Splitting the exhaust line through two thermal masses

I'm fairly new to the rocket mass heat concept, but I have worked with potbelly stoves before and I have a small coal forge, and have had a few years of work as a mason. I am currently in the planning process of building a small, primarily adobe, RMH in my tiny greenhouse. I intend to use a U-shaped thermal mass running along the sides of my greenhouse. I was wondering if anyone had successfully split the exhaust line and run it through two separate thermal masses, then back towards each other to come out through one point. I just watched a video where one had failed due to the 4 inch line used in the thermal mass, but I wondered if such line could be reduced and split from 8 inch and still function correctly.

What I gather in my own research into how these function, after the combustion chamber the name of the game is air volume (CSA, cross sectional area I believe it was referred to)...too big and the air doesn't flow fast enough resulting in ash particle build up, to small and not enough volume can be moved resulting in back draft or similar effect. Is there an optimal length for the duct running through the thermal mass? Also, is it preferable to vent the exhaust vertically or horizontally?

My plan is to use structural steel (have some tubing and C channel laying around) for the fuel tube leading up to the combustion chamber. A riser built from fire brick and topped with flue liner, a 15 gallon drum (also laying around), and adobe brick for the vent through the thermal mass (likely I'll use stovepipe for anything in livable structure, but I think it should work for the greenhouse build). The thermal mass will be made of adobe and pea gravel/sand mixture (creek rock and sand collected locally).

I'll post photos of the build once I've settled on a method for my madness, updates as the project is progresses, as well as a follow up to show what it does next winter. The goal here is to grow my seedlings in late January/February in Zone 5. Most winters here in Central Missouri are fairly cold, but recent patterns make already unpredictable Missouri weather even more elusive. If it works, I will be building one or two in my home for supplemental heat (and so my wife, who freezes every winter in this turn of the century house, will have a warm spot to sit).

Seth Hofstetter : While it is possible to split the flow of the hot exhaust gases down two sets of Horizontal piping within the thermal mass, and has been done anecdotally
one of the pipes will dominate, and you probably will never Know why! While there is potential for some savings in materials/parts not used, your design must be
clever to deliver such a savings!

A little physics, a little Math, when you start out with an 8 '' pipe with a Cross Sectional Area of 50.24 sq. in. and reduce its size to 4 '' pipe you end up with a Cross
Sectional Area of 12.56 sq. inches. Thats right, cut the size by 1/2, reduce the Area to a 1/4. Now even with (4) 4'' pipes having the same amount of sq. inches as one
8 inch pipe we are not flowing the same amount of material - liquid or gas - through them. This is because the ratio of the total surface area to the total volume
( 50.24 sq. inches ) is higher in the (4) 4 inch pipes, and we have to account for the higher frictional losses and loss of flow in the (4) 4 inch pipes !

If we take a Rocket mass heater RMH and give it a cross sectional area of 8'' we say we have a 8'' system. If at the Transitional Area (where the vertically falling Exhaust
Gases are funneled and turned at right angles) we divide the flow into (2) 6 inch pipes to flow into the horizontal chimney of the Cob Thermal Mass, they will each have a
Cross sectional area of ~28 inches~ with increased friction losses and a larger area in sq. inches to flow through we will have some slow down of gas flow ! This can be
dealt with by adding height to our Heat Riser and barrel and also by adding onto our final Vertical Chimney !

Anything that gives us the maximum reduction of the temperature of our hot exhaust gases ~140dF~ should be considered optimal, with an 8'' system that would be
about 40 feet, subtracting 5 ' for every elbow and 'T' in the system (a six inch System should run 30 ' w/ a subtraction of 5 ' for Elbows and 'T's) Any material lining ether
the Burn Tunnel or the Heat Riser and all of the Thermal mass should have walls as smooth as glass, rough walls will increase friction and decrease flow and distance !

It is always preferable to vent the final exhaust vertically to a point 3-5 feet above the peak of the roof or past the top of any other nearby tall object !

Steel is Doomed, it is unlikely that you will get through even one heating season with combustion zone parts made out of Steel, I am sure our fellow members will repeat
that for you regardless of what you may have seen on You-Tube !

I have no problem with a Thermal Mass made to be removable for one or two seasons! After two green house heating seasons if you have not finalized your RMH / Green
house design and switched to a more efficient Cob thermal mass you are just being lazy!

My Best piece of advice : Go to rocketstoves.com to download your PDF Copy $18.oo of the brand new third addition of Ianto Evans' Great Book 'Rocket Mass Heaters'
with over 100,000 RMHs built world wide, most have been made following 'The Book', and 95% of all the 1st time builds (that worked!) were made from 'The Book' This
will allow you to speak the same language/vocabulary when speaking of the parts of the Rocket Mass Heater, its shape, size, and orientation to the other parts, and better
allow you to calculate the Specific Constant Cross Sectional Areas of all RMHs with No more than 5th grade math you will learn to Do in your head! The Time, Effort, and Monies
not wasted, will more than pay you back, and it will always be handy to refer to when reading these Forum Threads ! For the good of the Craft ! BiG AL

Allen,

Thank you for the input! I am working from a hard copy, first edition (I believe) of the book I bought at a garage sale some time ago. Is there alot of additional material in the third edition?

Has anyone ever tried lining the hot spots on a riser made of steel with fire brick or perlite/clay cob? To my understanding, the hottest point for the riser is where the "jet" slams into the side wall after the burn tunnel. I assume, from your response, that most attempts at these result in the steel slowly eroding and combusting from the heat and temperature change. I think I will just build the fuel chamber from steel, and then fire brick for the burn tunnel and riser...pretty much following the books design layout.

Thank you, as well, for the information on the exhaust tube. I suppose it stands to reason that one of two exhausts would exhibit less resistance to the air movement. I will attempt to make the ductwork with cobb and a PVC pipe which will pull out once the adobe dries.

Also, I'm thinking of welding on fins to the side of the heat exchange barrel to act as a heat sink and hopefully help disperse more heat into the room, possibly cooling off gasses inside a little faster...my only concern is losing some of the clean combustion if the gasses move out of the barrel too fast.

Seth Hofstetter : I Am still waiting for my copies to come, I have not seen the new 3rd addition. I expect an improvement in the way that the Transitional
Area (that carries the vertically failing gases funneling, and turning them to flow into the horizontal Chimney)is described along with ' suggesting ' a
much bigger Cross sectional area at that location, a few clearer pictures added. If you have the language down correct you will be able to talk with your
fellow members here at Permies and quickly clarify any issues you have !

Steel really is doomed, about every conceivable combination has been tried several times, With the 6'' system you will have a feed tube opening of approx.
5.?'' by 6'' and the purpose of the larger surround/opening made from a grease drum is to allow you to burn wood a couple of inches longer than the depth
of that FeedTube, and to give you a way to cover up the Feed Tube opening after the fire is out ! Ernie and Erica Wisner our moderators here at Permies
Rocket and Wood Heaters Forums have abandoned this practice as it is false security at best and can allow the air just above the opening of the Feed Tube
to get hot promoting smoke back.

I have not heard of any one successfully pulling a dummy core out of Cob, Someone must have done it. Several years ago there was a Chimney Sweep
franchise that sold its members a long blow up bag that could be inserted into an existing older large-bore chimney. After careful placement you were
supposed to pump in a liquid ceramic material that was both insulating and refractory, and made a perfectly sized, re-lined, Chimney. Very few Technicians
ever took the training or successfully installed one of those chimneys and they were not cheap enough and were priced out of their market !

Unless you go out of your way to create a very high Heat Riser there is little chance of the Hot Exhaust Gases going through too fast! Weld on all the fins you
want, most of your heat- 60% ish will still have to be dealt with in the Cob ! Big AL !