Is 6" or 8" flue better

[700renegade]

I'm installing a 140K BTU Buderus ( Bosch ) wood boiler in a remote pole shed about 150' from the house. The flue collar on the back of the boiler is 7" diameter.

The O&M;manual states for my furnace the minumum flue heights are as follows:
> 7” flue - at least 32.4 feet
> 8” flue - at least 29.4 feet
> 9” flue - at least 26.25 feet

In an earlier thread of mine it seemed the concensus was these heights are to be ignored - another member is running 7" at 16' heigh on the exact same boiler with good results.

Since 6" and 8" class A flue is readily available at the local home supply stores, I'd like to stick with one of those diameters rather than paying 2X the money to order in 7" flue from a specialty store. Which diameter am I best to go with? I plan to throttle this boiler back to get extended burn times as this is a secondary heat source in my loop. 30,000 BTU for 8 hours is a lot more desireable than 140,000 BTU for 2 hours.

It would seem the 6" would run slightly hotter, have better velocity in the stack. Am I missing something?

 

[stee6043]

Tough call. Not meeting the minimum recommendations for flue height can have significant impacts on your draft and as a result also effect your efficiencey.

Velocity and volume are not mutually exclusive terms. Faster moving gases in a 6" flue may well move less overall volume than an 8" flue depending on the situation. Or they may flow the same if you find the sweet spot.

I'm running 6" pipe on a "roughly" 140k boiler. I have 24' vertical pipe once I exit my basement. I struggle getting the flue temp up when the outside temps are very cold (below zero) or when it's mild (40's-50's). But on an average winter day I get good draft and all is well. FWIW my boiler installation manual recommended 8" flue as a minimum. I chose 6" because of it's lower cost and the fact that there are several folks running 6" on my exact boiler.

 

[ManiacPD]

From my experience:

My 25 year old Memco had a 7" outlet and that's what I went with. I didn't want to go with a 6" because I didn't want to put in a reducer and I was concerned I might not get enough draft with an 8". The 7" was cheaper than an 8" even though it was a bit of an odd-ball size. My chimney is 22 feet high. It is on the back side of my garage, which is the gable end, and it is about 2 feet higher than the peak.

I have absolutely no issues with draft. We live on top of a hill and it is quite windy, especially in the winter, so my barometric draft gets a workout.

I would put in a 7" without hesitation.

 

[700renegade]

Thanks for the replies.

Just so I'm clear, this boiler will be in an unoccupied, unfinished pole shed. If it's -10* outside it's the same at my boiler. I'm not sure how that affects the selection.

My gut tells me if the automatic dampers are clamped down low most of the time, I'd be better off with the smaller flue. Seems like an 8" would have a 'lazy' draft.

 

[jebatty]

I get excellent to sometime too good of draft with my 140k Tarm installed in my shop. 5' of 6" black steel from Tarm to ceiling, then 16' of 6" class A through an unheated attic space.