Horizontal duct and chimney length for 6 inch RHM

[color=black] [/colorThis is our first build and any useful advice from experienced builders would be greatly appreciated.

Our situation is this: Our cottage has a 576 square foot slab for a floor, our existing 6 inch wood stove chimney is about ~ 14 feet long from the top of wood stove to the chimney cap, We have 8 inch SIP paneled walls and 12 inch SIP paneled roof so the cottage is well insulated. The 6 inch chimney dictates the size of the RMH we intend to build be a 6 inch system since everything is set up for that and we are not heating all that big an area.

In  “The Rocket Mass Heater Builder's Guide “  that I got from our local library system ( obviously they are in on Paul Wheaton’s  plot to rule the world)  in Chapter 4 , Step by Step Construction Example, they use an 8 inch RMH as the model so not much use in my situation. For some reason I recall 42 feet of duct work being the absolute limit for a 6 inch system but for the life of me I can’t seem to find that specific info. in the book. If anyone can guide me to that page or quote me the right number I would be grateful.

So here is the question, is the maximum allowable amount of duct work (whatever the number)  considered the total of both horizontal duct and vertical chimney or is the chimney considered be less of a drag coefficient since warm rises and is not struggling to go sideways and therefore I can have a greater amount of horizontal duct work in the thermal mass.

Our goal is to have 2 lengths of 6ft. - 6 inch duct plus 1 length of 3ft.- 6 inch duct and 5 90 degree 6 inch elbows/tees.
All total this would come out to the equivalent of 54 linear ft. 5 90 degree elbows/tees = 25 ft. , horizontal duct = 15 ft., Chimney = 14 feet.

Is this practical? Is I don’t want an RMH with slow to no draft.

Any feedback you could give would be greatly appreciated!

Thanks

Hi Roger, from my notes (via E&E Wisner) the maximum horizontal ducting length on a really good quality built 6" RMH is about 40 feet with no turns. Subtract 10 feet for each 180 degree turn, 5 feet per 90 degree turn, 2-3 feet per 45 degree turn. Don't count the last 90 degree turn exiting your mass that goes vertical to connect to the chimney pipe, nor is the vertical chimney pipe length counted.

One way to help avoid a sluggish system is to test for natural draft before making your build permanent. I.e. lay out all of the horizontal ducting where it's going to be, all connected up with elbows, Tees and etc. and connect it to the chimney. Test for a natural draft at the other end that will be connected to the RMH barrel manifold. With your 14 ft. chimney you should have a fairly good natural draft, and with that you're good to go. The draft can always be improved by adding additional chimney height too.

Thanks Byron,

I will definitely try the draft test. Your reply took a load of concern off my mind.  I finally found mention in " The Rocket Mass Heater Builder's Guide"
on page 229, Heat exchange-Channel Length. This is in total agreement with what you say.   You mentioned  notes via E&E Wisner, did you attend one of their workshops? I'm glade to have the my confusion regarding distinction between chimney and heat exchange duct work clarified. About 11 feet  of the chimney is fully exposed on the inside and should be nice and toasty compared to outside temps.

Thank You again

Roger

I would like to have, but no, my build notes come from E&E's early permies posts, that have long since been buried in all the thread traffic over the years. If you dig deep enough you'll find them. Good that you have the Rocket Mass Heater Builder's Guide -- that's a must have and simply the best J-tube RMH publication ever!o) IMHO. My own stove is patterned after their compact Cabin-8 model, via their plans.

I am working on a system for my RMH to be built in my place. I have a sketch of the design, but would like some input the 6" horizontal piping is a little over 40' and the chimney is approximately 31', but I have the horizontal pipe slightly angled up in the mass to enhance draw. I would like to know what others think about this design. I also have an infeed which is a "J" design, but also has a cleanout or priming access which is located at the bottom of the J with a damper to control flow until all is warmed up. Please review and respond .

Thanks,
RockinOn

https://www.dropbox.com/scl/fi/dr74bab81njsdr4h7xyqe/RMH-Layout.pdf?rlkey=iodyk103npyoxj2irx3x5g1ov&st=zu0xt2p2&dl=0

Hi Rocky;
A 6" J-Tube can push 30' of horizontal pipe.
Each 90-degree bend is a 5' deduction. The vertical chimney does not count against that 30'.
Angling your pipe will not make much of a difference; it is not recommended.

So I just looked at your drawings.
That is a K-tube design, and it appears you plan for it to be metal.
There is too much piping with corners. (Too long)
A J-tube should be made of brick if it is expected to last.
30' with deductions is about the max if you expect trouble-free operation.

As Thomas says, you have far too much horizontal duct and too many bends for this layout to work, and the metal K-tube will burn out probably in the first season of real use (most likely on the coldest day of winter).

Build a brick J-tube according to dimensions published in many places, and it will last for a number of years even if you don't get firebrick but use old red brick. Use firebrick in the core and it will probably outlast you.

Instead of the original RMH duct-in-mass design, build a stratification chamber or "bell" - just a hollow brick/cob/masonry box with the J-tube feeding into it and the exit from the bottom of the box to the chimney. This will have almost zero friction and work fine. For sizing the box, there are tables of batch box capacity, and J-tubes can be sized relative to those. I have found that an 8" J-tube requires about the same ISA (internal surface area) as a 6" batch box; a 6" J-tube would probably match with a 4 1/2" batch box more or less (around 30-35 sq. ft.).

I have also found that, with a well-drafting chimney, an 8" J-tube runs fine with a 6" chimney. If you are not certain of your draft, better to be sure and build a 6" J-tube.