8" flue into a 6" chimney by reducer coupling

A fellow asked me if you could do this. I said no - would think poor draft, smoke in stove room. Can someone please explain the physics of this? Thanks, Doug

I run 8" singlewall into 6" Metalbestos by means of an off-the-shelf reducer on my cellar stove. It works well with two minor problems. (1) Chimney can be a little hard to start when cold, but top down firestarting has solved this problem, and the chimney drafts very well when warmed up. (2) The stove, a Defiant copy, can be burned with front doors open like a fireplace, but that calls for big draft and the 6" pipe is not quite up to the task all the time -- I occasionally get a little smoking when I run the stove this way. Stove works fine otherwise, no smoking when loading or operating normally for heat, doors closed.

Physics? Flue gas velocity increases and pressure drops as the flue gasses move from 8" section through the reducer into the 6" section. Like putting your thumb over the end of the garden hose to make it spray further, water speeds up across this restriction, moving from an area of high pressure and low velocity (in the hose) to an area of low pressure and high velocity. Bournelli's principle if I recall correctly.

Thanks, VTZJ. I didn't know if this was a definite no-no.

I have a friend that also did this. He had no other choice, he did not want to buy a new stove. Everyone said it would not work well at all. He had this set up for years with no problems at all!

this principle is feasible in its basic state, however it would depend on other factors, the amount of resistance provided to the draft by the stove and dwelling for instance as well as the "air budget" of the stove itself. an older type unit with little resistance to draw may work well enough in this situation but not necessarily will a newer higher efficiency unit be able to function the bigest caveat to this is the water in the hose is forced by positive pressure, a chimney draft is pulled by negative pressure, so with this you may find that the volume of air required for the stove to burn effectively (hence the 8 inch collar) may not be present, remember a 6 inch chimney will pull "x" air a stove with an 8 inch collar may require"x"+1/4 air to be able to burn as designed. so the air moving through the 8 inch collar may not pick up enough velocity to compensate unless the overpressure of the flue is exceptional. it certainly will not work with a marginal chimney. not to mention it violates nfpa211 code requirements that a flue servicing a solid fueled heater shall be at least the same cross sectional value as the units flue collar.

This kind of interests me. I've had friends with older stoves that were suppose to use 8 inch stove pipe and chimney want to do this too and I've always said that I would not think that would be a good thing. Based on really no real knowledge of my own but just always reading to follow manufacturers instructions. Are there safety hazards involved in doing this? Would a set up like this pass an inspection? *Just wondering* Oh and while I'm thinking about all of this........what about a 6 inch converted to an 8 inch? Someone, at sometime told me one of these is OK and the other one is not. Heck if I remember which one was suppose to be OK! lol

technically you cannot get this inspected as code doesnt allow it. you can go bigger up to 2X cross section of the collar , but not smaller

Are you saying if you have a 6 inch you can go to an 8 inch and it's legal or not? Sorry I'm a slow learner at times. lol

I had an old smoke dragon that used 8", so my ceiling box and exterior stainless pipe are all 8". When I put in my new EPA stove (requiring 6" pipe), I used 6" (about 12') to the ceiling and then used an increaser piece to get it to the 8" at the ceiling. I have had no problems with draft, flue temp, or creosote build-up. Works great.

yes 6 to 8 is legal, 8 to 6 is not

Thank you......