Need for conventional smoke stack on RMH

I've seen the idea referred to over and over in many of the posts around this topic that there is a need for a conventional flue,,going so many feet over the closest roof surface within so many feet...---also the need for expensive insulated pipe and the like..

And after putting together and running my 6" rmh this winter with no such worries --in fact i was under the impression that tall chimneys could actually be a hindrance to effective operation-- i still wonder every time i see the suggestion of adding a standard flue system to what i thought was supposed to be a way to eliminate the use of such costly materials.

Is this the sort of thing where initial design can eliminate the need for a tall stack, and if so, maybe it needs to be given equal time.

I did see one post referring to Ianto taking prevailing wind, house structure design, and exhaust exit placement all into account when installing his RMHs

in my case, prevailing winds are from south west--with occasional gusts from other places, i have a 12 sided house and my exhaust exits on the north side of the house,,My RMH performs well, very little to no smoke at startup, which is not to say that it did not act like a flame thrower when at one point there was a freaky wind gust from the north, but since then i have added a baffle outside , and it would never occur to me to go back to a conventional strategy for exhausting the stove. In fact, i have run wood stoves a long time and often seen similar gusts push smoke into the room while using a code approved stack system in an airtight stove.

My understanding was always that the basic design of the RMH eliminated the need for the draw/pull/draft required for a conventional wood stove,, and instead was based on the push of gasses out of the stove

That being said, i have seen numerous stove designs that make me wonder if the people were reading the same stuff i was,, with 40 feet of pipe and 40 feet worth of elbows on a system that should only have 30-40 feet total

anyway, from my perspective it seems that the RMH is well thought out, extensively tested, with simple but explicit directions on it's design parameters. Follow them religiously and enjoy the fruits of an easy, inexpensive system,, ignore them and redesign and get caught in the same old stuff. I mean, it's one thing to push the limits of knowledge by testing new designs, and another thing to depend on an unproven system just because you want it to work and then get caught in disappointment and wasted time and energy

Bob Day : I agree with everything you say, but feel it is important to add a Large Caveat to your otherwise Clear and complete coverage of this topic !

I truly outclass everyman in being tight with a buck, and it Does pain me to erect beautiful Rocket Mass Heater RMH, with mostly Scrounged parts, and then
spend more on New insulated chimneys than I spent on the Whole rise of the system ! This Could Be a forum topic by itself ! However -

I live in an Economically depressed area that while not as Bad as Detroit has been depressed Much longer, and most people around here are used to trying
to do more with less, If I use an old vertical chimney I found in my House Because it is safe to use for the Low temperatures I will be drawing thru it, I am
being very cavalier about the next guy, Who could be my Dumb-assed Son-in-law, who Thinks one wood stove is as Good as any other and hooks up his
" Blast 'O Heat 5000 '' up to that chimney my carelessness could potentially be fatal to my own Grand Children, ANYONES grandchildren !

Some places I am willing to go Cheap, some places the Very best ( and Smoke, Heat and Carbon Monoxide Detectors ) are the cheapest way to go !

Thanks for letting me borrow your soap box ! I just wanted to get that off of my chest ! 40 + years Paid and Volunteer Firefighter E.M.T. Big AL !

allen lumley wrote:

I truly outclass everyman in being tight with a buck

--- You sir, have insulted my honor --- I am by far the tightest with a buck. I hereby declare a thumb war. Young Landon may also feel the sting. I think he can out-cheap us both.

People used to call me a cheapskate, or to be kind "thrifty" but all I can say is I envy you guys that have a buck to be tight with.
I've been disabled (no disability pay) and unable to work more than a few hours a week for the past 7 years.
Almost everything I do to be self sufficient is out of absolute necessity. I've scrounged 80-90% of what I have put into my aquaponics green house. Including the fish I caught in the river.
I certainly don't feel sorry for myself I just deal with whatever comes down as it happens. I see people worse off than me and try to do what I can for them when I run in to them.
Not exactly on topic, I just had to spill my guts between posts helping those I can on the forum
I might revisit this thread and post something relevant.

I believe it is referred to Stack effect, As the chimney or exhaust heats up it draws faster as heat rises, if your chimney rises above the house it creates a better draw and drives the exhaust away from the house. But depending where you live and the design of RMH. Snow could block the exhaust if it gets covered.

yes, snow can block a low horizontal output--that's why i went out and shoveled the snow away from the exhaust after this last snow--just enough to get the stove started, then the minimal heat did the rest keeping the snow melted as it fell


i suppose it could get so cold and the snow be falling so fast that operation of the stove could be threatened in which case a vertical stack would be required-at least until the snow covered that as well

I think the way I've come to summarize it is:

- If you live in a conventional building (especially a tall one), a conventional chimney may be the cheapest reliable way to exhaust any wood-burning device including a rocket mass heater. For some rocket mass heaters such as Peter van den Berg's batch box, the system depends on this exit chimney to maintain under-pressure throughout, allowing things like his secondary air feed to work, and producing one of the cleanest-burning systems ever seen. For some buildings, the building itself is such a good warm chimney that any opening installed below the roof line will end up serving as a de facto air intake.

- If you live in (or are heating) an unconventional building, especially a short one without upper roof vents, you may find it's worth experimenting with a downwind, baffled, lower-type exhaust. I encourage leaving an access such as a cleanout port where you could convert to a conventional chimney if needed, but I've seen some clever problem-solving people who have made it work. So much depends on the building more than the heater itself; there's only so much you can do, and you need to stack the odds heavily in your favor.

- There are a few situations where you might be able to build a rocket mass heater that would draft successfully with a 'drain' type vent, but not with an upward conventional chimney. I am not convinced that such a system would run reliably under many conditions, and I would tend to think of it as a 'parlor trick' more than a reliable heater. Example: a greenhouse heater where the greenhouse beds routinely stay below the dew point of water due to a lot of damp soil, and the exhaust condenses and runs out denser than air. Especially if it's warm or mild weather ... meaning a conventional chimney would need to be pretty hot to get any lift ... and you are running the heater in a greenhouse for some reason anyway. Incrementally working the warm end of the bench toward tropical germination temperatures, perhaps, and running the cold exhaust through the cold end of the bench despite diminishing returns because it allows you to exit downwind or something. I can conceive of the system, but it's hard to see myself wanting to build it. I think you could get equally useful results with a more reliable heater, or possibly even a sheet of tinfoil to double your solar collection for the warm end of the growing bed.

What's involved:

There are so many variables including wind direction (or unpredictable gusts), the weathersealing or lack thereof at different elevations of the house, how much warmer the home is than outside air and how this translates to the positive or negative pressure at different heights in the building.

In general, the most negative pressure in any warm chimney-like structure, like a house, is down low. Stuff is sucked in down low and pushed out up high, in other words.
When considering relative pressures, the height of the through-wall doesn't matter as much as the chimney opening. A chimney opening that is in the top half of the house, or comparable to whatever the highest other vent in the house might be (the eaves, in the case of Cob Cottages), is in much better position w/r/t the house's own positive/negative pressure gradient. An opening at a level below the lower third of the house will likely want to draw inwards (backwards).

If you try to go out low and then build back up to a proper height outdoors, with the available, cheap, single-wall pipe, you can have chimney stalls and backdraft problems. An exterior chimney will get chilled below the dew point of water, and the exhaust gets too dense to rise. Wood combustion exhaust contains more than half a pound of water for every pound of wood burned - more if the wood was wet to begin with. So it doesn't need to get colder than outside air, just cold enough for water at whatever concentration (it varies with the air intake and fuel burn rates) to reach its dew point, and start condensing on the cold chimney walls. The water runs down, evaporates when it hits the warmer parts, the exhaust gets steamier and more water condenses, and eventually you have a super-saturated water fog that just falls downward instead of being able to rise at all.

If you need to keep the chimney warm enough to get up just a few feet outdoors, you can build an improvised LT exhaust by insulating your own stovepipe or ducting. The insulation needs to stay dry, so you're looking at not just wrapping some rock wool on there, but cladding it with more sheet metal and doing storm collars (flashing) and so on. If the insulation gets wet, or God forbid freezes, it's a chiller rather than a warmer.
We've seen this done, and it did help, and that plus a weathervane chimney cap saved that particular owner purchasing another 5 feet of chimney, or doing a through-roof at a complex point on the roof where he'd had problems with ice dams before. And they live locally, so if they sometimes have to invite Ernie over to get the stove going for the year when its chimney is too cold to draw properly, well, it's tolerable. They have a furnace for back-up, so the stove is optional.

This process involves basically working your way back toward a proper chimney incrementally, and stopping whenever you hit your budget or personal tolerance for how reliable the stove needs to be.

The problem is that retrofitting a poor chimney so that it resembles a proper chimney, and can safely be used as one, sometimes gets more expensive than the proper chimney would have been. You can use single-wall pipe inside the room, for example, and just do a short section of insulated or triple-wall with proper clearances through the roof. But if you do the same length of chimney outside you may end up needing to insulate the whole height of the chimney, and it still will lose heat faster than it would have inside the building.

The neighbor above at least followed our advice to go out as high up the gable-end wall as possible, and as a result, he would only need a few more feet of insulated exterior chimney to get back up to a conventional chimney height. His heater's in an addition under a low eave, so running the chimney out near the center of the building and exiting at the ridge didn't seem feasible in any case.

For most of our clients and people who ask online, I tend to assume they will be better off considering how to do a proper chimney, or leave themselves that option at least, rather than selling them on the idea they don't need one.
They might need one, and it would be a shame to encourage them to put in all that work and effort and then have it not work for a predictable reason.

For people who flat-out can't afford a proper chimney, and have the resourcefulness to scrape by anyway, a thorough understanding of the forces involved will just be more tools in the arsenal of unconventional success.

Hats off for making it work. I'd like to know if you believe your system has proven me wrong about any of the above. Always interested in learning something new.

Yours,
Erica W