In the other thread on shortening the horizontal run, Elk mentioned that code dictates an uphill pitch to stove pipe. Of course it makes sense that you'd want the smoke to rise, as that's it's natural tendency. The sort of confusing part is that in some masonry heaters (Counterflow, Finish etc...) the smoke path goes up to the flue exit then DOWN along the sides of the firebox followed by another turn up to the chimney. Some coal stoves work this way also...Harman Mark I is an example of this (II and III are not however) So... 1. Why is a down slope or even a severe small downward smoke path not allowed? Consider a tall stove in front of a hearth where the install might dictate a short downward path out of the stove followed by a u-turn up into the flue. 2. Does a masonary heater essentially PUSH the draft through the heater via pressure from the hot gasses passing through the flue area? or is the chimney still nominally generating draft thus pulling the downward flow through the heater? 3. Keeping the gasses hot through use of insulated pipe would seem prudent so that when the still hot exhaust does go vertical it want's to rise, yet masonry heaters entire goal is to EXTRACT heat from the flue gasses. What principles are at work here.