Reducing 8" vertical stack to 6"

Getting ready to install my vertical chimney stack for my 8" RMH. Will it create problems to reduce the 8" to 6" near the top of the chimney where it exits the roof ( 1 1/2 story barn ). I have about 30' of horizontal duct ( accounting for all elbows ) and vertically will be about 25'. Or would it create draw issues? I'm using HVAC duct and planning on using a 3' section of double wall pipe where it exits roof to keep it insulated. A 6" diameter section would be much cheaper, but certainly don't want to shoot myself in the foot at this point. Anyone have any thoughts on this? Thanks for any advice!

Robert; This is only a guess but I think you would be ok . You're only running 30'of horizontal then your 25' vertical of interior 8" pipe "should" give you plenty of push to go thru a 6" pipe at the very top. Keep as much inside the roof line as you can with maybe a foot outside , this should help keep it nice and warm. If I am wrong... hopefully somebody who has tried this will post and let us know.

Thanks Thomas! You've been a great help! I actually owe you some pics. I fired it up with a "practice" stack over the weekend and it fired beautifully the first time! I laid out the duct almost 25', ran it under the garage door, hit a 90*,another 4', another 90*, and 15' vertically with a tee at the top ( all 8" ) and braced with garbage cans. My neighbors must think I'm nuts. It just roared, putting out mostly steam. It was a 30degree day and the condensation was excessive and water was running everywhere. I figure the with all the draft issues I had going on (open garage door ) and unsealed duct, I should have a strong system with everything sealed and inside. I'm building the bench this next week after I'm done running the chimney. Thanks again!

Hi Robert; Nice looking build ! When you build your bench if... its cob then expect much more condensation and that your stove will not roar well until your mass drys. It will take weeks ! If its cold enough outside then run it as you cover horizontal pipes , this will help with the drying process .

Hi Robert,

Why the "splitter" at the top of the flue?

I understand "diffusers" on some historic chimney flues from different cultures, yet most turn out to be decorative or inhibitors to wildlife (and a proper drafting of the system) than anything else.

Regards,

j

I plan on using SOME cob. I'm going to use brick to build the outer enclosure with urbanite under the duct itself, with some brick on top of that, and then 3/4" angular gravel to fill in and top off with 16" patio block to cap it. I'm wanting to keep it as portable as possible (and to save time). I have a finite supply of the clay and I'll use cob to further insulate the combustion unit. If I ever feel the need to rebuild it in my basement I'll be able to dismantle it without too much destruction - hopefully. Thanks Thomas!

Hi Jay. I just put the tee at the top to help eliminate any back drafting and just to see what would happen. I got the idea from Ianto's book. I'm hoping it might save me from buying a rain cap - anywhere I can save some money!

On vacation and finally building my bench. On the downside, winter decided to show up in central Illinois this week. 5-7" of snow with sub-zero temps oh, why didn't I get on this sooner?! The bench is coming along well and I'm looking into using cam-6, which is used for road underlayment @ about $8/yard. I'm still planning on using urbanite under the duct, filled in with cob to reduce air gaps. I have it raised up on brick rows, Bonny style, to tap into passive air flow. The hollow cores on the block that I'm using for containment, I'm going to fill with either cob or vermiculite- (I got the block CHEAP). My exhaust configuration should be a winner as I'll have the vertical stack through the roofline. But since old man winter decided to show up, I might have to wait. The roofer down the street who was going to do it this week says ill have to wait till the temps get to at least 25 degrees to do the flashing. In the meantime I might just run a temporary stack out of the upstairs window and just pack the space around the duct with rock wool to keep from freezing or burning my barn down. Will post further updates and pics!

Jay C. White Cloud wrote:Hi Robert,

Why the "splitter" at the top of the flue?

I understand "diffusers" on some historic chimney flues from different cultures, yet most turn out to be decorative or inhibitors to wildlife (and a proper drafting of the system) than anything else.

Regards,

j

I don't know if it is cheaper than just getting a decent wind-vined, rain cap, but with three T's you can make an H-shaped top. This is an old method, been around a long time, to create low pressure up there at the very top of the chimney pipe, helping the draft. You also give the arms of the H a little tweak to help ensure the rain water drains away from the central vertical pipe, instead of inside of it.

With the T you have in your pic, just add another T to each "arm" the existing T is offering. If you ignore the vertical chimney pipe, the three T's end up looking like an H. I have also seen folks use two small pieces of connecting pipe to make the fit easier onto the "arms" of the first T.