Could an 8-inch J-tube system have the vertical exhaust reduced to 4 inches

First, the firebox of an 8-inch j-tube is really small.   Smaller than the teeny tiny conventional wood stoves that have a 4-inch diameter chimney.

Second, the batch box systems have a much larger firebox, and still have an 8-inch exhaust.  

Third, by the time that the exhaust is ready to be taken out, it has cooled dramatically - so it has shrunk a lot.  

Fourth, when I look at the exhaust during a roaring burn, I see a tiny piddle of exhaust coming out.

I have two motivations for considering this idea:

601:  4-inch duct or stovepipe is a helluva lot cheaper than 8-inch.

602:  A 4-inch exhaust has one quarter of the CSA - so less cold coming back from the outside; smaller overall hold to the cold outdoors, less metal conducting cold to the indoors from the outdoors, less heat that might move up from the mass up through the exhaust when we are not burning.


My spidey sense is suggesting that this might improve the efficiency of an existing rocket mass heater by 15% to 25%.   So if a rocket mass heater currently burns 1 cord of wood each year, this might take it down to 0.75 cords each year.  

What am I not thinking of here?

What about insulating the chimney with a non-conductive coating? Rated to 150F is plenty for an rmh.

Over at Donkeys, member Wolf1004 heats his house with an 8" J/bench RMH exhausting into 6" round (class-a) chimney with good results, for over 4 years now. Following that example, one would think a 6" J, 6" bench ducting, exiting vertical into a 4" chimney may be doable. But I wonder about the friction in 4" pipe especially if it's really tall chimney. Maybe lash together a test chimney using the cheapest available HVAC ducting, roll it up overlapping the seam so it's 4" ID, high temp aluminum foil tape it up, and try the thing out on a test stove. To keep the exhaust gases hot for improved draft, do some kind of insulation wrap. Cheap building R11 fiberglass comes to mind, just for a quick test of course.

Draft, in a box stove is induced by a very hot exhaust temperature. Counteracting the friction in the 4 inch pipe.

I'd say, 8 into six is doable, 6 into 4 too. But 8 into 4, i would be careful. 6J into 4, i have done. With lots of trouble for startups.

6 batch worked better, butr with just a 3ft restriction to 4 inch. And i still had to have a fan for easy startups.

As well, imagine, the hot'ish gases out of a rocket, let say 80C° entering the chimney, going down to may be 60C° at the top of the chimney. Those densifying gases have to be pushed out, by gasses buoyancy and by the push of the unmixable gasses bellow.

I think 300C° entering the chimney after a box stove, and 200C° at the top work better fighting the friction of small pipes.

JMHO.

My recent build thread on the match box mass heater has a firebox bigger than 8" and the final stack is only 4".  

I also mentioned weeks ago in that thread that the exhaust should cool and contract enough by the time it reaches the 4" stack, after the 5" mass.   It works well but the main difference is the batch box design is already restricted with a tiny flow rate due to the small intake opening and the Venturi, which both only allow so much CFM/flow.  

Have you tried this idea in question?   Usually if I want to find out if something works, I just try it.   😋

My 8" J-tube RMH with brick bell goes to a temporary 6" stovepipe exhaust, with a few sections of salvaged high-grade insulated chimney, because that was what I had on hand. It also jogs through the wall at 7' from the floor, and only goes 4' above that, 7' short of the proper roof-clearing height (I'll get to that when I have enough clear dry days to be three stories above grade on the roof.)

With all these factors, I have always had some draft before lighting the fire, and it has always taken off immediately, even before there is any warmth going up the chimney. I have not yet observed the draft to be hindered by any factors beyond the feed.

Inside a vertical, circular cross section duct which is hotter inside than outside (this difference is called delta-T) there's a fast streaming core. The rest of the gases in the duct are built up in layers, the closer to the wall of the duct the lower velocity and the lower the temperature. Against the wall, the last microns are effectively at a standstill, causing all the drag that is evident in a round stove pipe, apart from temperature differences.  It's easy to see why the 4" pipe is not as capable of transporting gases compared to an 8" pipe, the high velocity core is much smaller and the wall is closer by. So drag is a comparatively large factor. I count this as a minus point.

The volume of gases which are inside a J-tube does heat up and expand, cools down and contract, that's true. But by how much and how effective is that? Complete combustion is a misleading phenomenon, since only the moist is visible we tend to think it isn't much. Air is expanding not as much as one would think, from 68ºF to 1900ºF about 4.5 times. In the chimney there need to be some delta-T left for to keep the draft going, say 200ºF. At that temp, air has been shrunk to 1.2 times it's original volume, so inside the J-tube it's 3.3 times its volume as compared to in the chimney. So this seems to predict the chimney pipe shouldn't be smaller than the rocket's system size by a factor of 3. An 8" pipe is roughly 50"sq., and a 4" pipe is 12.6"sq. csa, this is a factor of 4 smaller, fat chances are a 4" pipe would strangle the 8" system. This would be the second minus point in my opinion. But a 6" pipe looks like it should work in this respect.

But... this is only about drag and air expansion, what about water? This is another story altogether, at 212ºF it will expand roughly 1500 times! In the fuel itself is only the left-over moist from the drying process, which could be anything from 25% to 10% of fuel weight. I am not as good in calculating in cubic feet so I leave that to somebody else. On top of the fuel moist there's another effect, the combustion process itself produces heat, carbon dioxide and... water! This is quite a bit more, some people say it's close to half the original weight of the dry fuel. If this need to expand 1500 times also there's no chance on earth one could vent an 8" system with a 4" stove pipe, even when it is free. I would count this as negative 3.

When I had to choose and being tight on money I would choose a 6" pipe to vent the 8" system but take every precaution to never restrict the system anywhere else. The slightest constriction would then muffle the rocket because there's no more slack left. By the way, a batch box is quite another animal, not really comparable with a j-tube although they make the same peculiar noise most of the time. The burn rate of a BBR is much higher so it is better to have a chimney that's bigger as opposed to smaller than system size.

Nearly forgot: even when it would work to vent an 8" system with a 4" pipe starting it cold would be a nightmare. A rocket heater isn't a self-starting apparatus, there need to be a delta-T before there would be any draft in the system so the fire wouldn't smother itself.

For temperatures and expansion rates: see this very comprehensive piece of text from Erica.

John - your match box firebox dimensions are not comparable to a J-tube cross section; it is sized more or less for a 4" system, so is right in the recommended zone.

But generally could you please explain how come this is opposit the theory described in Ianto Evans and Leslie Jackson book ("... The  size  of  the  cross sectional  areas  of  all  parts  of  the  stove’s internal  ducts  or  intestines  should  never decrease  below  that  of [the exhost]. Thank you.

arnon raab wrote:But generally could you please explain how come this is opposit the theory described in Ianto Evans and Leslie Jackson book ("... The  size  of  the  cross sectional  areas  of  all  parts  of  the  stove’s internal  ducts  or  intestines  should  never decrease  below  that  of [the exhost]. Thank you.

First, Ianto's writings are pretty old and the book is pretty outdated.  It is a lovely book painting a lovely picture.  And it was a magnificent foundation for rocket mass heaters ten years ago.   But so much has changed and Ianto's book does not reflect these changes.  Erica's new book is now the foundation.   And there is a lot of room to optimize rocket mass heaters, so Erica's book might get rusty and dusty in a couple of years.

Peter,

Part of what I am reading in your post, between the lines, is that "if you are going to use a 4-inch exhaust on an 8-inch system, you MUST make sure that you have extracted so much heat that your exhaust temperatures are very close to room temperature.  A temperature of 95 degrees could work, but a temperature of 140 would not."

The next point is the point you make about water ...   let me try some wording to see if I can get some agreement from you:  "this choice might reduce overall wood use by 10%, but this type of optimization comes at a price - the system could smoke back with wet or green wood."

??

I am currently doing the tests to measure my wood use for this winter.  There is a possibility that it might be just half a cord.   As I measure the wood, I am looking at things to improve the efficency.  So if I end up using 0.7 cords of wood this winter, what all can I do to get that down to, say, 0.4 cords next winter.   Already I am thinking:  drapes on the windows, a covered porch for the front door, and optimizations to the rmh.  

One thing I am thinking of is re-routing the duct in such a way that I make something of a stratification chamber.   And another is this idea of dramatically reducing the size of the vertical exhaust.


This particular 8-inch system has already had the wood feed reduced to the size of a six inch system.  So reducing the exhaust to a 4-inch exhuast would be extra safe on this system.  

Basically, this system runs a little too fast now.   We reduced the size of the wood feed and added loops within the mass to slow it all down.  It used to burn so fast that it would suck the flames off of the sticks.   It still roars pretty fast.   So I want to slow it down a bit.   I can feel quite a bit of heat going up the exhaust.   Heat I would like to keep inside.

paul wheaton wrote:The next point is the point you make about water ...   let me try some wording to see if I can get some agreement from you:  "this choice might reduce overall wood use by 10%, but this type of optimization comes at a price - the system could smoke back with wet or green wood."

In general, I think that is true. When most of the tolerances are taken out, every hiccup would stall the system. Your home system is running too fast, probably because its heat extraction isn't optimally. The barrel is stainless for example, which is not as good in extracting as a mild steel barrel. As a result, the chimney temperature is probably on the high side of things. Wouldn't it be better to extract more heat instead of slowing down?
Not for this winter of course, a small barrel on top of the pipe run could make already a world of difference. For that, digging in the gravel box is in order, not very sensible in winter time I would think.

Peter van den Berg wrote:Your home system is running too fast, probably because its heat extraction isn't optimally. The barrel is stainless for example, which is not as good in extracting as a mild steel barrel. As a result, the chimney temperature is probably on the high side of things. Wouldn't it be better to extract more heat instead of slowing down?
Not for this winter of course, a small barrel on top of the pipe run could make already a world of difference. For that, digging in the gravel box is in order, not very sensible in winter time I would think.

I'm thinking that this could be things to try this summer.

Yes, the stainless barrel does not transfer heat as well as a regular barrel.   However, I would like to complete the overall aesthetic before making a switch like that.

extracting more heat:  that is why I am thinking of reworking the innards to be more of a stratification chamber.

I like the idea of trying the 4-inch vertical exhaust, the stratification ideas, the insulated curtains and the enclosed front porch - to see if we can get our overall wood usage to be less than half a cord for a winter.  If it doesn't get us under half a cord, then I would like to try adding rocks and more granite to the mass.  And then maybe come up with some more ideas.  And then maybe switching the barrel.

----

Back to the 4 inch vertical exhaust ....  forget about my system for a moment and just think about a regular 8-inch j-tube system ...  I think that a 4 inch vertical exhaust would work given a few important criteria:

901:  a well made rmh with a strong draw.  Possibly a ceramic fiber core.

902:  the vertical exhaust must be placed close to the barrel.  Probably within 2 inches.

903:  the system must be quite efficient, thus keeping the exhaust temp low.  Typically not exceeding 120 degrees.

904:  the 4-inch vertical exhaust would make the system 10% (possibly 15%) more efficient, but now wet or green wood may lead to smoke-back.