RMH exhaust

Does the outside vent need to be higher than combustion chamber, and with an 8" heat riser, how long can the horizontal "mass" part be?

Rob Carlisle : Welcome to Permies.com and sister site Richsoil.com. A Big Welcome to The Rocket and Wood stoves Forum threads. As 1 member of ~35,000~

Fellow Members World-wide, you can come here to Permies 24 / 7 to talk to someone who wants to talk about what you want to talk about .

Assuming you are talking about retro-fitting a Rocket Mass Heater RMH into an existing home then there will automatically be compromises, Expect that you will

need a new Final vertical Chimney, preferably inside your house and exiting on the downwind side of your house very near to the peak of your roof and extending

3' - 4' higher than that peak and all nearby objects bigger than a TV antenna. Often a well built final vertical chimney will be the most expensive part of the build !


A good well made 8" RMH should run 50' horizontally, It is important to remember that when laying out your RMHs placement that every elbow gets counted as -5'

of pipe, You will also need several clean outs with covers and as they are often substituted for elbows the -5 Feet applies to them too.


This is where I ask if you have been to Rocketstoves.com to download your PDF Copy of The brand-new 3rd Edition of " Rocket Mass Heaters ".

This is the goto Book, it allows you to come here to Permies sure that you will be using the same Words to Describe the Sizes, Shapes, and Orientation of all

theRMHs parts. This is a must have book saving you time, money, and frustration. It ensures you get the right parts, it will quickly get a little shop-worn, and

will be the 1st thing you loan out to good friends!


Please share some more information on location/climate/elevation. Include the size of your home and remembering that all wood stoves including the RMH are

space heaters give us some idea of the core area you are hoping to heat and layout ! Also remember the RMH is Heavy, and the floor and surrounding Exposures

must be protected from heat Radiation Convection, and Conduction !

I hope that this is timely and useful, For the good of the crafts ! Big AL

Thanks for the info,
I live in hot/cold climate down to minus 30 upto 100.
I want to build rocket outside in my ply walled In gazebo,no insulation(as of yet). It's not a big space 200sf, but it gets real cold. Have a tin roof. 50' of horizontal exhaust would be too much. Should I go with a smaller riser diameter?

Rob Carlisle : OH MY, What you said ! This will give Me an excuse to climb up on a Sandbox and Pontificate ! This is where I recommend that you go with a 6'' RMH.

This is my opinion based on the fact that all Rockets get much harder to Successfully construct in the sub 6'' RMH sizes. Many of your fellow members started

out with a goal to create a 6'' rocket as a 1st time build , fully expecting to try later to build a smaller unit, most found that there was more than enough flexibly in the

range of ways you can fire the 6'' rocket mostly depending on the amount of Small, finely split, very dry wood you Feed it !

A Well built 6'' rocket can easily handle 30' of horizontal Chimney, and indeed needs about that much to not end up losing most of your heat up Your Final Vertical

Chimney ! For th' crafts Big AL