RMH length of mass tubing?

Is there a length consideration for the function of a rocket mass heater tubing?  My concerns are:  

1.  Will it fail to draft properly if the length is too long?  

2.  Is there a diminishing return on length, meaning there will simply be a lower amount of heat in a larger mass?  

And a bonus question:  

3.  How often does a RMH need to be tended (adding wood) while burning to keep it going?    

Hi Christopher,
yes, the overall length, roughness (material) and the number of elbows will influence the draft.

1. Yes, that will happen. You want enough length to extract a lot of heat while leaving enough to have a draft. There's some calculations in the Wiesner's book. Also there's some rule of thumb lengths. But it depends on the size and type of heater you're planning

2. If I understand you right your question is whether there's a choice between a small and hot mass and a larger, not so hot mass? It mainly depends on your needs and site, as for example in warm climates a smaller, more reactive mass would be preferable

3. It depends on the size and type. A 6" J-Tube roughly every 20 mins I would say. But you often only have to run it for 1h or 2h. With a Batch Box it's liek a normal wood stove, depending again on the size.

Hi Christopher;
An 8" J-Tube can push 50' of pipe with deductions for any bends.
A 6" J-Tube can push 30' of pipe.
An 8" once heated up will burn for 60 minutes.
A 6" around 30 minutes.
A  Batchbox has a door and will easily have coals for 2-3 hours.

If you try to push it too far, your rocket will stall and smoke back into your home, (But at least it will not blow up on the launch pad...)

Almost all new builds have changed from a piped mass over to using Stratification chambers (Bells)
They are much lighter, easier to build, take up less space, and you never need to clean out ash-filled pipes.

Rocket Mass Heaters in an insulated home are only lit a few times a day or less depending on weather conditions.
An RMH in a larger area can be burned back to back all day if wanted, I call these, Extreme Burning Batchboxes.
I run two of my four Batchboxes as extreme burners, lit off in the morning they burn all day until evening and then are allowed to go out for the night.
Extreme burning RMH cores will require more maintenance, as bricks move or mortar cracks, I expect to do partial rebuilds every other year or so on them.