Flue diameter vs chimney diameter

[Stihl025]

I have been researching a new wood furnace and came across something that I don't quite understand. A few furnaces have a 6" flue diameter but require the use of a reducer to bump it up to a 7" chimney. Here is an excerpt from the US Stove 1950 manual:

A 7” diameter chimney listed for use with wood burning heating appliances is highly recommended for the
Hotblast 1950. In this case, a reducer from 7” to 6” is required. If draft exceed –0.06 in. W.C a barometric damper
should be installed on the smoke pipe. Never install a manual damper.

I currently have a 6" stainless liner in a 6" terra-cotta chimney running through the middle of the house. I don't have an option to go larger. Why do they require the larger chimney coming from a 6" flue exit???

 

[fossil]

It's possible their concern is too much draft. The wording also says "highly recommended", rather than "mandatory". I'd say contact the manufacturer's tech support folks and lay out the situation for them, and see what they have to say. Rick

 

[Beanscoot]

How hard was it to get the six inch SS liner in the six inch clay pipe? I have the same size clay liner and am considering putting in a stainless six inch liner, if possible. Although there is a little clearance, maybe a quarter inch, from what I have read I thought it was very difficult to do.

 

[Stihl025]

How hard was it to get the six inch SS liner in the six inch clay pipe? I have the same size clay liner and am considering putting in a stainless six inch liner, if possible. Although there is a little clearance, maybe a quarter inch, from what I have read I thought it was very difficult to do.

Umm, it was tedious. We just took our time and slowly wiggled it into place. I put the tee connector on and then used a piece of Gorilla Duct tape looped acrossed the bottom as a strap. Used some clothes line tied to it to pull from the bottom, and two of us, slowly straightened the pipe and guided it down from the roof. Surprisingly, it went in very easily. If I were you, I would go a little smaller if you can, just to be safe.

 

[Beanscoot]

Well I've got the six inch flex as I bought it slightly used for $40 so will really try to use it. I still am waiting on getting a new baffle for the stove before installation, though. I am leaving the old stove in until I am all ready to swap.

 

[gpcollen1]

I think if you do the math that 7" round is quite comparable to 6" square. I would be open to the option of just using the 7 inch flu if it is in good condition.