How long can the exhaust duct be for a RMH?

Hi all, long time permies.com lurker here posting what I expect will be the first of many questions I just can't keep to myself any longer!

I am interested in building a Rocket Mass Heater, and have various ideas for how to incorporate one into a small home structure. One question that keeps coming up for me: how long can the exhaust duct be? Can parts of it go upwards and if so, how far?

If someone can direct me to further info on understanding the calculations and physical specifications necessary to make a RMH work properly, I would greatly appreciate it!

Hi Joe;
Big Welcome to Permies!
And a Big Welcome to the wonderful world of rocket science!

So you want to build an RMH , and your hoping to put it in a small home.
How small?  Like a tiny home size?  Stationary or mobile?
Do you know what style RMH you want to build?
I suspect an original style J tube?
It sounds like your wanting a piped mass?
Do you know about using bells as a mass in place of fixed piping?  

We can give you all the specification's you need to build any style RMH.

Question?)    Have you built a J tube rocket stove out in your yard yet?
Practice building outdoors with no mass is a good way to feel comfortable building indoors with a mass.
You can build with any clay brick for practice but you'll want heavy firebrick when you build indoors.

Give us some more info on what your situation parameter's are and we will tell you the options.

Oh in answer to your question)   An 8" J tube can push thru 50' of horizontal pipe with a 5' deduction for every 90 degree turn  until going vertical.

Hi Joe, there is no one answer … it is a bit like saying how fast is a car.
We have small, basic J tube heaters made with straw and clay with a short chimney or a massive 10” batch box, made from high tech materials and a tall insulted chimney.
It may take you a bit of time studying  the forum to discover what sort of heater would suit your circumstances and budget or perhaps you could tell us more about what you want, where you live and how big a space you want to heat?