Why using an external chimney with RMH ?

I did not build a Rocket-mass-heater (RMH) yet. So my thoughts published here are of theoretical nature. I hope some people with practical experience will contribute to this discussion.

Question: Why using an external chimney with a RMH?

The function of an external chimney (classical, usual, conventional chimney) is to suck fresh air in the combustion chamber and transport flue gas out of the oven and expel it out of the house over the roof. It ensures a steady burning and prevents flue gas, especially the very toxic Carbon Monoxide (CO) from entering the living room.
This all works good as long as the gas inside the chimney is hot enough compared with the ambient air. Than the air has a higher density and presses the lighter (less dense) flue gas skyward. The same happens if you dip a piece of wood into water. Water has the higher density, wood is swimming on it.
But this also means the usage of such a chimney is associated with a loss of heating energy! The gas has to be hot when entering the chimney to ensure the function of the chimney. The energy contained in this gas is lost. As I know the flue gas of a simple iron oven enters the chimney with approx. 330°C / 626°F. And the one of a good masonry oven has still approx. 100°C / 212°F.
At these temperatures the water contained in the flue gas is vapor. The water comes partially from the moisture of the wood and partially it is formed in the chemical reactions of the burning process. Vapor has a much higher internal energy than liquid water. It is not only the difference in temperature but also the difference in the state of aggregation. To evaporate water at 100°C an energy amount of slightly more than 2000 J is needed for every gram. This energy is released when the vapor is condensed but it is lost when the vapor exits the chimney. To condense water from the flue gas it has to be cooled down below the dew point, that is in this case about 50°C / 122°F. At this temperature the flue gas is to cold to raise the chimney. Commercially available condensing heating systems use an electrical fan to blow it out.
The RMH should not need this because the internal chimney (heat riser plus drum) acts as a pump. As long as the temperature inside the heat riser is much higher than in the space between the insulation and the inner surface of the barrel, the pump should work. So the exhaust gases can be cooled down to any temperature without affecting the function of the RMH.
I think this is one of the advantages of the RMH compared with usual wood stoves. It should be possible to reach degrees of efficiency comparable with modern condensing heating systems but without computer controlled air and fuel flow and the whole complicated and expensive technology.
By the way - these systems use a technique that can be incorporated in the RMH namely the preheating of the fresh air (from outside) by the exhaust gas. This way the exhaust gas is cooled below room temperature. Its energy is transferred to the fresh air that is than conducted in the combustion chamber.

Attached are 3 figures trying to explain what I mean.

Again the question:
Why using an external chimney and renouncing to a substantial part of the energy?

Peter Peterson : Welcome to Permies.com* and a Big welcome to the wood burning and Rocket stove Mass Heaters Forum Threads ! i expect that you will gain a
lot of members eager to follow your New Thread as possible add-ons to their Wood Stoves, but you will have a harder sell amongst the 'Rocket Mass Heater R.M.H. Crowd. You
will certainly greatly disturb the ' but we've always done it this way group " Good on You, We need new members that break us out of our preconceived Ideas.

I may not understand what you are suggesting, we need a complete exhaust system with a discharge Chimney, for the same reason we need an 'other end ' to our Alimentary
system, to keep from being constipated

I will let others tackle the many reasons pro and con for Not deviating from our present way of pre-heating our make up air ! I believe most of the systems you are talking
about use an external exhaust fan !

There is a National contest to evaluate a new crop of High efficiency Wood burners just starting testing in Washington D.C. NOW, a few will have similar technology, My Bet is
down on a R.M.H./ Masonry Heater Hybrid! Watch this space for further developments !

I went to www.condexenergy.com/how it works for the information that I am paraphrasing and thus is subject to possible error !

Reading Their graft, it seems o show a nearly flat line below 150 degrees F, which is in the upper range that we find acceptable or R.M.H. Exhaust !

While they claim a 22% savings with a Conventional Fossil Fuel Fired Furnace With a typical exhaust temp of 500 degrees F reduced down to 80 degrees F, at our upper range
of 150 degrees F a drop to 80 degrees will not show that much improvement - we often see condensation occurring within the Horizontal piping of our thermal benches, new
builders are always cautioned to slope the last 10 feet towards the last clean out !

Do we get the latent heat of evaporation constantly at 100% conversion ? No Outside Temps, prevailing wind direction and Barometric Pressure fluctuations will all have their
effect but our system rarely if ever gets out of whack, as compared to The mechanical marvel that my simple google search re-introduced me to !

Again, this is a very promising line of future research for more conventional Wood Stoves, and some day we all may gain from this line of research ! For the Craft ! A. L.

As always your questions and comments are Solicited and Welcome, Think like Fire, Flow like a Gas, Don't be the Marshmallow ! PYRO - Logically, Big AL !

* Have you been to our sister site richsoil.com ? A.L.

Sounds like the question is more about, why take the exhaust upward, when a downward exhaust might extract more heat?

This remains a controversial question in rocket mass heater design.

The original author Ianto Evans published his contagious excitement about a heater that can pump through a horizontal chimney, allowing much greater recapture of the waste heat from the exhaust.
The idea is practical in a limited range of settings - particularly when the pressure of the building, wind, and other factors does not exceed the pressures of the heater itself. Ianto Evans did much of his research in a sheltered, coastal valley in southern Oregon, where winter temperatures are fairly steady, winds are predictable in direction and strength, and in buildings with membrane-lined roofs that were not very tall (less than 15 feet to the top of the roof).

We find that for most buildings, both traditional and modern, a more conventional exit chimney is more reliable. Most buildings work a little bit like a chimney themselves: the interior is warmer than outdoors, so the building 'breathes' in at the bottom and out at the top. Roof vents and other ventilation prevent mold, mildew, rot, and structural problems that are known to occur in airtight buildings, as well as providing good fresh air to the occupants. The minimum for the rainy Seattle area is something like 1/3 of the volume of the house per hour, and of course it can be a lot more if there are kitchen fans or bathroom exhaust fans running. In a building that naturally draws air upward, any opening in the lower part of the building will experience pressure pushing air inward. The pressure affects the draft, making horizontal exhaust especially vulnerable to any secondary effects like wind gusts, eddies around corners and eaves, and pressure buildup on the windward side of the building.

Even with the vertical chimney, the pump effect of the systems means that the exit chimney surface temperature can be around 100-120 F (40 C), releasing mostly clear exhaust but sometimes white vapor, and there is still enough heat in most climates for the draft to work. For warmer climates, we recommend adding 50 F (20 C) to the hottest temperature you expect to run the heater (for example, if you want to run the heater when it is 65 degrees F outdoors, then the exhaust should be above 115 F during test firing). This is still far cooler than most solid fueled devices; in Kelvin, it just isn't that significant (the difference between 70F, and 100 F, is only 15 K, out of a total of maybe 700-1000 K that are being generated and then shed by the system).

Some builders LOVE the horizontal exhaust, and swear it's way more efficient.
Others (and this has been the majority in my experience, especially for home users as opposed to research tinkerers) have found that their heaters have unreliable draft problems with a horizontal exhaust. When they get the exhaust above the roof of their home (even at the same temperature), the problems lessen. When the exhaust is put vertically out the house to begin with, (so the vertical chimney stays warm inside the house, and doesn't need to waste as much heat to warm up an outdoor, exposed chimney), then the problems never appear. Because fitting an insulated, after-market exhaust on top of a horizontal one is phenomenally expensive, compared with either a vertical exhaust or the original 'miserly' horizontal concept, we prefer to go with the reliable method or at least design it in as an option for a future fix.

Paul Wheaton agrees with you and Ianto about the horizontal exhaust being more attractive. Paul is determined to build some more systems with horizontal exhausts, and try to find other solutions so that the horizontal exhaust can be used by more people. We will be working with him on some systems with horizontal exhausts in the upcoming workshop, as well as at least one system with a vertical exhaust for contrast. I will be excited if we find that one, simple solution works to correct all the problems of the horizontal exhaust (well, as long as that solution is not a fan that makes the stove fail during power outages).

Ernie and I hear from a lot of folks who are interested in reliable heat now, not a fascinating engineering problem. If reliability is more critical than efficiency, then a vertical chimney is a good tool.
We have had a lot of people disregard our advice about chimneys (especially critical in tall, leaky buildings). A lot of these folks come back to us to sadly report that their heater doesn't work properly, and not many are interested in re-doing the chimney at that point. We prefer to help people build heaters without known problems / complications, if given the option.

Hope that helps.
Love to hear from other builders who may have tried both.

yours,
Erica W

Thanks to Allen and Erica for reply!

>“Sounds like the question is more about, why take the exhaust upward, when a downward exhaust might extract more heat? “
Just to make sure I am not misunderstood: of course it is not enough to simply remove the external chimney from Fig.1 to get a more efficient RMH. It is also necessarily to make sure the exhaust gas
loses its heat before leaving the thermal battery. This energy exactly would be the benefit.
One way to cool the exhaust gas down is with a longer tube in the thermal battery or much better with the trick from Fig.3. But of course, this is a more challenging construction. Any way, if there is no vertical chimney, the flue gas should pass the coldest part of the thermal battery just before being expelled, i.e. the exit opening should not be close to the barrel, or at least there should be a very good insulation between. In the case of a vertical chimney it is not possible to cool the gas too far because there will be no stack effect any more.

With “external” I mean external from the point of view of the RMH. This external chimney does not necessarily needs to be outside the house.

Erica – all you write sounds plausible. As I saw (in the Internet) several RMHs build by you and Ernie it is clear to me that you know what you are speaking about. But I still hope that future developments will make the external chimney dispensable.

Because a vertical chimney perturbs the ingenious elegance and beauty of the basic Rocket-mass-heater concept !
This says a guy, who never build a RMH and has no idea about the constraints one faces when really building one.

The critical question seems to be “how effective is the internal chimney?”.
If the pumping is strong enough there is no need for the “help” of a vertical chimney.
There are two ways to make the pumping stronger.
1. Make the chimney taller. Double hight of the heat riser (and barrel) gives approx. double pumping power
2. Increase the temperature difference between the gas inside the heat riser and outside. The inside temperature gets higher when dry wood is burned in sufficient air and the insulation is good. The outside temperature could be lowered when a heat exchanger (see Fig.3) is placed on the lower rim of the barrel. The barrel is cooled by the fresh air before it (the air) enters the combustion chamber.

>“Most buildings work a little bit like a chimney themselves” - good point ! The internal chimney of the RMH has to work against this “building chimney”. But also this problem can be avoided if the fresh air for burning is not taken from inside the house but through a tube from outside like in Fig.3. In this case the feed barrel needs to be closed with an air tight cover. If this cover is closed (after feeding the RMH) there is no connection between the inner space of the RMH and the living room and the “building chimney” has no change.
If the inlet for fresh air and the outlet for exhaust gas are on the same side of the house, the influence of wind should be lower because it blows in both openings and the effects cancel each other. Of course precautions have to be taken to avoid exhaust gas re-entering the inlet opening.

regards
Peter

~ I actually came to Permies this morning for a different question, but am glad to catch this post with Erica's response. We haven't begun to build our rocket stove heater yet. Meant for husband's Trapper's shed that we hoped we'd have put together by now. Nope. Not even started, but we have enough repurposed lumber and used materials laying around to create a 12'x16'. We also have several options for the stove ... ie: various old propane tanks and a 2001 water pressure tank.
We also have a cool-lookin' hefty old barrel stove on legs which was our only source of heat 10 years ago. I think there's probably some way to turn that barrel stove into a rocket stove. I'll keep mulling that over. We don't have a welder. Maybe I can use fire bricks, or an old steel bucket. (would have to fit through the door.)
Anyway, we have yet to formulate Le Plan as to which size/style/option will work best in the smallish space, but it sounds like we'll go for a straight up chimney to get the best 'draw' and function from the rocket stove. I had been leaning towards that horizontal design until I read your posts this morning.
Thanks for keeping us informed. *waving* from Chattaroy WA. (north Spokane county)

Excellent thread - clearly posed question and the reasoned (and not pontifical) responses I have come to expect from Allen and Erica. This speaks to my condition, as my workshop foundation was started with passages for thermal exhaust before I even heard of rocket mass heaters, and both the stove and the exhaust will necessarily be on the windward side of the building. I am planning to attach a pent roofed woodshed on that side of the shop to house the stove and wood supply, but haven't quite figured out how to capture heat primarily in the workshop's thermal mass, rather than have the barrel emit a lot of heat in the shed. I'm mindful that there needs to be heat transfer in the barrel to pump the exhaust through the mass. More details on all that when building proceeds and I can post pictures and ask detailed questions. For now, I am planning to use a vertical chimney, although on first reading of Ianto's book I was attracted to the idea of a horizontal exhaust. My aim is to have a system that works reliably under a variety of conditions, rather than one that works at maximum efficiency under "perfect" conditions. Nevertheless, I'd like to hear more from Peter as well. Phase change is worth paying attention to, IMO.

Like Vicki, I have parts collected because they might be useful - in my case a lot of flue blocks that went into the foundation to form horizontal channels and a couple of sections of triple-wall stainless flue pipe that I now envision sticking out above the shed roof. I wonder how many RMH designs have been constrained at the outset by the builder's desire to find a good use for materials at hand.

Again, thanks for this fine forum. Allen, I like your "practical zeal". Erica, your elucidation of potential variables is literally a breath of fresh air.

D.Elliott : L.O.L. Thanks! my hand is still shaking ! Big Al Actually, I have a Practical test, I run my initial reaction past a " But, But, But, we've always done it this way meter "!
The higher the reading the nicer I try to be, and the more times I re-read what wrote, The longer it takes me to answer the more I tried to give a reasoned answer !

Erica, I can't speak for - Maybe Ernie went fishin'

Collection of physical quantities related to RMH

I was looking in the Internet for some physical quantities of Rocket-mass-heaters but with little success.
Is someone here who can give me some hints where to find them?
Or did someone measured them and can tell me the values?
Please have a look at the attached image. Several points are marked which are interesting for measurement. Of course, not many people have the technical equipment to measure all of them but one might know one value and an other one an other value. If enough people will contribute we will finally get a general impression of what happens in an RMH.

Description of what quantity can be measured at the different Points:

Point 1. Feeding opening.
i.) How much wood you feed ? Unit = [Kilograms / hour]. Put the wood on a scale before you put in in the RMH. Cords are quite useless in this context.
ii.) Speed of air entering the feeding opening. Unit = [meter / second]. You need an Anemometer for this.
iii.) Flow rate of air entering the feeding opening. Unit = [m³/s] = volume /second. Some Anemometers can measure this too. You need to type in the area of the opening. If you know this its easy: Flow [m³/s] = speed [m/s] x area [m²]. But what is the area if the opening is blocked by the wood? In this case a feeding barrel is useful. If you do not have one put, for the time of the measurement, a short tube on the opening and use the cross section of that tube, see Fig 4.
iv.) Humidity of the wood. Unit = [%]. There are small and relatively cheep electronic apparatus which can measure this.
v.) Relative humidity of the air. Unit = [%]. You need a Hygrometer.

Point 2. Combustion chamber. Temperature. Do not forget to tell the Unit °C of °F or what ever you use. I would suggest °C.

Point 3. Exit of the heat riser. Between the top of the heat riser and the bottom of the barrel. Temperature.

Point 4. Gas inside the barrel. Temperature. At this point the gas lost a part of its heat to the bottom of the barrel (cooking plate)

Point 5. Gas inside the barrel. Temperature. Just before entering the thermal battery.

Point 6. End of the horizontal tube.
i.) Temperature
ii.) Amount of condensed water. Unit = liter / s = volume per seconds. Better would be kg/s



Point 7. Exit of the vertical chimney. Temperature, soot, CO content and iii.), iv.) and v.) of Point 1.

Point 8. to 12. Surface temperatures. An Infrared-thermometer is probably the best choice for this.

I suggest to use SI units (International system of units, kg, m, °C or K, liter …) but I know, not everyone is used to this. Therefore, what ever unit you use, do not forget to write it next to the numerical value.

Is someone here who measured the thermal efficiency of a RMH, or who knows such a value?
Thermal efficiency = (energy that remains in the living room) / (entire energy contained in the burned wood).
In other words the thermal efficiency is the ratio between what you really get, to what you would like to have. You would like to have all the energy contained in the wood in your living room but you get just a part of it. The rest is blown out the chimney.
With averaged values from Point 1, Point 6 and Point 7 the efficiency can be calculated.
I am afraid no one will read such a long text – so I stop here.

best regards
Peter

I have an 8" RMH with a near 4-foot riser(a foot taller than usual) and 25-30 feet of piping in the basement of a single story house and experienced low draw/suck/flow until adding a chimney outside which went up to about the mid to upper part of the upper floor. I had originally had a horizontal output at ground level(walk-out basement). I still have a few things to modify and I changed things from the designed plans so I'm on my own learning curve. Definitely need a chimney at least as high as the top of the riser pipe I would say, and higher if in a tall house(or basement for sure).

Vicky Barton : I want to send you to our sister site- richsoil.com , and click on -/- go to Rocket stoves, there is a video there about a real attempt to integrate a
conventional Wood Stove with a Rocket Mass heater/Thermal bench, with Ernie wishers explanation of why it was not ' could not be as good as a Rocket Mass Heater !
Hope this helps ! For the good of the Craft ! As always questions, comments are solicited and Welcome, Think like fire, flow like gas, Don't be the marshmallow A. L.

Gary Park : All basement locations with Rocket Mass Heaters R.M.H.s have problems, you seem to be doing better than most !

I want to suggest that you find the permies toolbox on the right side top of the page, click on search, and on the new page, find the 'Google Search field'
and enter 'Basement locations of Rocket stoves '' Basement locations of Rocket Mass Heaters' and search Within Permies! Ernie or Erica's reports on
problems with these locations. With a walk-in basement you are marginal, a full basement in a two story house rarely works ! For the Good of the Craft !

As always questions, comments are solicited and welcome ! Big AL !

Allen Lumley... thanks for the referral to http://www.richsoil.com/rocket-stove-mass-heater.jsp . I'd already seen that particular video you suggested, but I did watch it again. That wood stove is a small box-like thing that they attached to a rocket system. What I'm talking about is a very large barrel woodstove that we already have and the idea is to build the rocket stove INSIDE it somehow without cutting it apart. Leaning towards stacking fire bricks inside to create the 'stove', and maybe packing cob around the front edges to house the chamber where heat flows before exiting the chimney.
I should take a pic of our old stove. I looked for one online and can't find any even close. Ours is about 55-gal., heavy steel shaped like an oak whiskey barrel with 1/2" rivets at the seam. Someone had installed a door w/damper using a kit made for barrel stoves. There's a hole at the back end of the barrel where the chimney pipe sits. We had a friend create a flat steel unit to straddle the top making room for two pots. And the stove is currently lined around the bottom half with firebrick. I can vouch for the fact that this unit is a creosote-creatin' SOB, as I spent enough days on the roof cleaning the pipe. We had no other options for heat back then, but we won't be using this beast indoors again unless we can transform it into an efficient rocket stove.
Thanks for all input.

Vicky Barton, the barrel stove you're reffering to; is that an old pot belly stove, or a barrel laid horrizontal with a door? If it's a pot belly stove, my thoughts would be to use that over the heat riser, (you would have to cut the bottom out of it of course) instead of the typical barrel, which Would be quite attractive I think!

Vicky Barton : I just thought that you might have missed Jeff Ryckwa's very interesting 'whatcha-ma callit' as a working adaptation of the idea behind a frankinstein mish/mash of
parts that works, at least in this iteration!

Two points, Jeff is a renter, and can not do any thing his landlord will be unhappy with ! He actually has a working R.M.H. (without the Cob or Thermal Mass in the basement, this is
a second attempt to have the freakish efficiencies of a working rocket, and keep a roof over his head ! we can only hope for a story book ending like 'A Christmas Carol'

While something like this with your barrel stove could be superimposed on top of it, and certainly a horizontal run of Cob Bench/Thermal Mass start from where he has his discharge
elbow, you would actually want to strip out any fire bricks ! Still for what its worth, it is a 'Rocket Burner' sans Mass and it appears to work, time will tell,for the good of the Craft !

Think like fire, flow like a Gas, don't be the Marshmallow ! As always, your comments are welcome and solicited ! PYRO - Magically BIG AL !

Could I just use single wall chimney pipe? Jay

A good chimney has to be insulated above the roof line to keep the gasses warm and rising.