The following question came up in the Forums.

Can you guys settle a debate? Does it make any difference in draft if 2 45’s are up at the ceiling support box or down at the stove collar on the interior run of double wall about 6 ft. long?

An answer (opinion) by John Gulland

My reasoning is that the vertical portion of the flue is where the real draft is developed and I tell participants in workshops that horizontal sections are death to draft. True, 45 degree offesets don’t produce a horizontal section, but they don’t produce the same draft advantages as straight vertical. Plus, as you say, an offset does create some restriction and therefore turbulence which does cause greater heat loss from the flue gases. So, if you put the offset high, you get the benefit of high temperature and straight vertical off the stove, which should produce draft quicker.

What brought all this home to me was almost 25 years ago when CMHC in Canada was developing a computer model of flue gas flow in gas furnaces and water heaters. The model clearly showed the way hot flue gases creep along the highest level of a horizontal or near-horizontal section and when they finally reached a vertical section and began to rise, draft and therefore flow increased dramatically. As a result, I have always pitched the idea that elbows or offsets should be placed as far away from the appliance flue collar as possible. Same goes for increasers, which should also be placed at the chimney rather than right at the stove in order to create a higher flow rate, shorter dwell time, smaller surface area and higher flue gas in that portion. It is for all of these reasons that I have never been fussy about rear take-off flue pipe configurations for stoves. I always advise top take offs.