exhaust piping or lack there of

I remember reading at some point that parallel exhaust pipes was not a good idea for a RMH.  I don't remember why?  The reason for asking was I as looking at the neighbors burn pile and he has 2 dozen netwrap cardboard cores left over from round bales.  They are about 4" in diameter and just short of 8 ft long.   The cardboard is about 1/2" thick and the tubes are incredibly sturdy.   My thinking was suddenly wondering if cob could be build around these making a series of parallel pipes and simply burn the cores out when finished leaving the cob as the pipe.  You wouldn't likely want it in a house but wouldn't it work for something like a greenhouse build where minor leakage doesn't matter?

Hi; 4" is just a little to small for a good working rmh. Using cardboard as a form would work BUT could collapse when wet . HVAC piping is 5' long very cheap and works great.

I concur with Thomas' statements, but in your particular case I think they might work well. As long as the tubes are all used in parallel, and there is a plenum at the entrance and exit so they receive equal pressure, they should have sufficient combined CSA and any differences in flow should be minor enough to be inconsequential. The cardboard sounds heavy enough to stand up for some hours; I would put a fire in the system to burn out the cores right after packing fully around the tubes and before adding a lot of cob thickness. Two dozen tubes sounds like material for a wide daybed, probably too many to use for one project. It might do well for an underfloor heating project.

You would need to set up such a system so that the tubes would see fire at least for the initial burnout; even super-heavy cardboard will not hold up indefinitely waiting for wet cob to dry out on its own, and it must be burned away, not left to rot inside the mass.

The tubes are waxed or oiled on the outside so they are mildly waterproof as is.  But even without that it isn't a major problem.  Simply wrap the tube in a single layer of cheap plastic wrap and the moisture would mostly never reach the cardboard and the plastic wrap could burn out with the cardboard.  I think that problem is easily beaten.