lowering my manifold

Without sacrificing my Rmh efficiency what is the shortest my heat riser can be? My burn tunnel is "22

You haven't given enough information for us to answer usefully. What diameter is your system? What are the other dimensions of your J-tube core? How much horizontal run and elbows do you have/plan, and what is your chimney like?

Are you planning on doing a j tube or a batch box?

-Just sharing a random thought, If you were living in a one story house and only wanted the minimum amount of Final Vertical Chimney showing at your roofline

I have heard of Home owners Associations H.O.A.s where this was a potential problem ! Then the last thing you would ever want to do is to have a minimum

Height Heat Riser ! Big AL

The bottom of my barrel is 34 " off the floor because my heat riser is so tall. which also places my manifold that high. I don't want to loose the extra thermal mass placing the manifold that high off the floor.

The bottom of my barrel is 34 " off the floor because my heat riser is so tall. which also places my manifold that high. I don't want to loose the extra thermal mass placing the manifold that high off the floor.

Its an 8" system tunnel. 22" x 6x 8 feed tube 10"x6"x8 heat riser 54" x 6" x8" as much space as I neeed with as much chimney as I need. about 25 feet of thermal mass with 5 elbows . But I lose my space with my thermal mass 3' off the ground.

If you don't have something limiting the height of your barrel top, then you can just extend the manifold straight down far enough to get to your bench/mass level. A tall riser can handle an equal drop to the mass. Having the manifold taper gently from the bottom ring of the barrel, well cobbed and/or bricked in, will let the airflow transition gently which will reduce friction.

- Generally the barrel is centered over the Heat Riser, this distributes the hot exhaust gases equally around the entire barrel. The barrel can be offset, or as " The Book "

says given an ''eccentric gap''. One reason for doing this is that the Narrowed side will then run a little cooler, possibly protecting a flammable exposure, while the side

with the Wider gap will then radiate more heat out into the rest of the structure .

When this is done it is important to create the manifold area on the wide side every time .

In this case the likely need to reposition/reshape your manifold can be assisted by slightly off-setting the barrel to favor the manifold opening . For the Craft! Big AL

Phillip Baldwin wrote:Without sacrificing my Rmh efficiency what is the shortest my heat riser can be? My burn tunnel is "22

*** ANSWER from a Non-expert, not yet a builder, but a keen observer of others designs, requirements, and ideas ***

One could shorten the heat riser to the point that your "motor", your "engine", your source of power and push, your smelter, is dwindled, and doesn't create sufficient draft, or even enough heat for the mass in your RMH, for that matter. Rather than shorten your heat riser, you could make a sufficiently tall manifold to exit nearly anywhere you want, from the barrel bottom down to the floor, minus your base and insulation layer.

I see, and intuitively like, the LENGTH dimensions ratio of 1-2-3 for the J-tube, as seen in Ianto Evans book... add the heat riser that fits the barrel. Barrel length is fixed by the barrel size, and helps determine your height by necessity, having a gap of sufficient size to tune, not shut down the drive of the RMH. NOTE: measurement of J-tube sections is confusing because each section is part of the next...capeche. If your tunnel section was really 22 inches, your feed tube would be 11, and your riser 33, but I suspect you are measuring it differently than the book suggests.