Joe Danielek wrote:
It may seem the riser is pushing but it’s the downstream cooling effect that is pulling (“contraction”) the flame to behave the way it does in the J-tube. Even with the addition of the by-pass that is used to draw the combustion first horizontal then vertical up then vertical down to “PRIME” the system... to heat it up to establish a pull.
The way I understand physics, this statement would seem to violate the Laws of Conservation of Mass/Energy, as well as Newton's Third Law. Burning wood, overall, converts solids to gasses. These gasses have a much greater volume than the wood from which they come, so they must go somewhere, as well as the energy they contain. If, as you say, the contraction of the flames creates the pulling force, it could never make up for the fact that there are more gasses after burning than before, because the gasses (even at atmospheric temperature) will always take up more volume going out than coming in. Moreover, it would violate Newton's Third Law, as the pull cannot exceed the push generated by the expansion of the flames. According to Newton's Third Law, there can be no uncoupled forces; to every force there is opposed an equal and opposite one. If there is a pull, there must also be a push.
The way I see it, what makes the exhaust flow outside rather than into the house is buoyancy (in a naturally aspirated system). Since a RMH puts out more gas than it takes in, it cannot create this in and of itself. It is the differential of the much larger system (the atmosphere) with the smaller system of the RMH that creates the buoyancy needed for draft. A RMH releases heat, and this heat makes the gasses thinner and more buoyant in the exhaust in relation to the atmosphere. Whatever pushing and pulling is going on in the RMH should be inconsequential. Here it is the macrosystem that is important.
There's a quote from Shakespeare I try to keep in mind, "There are more things in heaven and earth, Horatio, than is dreamt of in your philosophy." I have found this to be true countless times in my experiences with making things. There are almost always factors that I fail to consider for which I must account later and modify. Sometimes, I never even figure out what the X factor was; I still have to modify my plan to make things work. I get the feeling that many things (like the bypass) regarding RMHs are a result of this same experience. They are work-arounds to help ensure RMHs work in the widest variety of circumstances possible. If a building has a healthy natural draft, it's easy. It's when it doesn't that things like the bypass can help.