My 1st RMH build!

Ok, let me just start by thanking every ingenious one of you for all your hard work and generous teachings; especially Ernie & Erica Wisner and Paul Wheaton, you are true pioneers and have shared so much that I am grateful for. I am so excited to plan this build and have been pouring over these wonderful forums, the numerous videos and many other online resources over the past 2 weeks and am ready to begin.
Now, with that said; I am planning an Rocket Mass Heater build in the next 2 weeks in my basement. There are clay dirt floors, a 7ft clearance and foundation walls 3ft thick of local fieldstone. My goal is to offset the heating cost of our oil fired steam boiler by keeping up the ambient temperature of the basement and circulating the boiler’s water through a heat exchanger on the 55 gal drum and preheating the inlet of our water heater.
My RMH will start with a preheated outside air intake and consist of firebrick sealed with refractory mud and level topped to form the feed tube, burn tunnel and base of the heat riser base all square with a 7in ID giving me a CSA of 49sqin. Atop the riser base, my 36in heat riser will consist of an 8in pipe encased in an 18in drum and filled with a 5in radius of clay & vermiculite. This will be topped off with the 55 gal drum with dual opposing 6in exhaust joining to a single 8in flue to heat my mass.

Questions I have about critical and efficient dimentions:
1) Is there a benefit to a burn tunnel having a lesser CSA than the feed tube and/or the heat riser; or a benefit to having a greater CSA in the feed tube than the heat riser?
2) Is there a benefit to having a heat riser that is 4X the height of the feed tube (vs 3X)?
3) Should my heat riser be measured to include the depth down into the base (in the firebrick) or just the 36in insulated pipe above?

FYI my CSA flows as follows:
49 at the feed tube, burn tunnel and heat riser base
50.24 through the heat riser column
expanding to 113.04 rolling over the top outer side of the column (2in gap between riser and top drum to maintain 50.24 CSA at center riser top)
197.82 down the sides of the heat exchanging drum (due to 18in OD on heat riser)
56.52 barrel exhaust (2pc opposing 6in pipe joining into the 8in flu pipe)
50.24 through the mass and ouside flu (5ft rise to end)

In addition, I am working on an extended feed tube concept that will allow for the stacking of greater fuel without impeding the air flow intake height at the feed tube.

I look forward to all your comments and critiques.

hey martin, ya might try your local 'heating & A/C business' ...I lucked out (maybe you will too!) and was able to get about 20 feet of used duct pipe, it had fittings & screws and even insulation...the occasional dent & bent was easy to fix, this stuff was free for the asking, in the junk pile. After I got all my duct plumbing done, I was happy to shop with the same business man, for a couple adapters I needed.

james beam

looking good martin! will you be able to 'build it wet' soon, or do ya have to wait on the building inspector's approval of the dry mock up? 49 csa that thing ought to run real good, and with your smoke stack going UP & out of the cellar, I'm expecting it to draw well. Maybe you can get the smoke stack into the natural wind currents above the peak of the house? Would you consider a normally servicable 'condensation drain port' at the bottom of the smoke stack, if nothing else a weep hole. Also your inspector will probably expect 'insulated flue pipe' at least 24" from combustible materials, and probably expect a 'spark arrestor/rain cap'? More pixs please, looks like your building in high quality.

james beam

nice brickwork, I like the 'coal pit' ya made in the bottom~~good idea!, the special blade on a grinder is a good idea too.

james beam

james beam wrote:nice brickwork, I like the 'coal pit' ya made in the bottom~~good idea!, the special blade on a grinder is a good idea too.

james beam

Thanks JB, it was my first time layin brick so I had help and took my time, reviewing every detail we could imagine. Other than the back strain, it was awesome, I'm pleased and can't wait to see/hear it burn

ya back strain, well you have handled the brick at least 3x's now....takes it toll. LOL,~~~but not like the 100# bottle will weigh when it has liner and packed full of cob or brick?. not sure if this will help, but your inner pipe of the heat riser, I guess there are several ways to 'connect it' to the burn tunnel, I like the outboard flanges (022) vs. 'slip fit into the tunnel (015)', I'm sending a couple pixs...maybe you have something different in mind, keep sending pix, nice project so far!

james beam

This looks awesome, any updates?

Made a vid on her

And here comes the challenge, I have serious draft/draw issues! I had opened my battery to inspect and found there was settling so I dug the channel out to ensure minimum CSA and ended up with 60-70" CSA inside. That didn't solve it so tonight I opened the drum to no answers. What I did find was that the drum lid concaves 1/4" from cold to hot but with a 2.5" gap atop heat riser, this shouldn't be the culprit. I have ideas but am looking for feedback as to why she only drafts by force (fan).
Made a vid of tonights burn:

Hi Martin! Great project and great documentation! I feel pretty certain that what you are dealing with currently is only temporary. I think your system is behaving this way because the flue gases are directly exposed to massive quantities of moisture evaporating off the considerable mass in your system. The heat generated in your combustion riser makes the incoming air less dense than the surrounding atmosphere and creates the draft. But as the combustion gases hit your barrel and begin to cool they are forced down and out. Because you have built this as a masonry flue with lots and lots of surface area, all the moisture will take quite some time to be purged from your system. The difference in density that you need to drive your system in the burn chamber is currently not sufficient to overcome the supersaturated flue gases through all the chambers, and then push that dense air up and out of your basement.

To test this hypothesis, you might want to check a few things. 1 What is the temperature of the duct at the exit of the masonry? If it is cool to the touch or cooler than the surrounding air, that's probably a good indication that my hypothesis is correct. 2 Do you have enough duct to be able to temporarily reroute the flue gases out the basement door (assuming it's a walkout basement)? If you can keep the flue exhaust lower than the heat riser, your system should behave normally immediately. If not, you will need to use the fan until you have driven off enough moisture that the evaporation inside the system does not produce an exhaust that is more dense relative to the atmosphere than your combustion gases are light (relative to the atmosphere).

I hope this helps! I wouldn't pore over your numbers anymore. You were meticulous and I trust that those were correct. I think its just a light air vs heavy air thing. Good luck!

- Colin

hey martin, real nice work, I like the little flower flange you cut on the end of your heat riser inner pipe...real pretty. LOL

Ok what about trying this, find a piece of sheet metal or flashing, an put it on the underside of your baffles. I think it is your baffles that are disrupting your exh. flow, and causing 'back pressure bubbles' much like beer bubbles in your belly...that makes you burp back. So if you found a length of sheet metal to temporarily block off your baffles, & promote flow straight thru to your first Tee. It might be about 10" wide and about 4 or 5' long, fit it in there to block off your baffles, an yet still might transfer some heat to your baffled radiator thing, but mainly to get uninterrupted exh. flow for testing, its temporary, see if that will improve natural draft.

As you know, as Colin said, there is the moisture you have to burn off, and really...your firewood has to be dry stored for like a year to burn with least moisture in the wood, but that will come later. You said in video your using pallet scraps but you also said it was kina green & hot, you eventually want very dry wood.

So how is that 'coals hole' thing working out? Is it catching & holding lots of coals like you expected, or are the coals getting blown up the burn tunnel? I also think your little fan on the exhaust to pull, might be a easy change you could try...yanno if you haven't burnt yours up yet. LOL

Can we get a pix of the outside stack & rain cap, is it upright or laying on the ground, and you also mentioned a 'damper'...but I don't think you put in a damper valve, so that is alittle unclear. Also you said 12' of steam cloud at the exh. port...but that's with a box fan forcing it right?

Also there has been some RMH threads talking about using house temp. air, is better than cold air...I think you said it was 20's in the basement, so that cold source air may have something to do with it.

James Beam

james beam wrote:hey martin, real nice work, I like the little flower flange you cut on the end of your heat riser inner pipe...real pretty. LOL

Ok what about trying this, find a piece of sheet metal or flashing, an put it on the underside of your baffles. I think it is your baffles that are disrupting your exh. flow, and causing 'back pressure bubbles' much like beer bubbles in your belly...that makes you burp back. So if you found a length of sheet metal to temporarily block off your baffles, & promote flow straight thru to your first Tee. It might be about 10" wide and about 4 or 5' long, fit it in there to block off your baffles, an yet still might transfer some heat to your baffled radiator thing, but mainly to get uninterrupted exh. flow for testing, its temporary, see if that will improve natural draft.

Nice idea JB! that would be a great way to diagnose if the baffles are causing the smoke back. As much as I dont want to open that big (270lb) end cap, it will be for good cause and that's why I put them cleanouts in, cause all the experts said you'd need to get back in for mods.

james beam wrote: how is that 'coals hole' thing working out? Is it catching & holding lots of coals like you expected, or are the coals getting blown up the burn tunnel? I also think your little fan on the exhaust to pull, might be a easy change you could try...yanno if you haven't burnt yours up yet. LOL

The ash pit works well even without a grate yet. The exhaust fan was the first thing I changed as it was a complete choke point and smoked back even with a fan in the feed. The lil fan def didnt push like the package said :(

james beam wrote: Can we get a pix of the outside stack & rain cap, is it upright or laying on the ground, and you also mentioned a 'damper'...but I don't think you put in a damper valve, so that is alittle unclear. Also you said 12' of steam cloud at the exh. port...but that's with a box fan forcing it right?

Also there has been some RMH threads talking about using house temp. air, is better than cold air...I think you said it was 20's in the basement, so that cold source air may have something to do with it.

Pics are as follows, although the outside has been mod'd as stated before with no changes to the lack of draw.
No damper in this duct. Yes, massive fog on first burn with this 8" exhaust. That was with pusher fan at feed tube. Subsequent burns have not produced nearly as much steam, a sign the mass is drying.

james beam wrote:

Also there has been some RMH threads talking about using house temp. air, is better than cold air...I think you said it was 20's in the basement, so that cold source air may have something to do with it.

James Beam

Upper 20s outside, 54-56 deg in my basement.

Today's video update
http://www.youtube.com/watch?v=uCIyZzaNcZo

My "downhill syphon" exhaust has not produced results favorable to increased draft. JB's suggestion is on my todo list. I am hoping more of y'all have some other ideas to diagnose this dilemma.

Keep on burnin,
~Marty

I received a great comment/suggestion on my youtube vid:

it'll be smoky for a few minutes,but you should try unhooking the flue the first place you can,after the mass to see if it picks up any draft.if it helps maybe you should move that last upward 90 closer to the mass then go out of your dwelling with the flue at level. more of your heated exhaust will flow upwards sooner that way.as it is now that last 10 or so feet before your 90 might be the problem
krowebmf76 30 minutes ago

This looks to be a good diagnostic tool prior to opening my mass again

Hi,
I think James' suggestion is your most direct test. Those cavities can cause much turbulence/vacuum (akin to Tesla's one way valve) to disrupt the flow. I'd try the sheet metal trick first...

try the sheet metal bypass first

Martin,

before you do anymore "damage" or add another variable (ie. multiple path manifold), try James' idea....if you can insert a sheet metal "shield" or duct to bypass/close off that "comb" I think you will have found part of the problem...radically changing the flow path by other means will take you further away from diagnosing the issues you have now...and if sleeving works, if the turbulence/local vacuums in the "comb area" is the issue at hand, there are still many solutions....

again, diagnose with as little surgery as possible before treatment

feeling hoppy here too, tryng to help, check your FB

Andor

Hi Martin, I was looking over the whole project again and wasn't quite sure about something. Did you end up deciding to leave one brick removed as an ash and ember catcher? if so, is this essentially the first brick you would see looking above from the feeder side? My thought is that maybe you should try replacing the brick and insuring that your highest concentration of heat is not directly below the feed opening. Is it possible that you have a large accumulation of oxygenated fuel causing an updraft there?

Also, I like the suggestions you are getting for troubleshooting! It's nice to see so much support! Good luck again.

- Colin

P.S. the good news is while you try all these things you are drying out your system, so it may start performing better anyway.

Hey martin, ya the heavy end piece may have to be opened to install the 'debaffle flashing test'...but maybe not......

Perhaps there is a way to bypass your baffled section altogether! LOL OMG! your '2 week adventure is now 2 months', just hook your exh. pipe right too the exh. port before the baffled area, as close as possible to the 55g barrel, I think you built it 'accessible' & convertible in that area. I'm also agreeing that the Utube guy suggestion, about bringing your flue pipe off the dirt floor quickly, perhaps a temporary connection to the exh. port, just below the burn barrel? I know that would temporarily bypass all the mass, but your stove is below or nearly below ground level outside, which kind of points to some of the plumbing problem: raising the smoke UP & out of a below grade level stove.

What about ventilation? Is there a consideration of venting any accumulated poison gas out of the basement, especially, CO carbon monoxide, which usually accumulates in low areas? You mentioned earlier there was some kind of 'negative pressure' in the basement, which must be overcome with a vent for the stove to run better. Kind of like a drain on a toilet, even that must be 'vented' to allow the proper flow of the plumbing.

It just seems to me that the J tube/heat riser/burn barrel is already the first 'plumbing trap'...as it should be, but as your currently running your exhaust duct work (routed well BELOW the burn barrel, then baffled, then run at basement floor, then UP & out, second 'trap', then back DOWN again on the outside leg, are you not creating a third 'trap'...which therefore cannot easily flow. I would definitely rotate your outside elbow joint, to change the outside leg, to discharge UP. And make sure there is a vent that allows outside air pressure to the feed tube.

james beam

Hi Martin, First off, stupendously documented project-- bravo, good sir, bravo! But when I was watching your videos... and you go from totally psyched in the first one, to completely depressed in the second, to desperately pleading in the last... my heart goes out to you, man. I want to try and help you fix your problem without all of your laborious intent becoming moot or discounted.

It is from that place that I ask you to bear with me as I am going to propose a radical solution to your system, a slightly less radical solution to your system, and then a low-brow solution to your system, but before I do, I want to try and bring you to a place of understanding about what's happening in the second half of your construction that's hindering your first half from rocketing right along. Believe me, if you could just burn your J-tube/riser, even without the barrel, it would easily shoot flames out the top in a matter of minutes. I've built J-tube dry stacks that draft properly just to illustrate the principles of 'sideways fire' to people and they work beautifully... as long as your geometry is right on, it's flame on!

Before I go any further, I must again commend you for your excellent video and photo documentation throughout the construction/test process, for it is that attention to detail that provides enough information for me to completely agree with Andor's assertion that your baffled mass is the major issue at hand. You don't have to test it with a baffle blocker because I'm going to explain to you why it's causing the problem and you don't need to waste any further effort at this point because I think you've already got some more work ahead of you to make this thing right. But not to worry, it is still a construction that will be beneficial in the end. Right now, however, it's a 'flow death chamber'. Disclaimer: short but relevant anecdote-- many years ago, I bought a (then) 16 year old BMW sedan from a friend who was moving out of town. He had purchased it from an Air Force major at the local base who had originally bought it in Berlin, hence, every word that was printed, cast or engraved on the car was in German. When my friend was handing me the keys and going over everything on the car that he had translated so far, we had the hood up and he pointed out this odd looking, multi-bubbled plastic chamber on the firewall that had a hose coming out of opposite ends-- one hooked up to the intake manifold and the other disappered back into the firewall. He said that the rough translation of the label on the side of it said' "flow death chamber" but that the intended meaning was something more like' flow-deadening chamber'. It was essentially a series of equally sized chambers that were interconnected to each other by a single opening on either side (sound familiar?). The car had an elaborate but effective vacuum-powered control system for all of the vent, A/C and heater dampers that was fed, like most vacuum systems, from a tap on the intake manifold. The cluster of chambers would allow the system to have a reserve of the required pressure differential to provide the user with the ability to make multiple climate control system operations without compromising the negative pressure in the intake manifold during low RPMs. It was a tank that could be taken from on the output side without the 'loss' being felt on the input side. That's exactly what you have with your multi-chambered mass-- it's a big flow death zone. That's why, as you mentioned earlier in this thread, having a fan on the other side, even if it was just a 4" pipe, seemed to provide no worthwhile improvement to the draft of the system; because the chambers are deadening the flow by making the gases do work to get in and out of each successive chamber.

Take a moment to visualize the fluid dynamics going on inside your system as the hot gases try to travel along the intended path. Your J-tube and radiant exchange are golden. Beautiful. But after your gases leave the bottom of the barrel, that's where your trouble begins. Your slab-topped clean out box is your fist flow restriction. As the gases leave the barrel, they are still quite hot and want to stay at the top of this first chamber, but have to flow down the end wall and then out the first triangle at the bottom. This area is not such a big deal as is the comb chamber, and will work to your advantage in the end, but more of that in a minute. Your chambered mass starts with a 'first' vertical dead end chamber that is larger than the rest, then continues with a series of smaller, yet equal, vertical dead end chambers, all with openings that have CSAs larger than the triangular opening at the bottom where the exhaust is expected to flow into the next. As the hot gases enter this first chamber, they want to flow upward and fill the chamber before they try to flow back down and out the next hole, all the while having to push out the gases that were in there in the first place, only to crash into the gases flowing behind them that are trying to flow up-- this whole process repeats itself in each of the other narrower chambers, By the time your exhaust has reached the end, it's done-- all the velocity has been turbulated out of it. It's like some kind of Heisenberg's molecular death march.

Okay, let me pause for a moment and apologize for that last remark-- that's a bit cheeky, and the Heisenberg reference might be a bit obscure, sorry, but I'm trying to add a little levity to the fact that you've done an excellent job of constructing a giant passive flow damper and you're going to have to bypass it in some way if you ever hope to get the glory you deserve of listening to your beautiful front end rocket up to temp. Really, Martin, brother, I understand... the chamber... it's your brainchild; your baby; a geometric inspiration that is elegant in its implementation and carefully constructed with durability in mind; you cut every one of those damn slabs that you bought expressly for this application and spent hours upon hours mortaring the whole #*$&! heavy thing together and you'll be damned if it's the problem... dammit! (<-- levity) But, I've been there; when you see this beautiful solution to a problem or an idea that just has to work and when you spend hour after focused hour on a singular vision that has seemed so right all along... but in the end, it either just won't work, or it actually winds up compromising something else in the scope of its dysfunction. ...you just have to let it go and re-invent what you have into a new brainstorm.

So, listen, I've got a few options you could consider to kick your system into overdrive, and all require cracking into your masonry in one way or another, and all will require a bit more work, but all of them focus on using what you already have... what you've spent so much time and effort putting together... just using it in a different way, and without it having to be a complete 'phoenix from the ashes' type of rebuild.

To begin with, I want to propose an initial change that will not only improve draft, and will mean much less labor spent on maintenance in the long term, but would be required in order for you to implement either of the first two options I'm going to describe in the next two paragraphs: you would need to re-configure the slab-topped clean out box such that it exhausts horizontally out of the top instead of the bottom. Pretty much like that photo that you posted where the camera is looking right under the end of the slab towards the exit from the barrel...right there is where the gasses should exit. This is the direction that they naturally want to flow. Plus, by doing so, you will have transformed this chamber from a once-every-year clean out box into a once-every-ten-years clean out box. To do this, in such a way as to be set up for the next options, you should: remove those two large stones in between the end of the slab and the beginning of your chambered mass, reach down inside and mortar off the hole at the bottom, dig out a little in the area where the stones were, and use some ducting to make the 45 degree turn remain level as it exits from under the end of the ash slab and meets up with the top of the first slab of the chambered section. As a component part to this process, I would also recommend that you cut a hole in the ash slab and just put an end cap in it or a smaller slab on top as an inspection/cleanout hole so that when you eventually do have to clean out the pit vault, you won't have to chip away mortar to do it.

Now, I'm gonna warn you before I go into the three possible 'chambered chamber' solutions; all involve removing one of the cap slabs that cover the ends of the chambers. You'll need the access and it's the most straightforward way that results in the least amount of work. Now, on to...
Option #1: This is going to be the most painstaking of the three, but I think it would give you the best results... cut a cradle into the the top of each slab so that an 8" duct can run through the top of the chambers with as much direct contact as possible, and the cap slab fitting back on flush. This would, of course, include cutting a new hole in the heavy end slab, which looks to have already suffered some damage...eek! Then, before re-mortaring the cap slab back on, fill the chambers with perlite/vermiculite/sand/whatever. This is going to transform your flow damper into a rather sophisticated thermal battery. You can then run your exhaust out your exterior wall without the 3-4 foot rise being a problem.

Option #2: Okay, so you don't want to have to go through the hassle of cutting a circular notch in the top of each slab... well then, how about cutting a hole in just the first slab, digging out the triangular passage a bit, and then plumb ducting through by coming in the top of the first chamber, 90 degree elbow down, 90 degree elbow into your triangular passage, and then out your existing hole in the end slab (no more futzing with a compromised component). You would then back-fill the chambers with perlite/sand and mortar the end slab back on. While this solution involves less concrete cutting, getting the ducting all shoved in there and connected could be a bit of a frustrating pain in the ass... but it may be your best option as it would allow you keep your existing exhaust along the floor in case you want to cob that in and increase your mass-- which you may want to do since you are going to see big jumps in temperature when you finally do get that 7x7 feed tube blasting away on its own.

Option #3: So, let's say that you don't want to hack into the masonry of the first chamber... fine. Then just keep digging out your triangular passage so that you can shove a piece of duct in there... but you still need to fill the airspace within the chambers with perlite/sand to get the same multi-plate thermal mass battery core benefit of the first two options.

So there you have it, man. I hope you can see the possibility of one of these solutions working out for you, and I at least hope that I've helped you understand a bit more about fluid dynamics. But if I haven't made anything clear enough or if you've got a ton of questions... just ask.

Chris

Wow! Just wow! Thank you for the great ideas!

JB: I installed a piece of sheet metal to block off the baffles and immediately saw an increase in air flow! See today's vid for the full 4 hr burn. As far as basement wide air flow, I have a 4" pvc pipe with that cruddy inline fan bringing in outside air. I have a floor grate to install in upstairs floor to feed "cold air return" close to feed tube also. I also considered the height of my feed tube off the floor (16") and the blower inlet height of my furnace, both "bottom feed" air from floor level. I have a CO detector mounted 18" off the floor at mid room. I did (re)upright my outside chimney. Whats your take on running the duct up above roof level?


Colin: Ash pit; look at my early pics of the base bricks. I built the bottom of the feed tube 1 upright brick deeper that the burn tunnel floor. No missing bricks but yes, that pile of coals really pushes heat up the feed. I need to install a "table" style grate to only allow ash to fall into the pit.

Chris: WOW! Thank you! your honesty is much appreciated. And your humor is just what I needed. Thank you sir. Your explanations are spot on, man. You are right, I get it now. It would be nice if I read your comment before I got to work on her tonight. Ehh. Seeing the results from blocking off the baffles has me tossed up between your #2 & 3 ideas. To start, Im ganna tackle that first drop next to my barrel side clean out stone. I figure, with the Heisenberg's molecular death march already semi blocked and the draw getting much better, if that first drop can be fixed to increase draw also, then I'm that much closer to my victory cigar

Check out today's burn vid. BTW I mentioned beer more than I drank, lol
http://youtu.be/mnFcNTl0cNY
[youtube]mnFcNTl0cNY[/youtube]

...looks like the sheet metal trick worked...sounds like anyways as we can't see your video yet (processing time). Kudos to JB for pointing towards the best test/diagnosis...