry, thanks for the clarification and link on bell vs. duct, very helpful.  I still think velocity plays a role in the dynamics.  It seems to me that a bell system is like dumping flow into a lake, where the warm gasses float to the top and the cold gasses to the bottom; yes, there is much less friction, but velocity drops off steeply; whereas the short duct system described in E&E's quote above maintains more velocity, and because it's a shorter run, more temperature.  The result is a medium temp exhaust (200 F or more) that sacrifices some efficiency in exchange for a reliable draft in all conditions.  The more often one experiences conditions that may disrupt of a low-temp draft (warmish exterior temps with a cold mass, high winds, etc) the more such a trade off of a medium temp exhaust may be worthwhile.  This is all new ground to me, so I am not certain and welcome any further clarification.  It's interesting that i haven't seen more posts on RMH's in mild climates, or at least climates such as mine that have a long shoulder season with wide daily temperature swings.  I have a lot to learn about all this.

Satamax - i agree about the need for more height in the chimney and have the extra pipe for it, as well as the rain cap, just haven't yet installed it, will do that soon.  I'm waiting on the permanent install in case i want to test the draft temp at different heights...

Thanks again, will be sure to post my results ass this develops!

 
thomas rubino
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Hi Mark;
It will all work for you it just needs a little time.
Cobbing or temporarily  insulating your barrels will help.
As suggested raising your outside stack will help on wind event days.
Installing a bypass would cure all your problems.
I suggested a way to Luke for you to preheat the system, that and covering your barrels would go a long way towards making your rocket take off.

One thought, is that if you cob your barrels now before you have seen your rocket roar. You will need that cob to dry out as well.
Use your water heater blankets to experiment , then cob afterwards.
 
Jordan Holland
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I wouldn't worry about velocity. Rather that seeing it as a stream dumping into a lake, I see it more as a stream flowing into a much wider section of stream for a distance, then flowing back into a narrower stream again. Such things happen, and streams do just fine. Lower velocity is typically good in the mass because it gives more heat transfer. When your mass gets a little heat in it, things should improve.

I did not obsess over efficiency when I built mine. People can poo-poo me all they want. I DID NOT WANT SMOKE IN MY HOUSE!!! I've been there, done that. The thing is, you have no idea how the whole system will work until it is built. So, I built erring on the side of getting the smoothest flow possible, and not trying to run the maximum length of ducting like many people I have seen who have problems. Luckily it worked on the first try. The hottest the exhaust on mine gets after a full day of running is just under 200 degrees Fahrenheit, so it still gets a pretty good amount of heat transfer even though I didn't try to maximize it.
 
Mark Dumont
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I'm creating a new thread for input on bench inlet, exhaust and bypass...
 
thomas rubino
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Hi Jordan;   Where are you taking that temperature?   And are you using an infrared gun on the pipe?
Do we have photo's of your build ?
 
Jordan Holland
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thomas rubino wrote:Hi Jordan;   Where are you taking that temperature?   And are you using an infrared gun on the pipe?
Do we have photo's of your build ?



It's a temperature probe inside the duct right before it goes through the ceiling. I haven't done a write-up on it yet. I'm about half-way through the rocket cooker one I wanted to get done first. One of these days...
 
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Hi Mark, Gerry Parent sent me over to your thread and I see that you and I are in the same boat, trying to get our rockets working before winter. I can give you some good news- despite causing major headaches and lost sleep, my build has burned perfectly sometimes, when the outdoor temps and wind conditions are working with me. I think my build and yours will work as intended when they are actually needed, when it's cold outside, and warmer air is naturally rising out of our vertical stacks.

It's a colossal leap of faith to build a huge, heavy thing and only find out at the end whether it works or not. It seems like that's the only way to do it with an RMH, though. My attempts to run tests at every stage of construction have caused more harm than good, so I backed off for a couple days to relax myself and think about it. I'm confident that once it's cold enough outside (and maybe with some priming of the vertical stack using a candle or electric heater), my rocket will take off for good, and I bet yours will too. Of course we're in different houses and different climates, but I hope these words can help your sanity in some small way. You're not alone, and you will survive this :)

Best wishes and good luck,
-Richard
 
Mark Dumont
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Thanks for the kind and supportive words Richard.  Yes, I've arrived at the same conclusion myself, after lots of reading, and receiving some encouragement on this forum.  I now feel like I understand the difference between low and mid-temperature exhaust heaters much better, as well as the the fact that a masonry heater needs to be surrounded with masonry to work properly.  I'm glad I for the experience though, for one thing it convinced me a of the need for a bypass, and even though it's entailing some reverse engineering, I think I'll be able to pull one off, and it seems pretty darn important for my conditions.  I'll be sure to post some results once I finish and start using my RMH.

Thanks again!
 
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Hi Mark,
Just wanted to chime in as I am nearing the test-firing stage of my new build, which is very similar to yours. 6” Matt walker CFB core with a half barrel bell system.

I don’t know all that much, but from what I’ve read it seems you really can’t get an accurate picture on how well the systems working until you’ve got mass on it and it’s dry. Add to that cold outside temps.

I am in New York State so luckily (or unluckily) we have already had some 32 degree days. I have that going for me, but I am expecting when it comes to dry running the system I won’t yet have a perfectly running rocket.

Looking forward to seeing how your system changes as you move forward.

Best,
Sophie
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Mark Dumont
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Thanks for the encouragement Sophie.  Yup, I agree with you, gonna have to slap some mass on this puppy before I can take it for a real test drive.  What's the height of your riser?  Looks a bit taller than Matt Walker's specs, which is a 24" riser over a 12" high burn chamber, about 36".  I've got some more room in my barrel and am thinking it would be ideal to have a bit more riser.
 
Sophie Gell
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Yes you’re right! My riser is 42” (including burn tunnel). Originally I was going to make an 8” system but had to switch to 6”. I kept the height of the riser the same from the 8” design.
 
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