Flue liner vs. thimble size?

Hello All. I am having a chimney built as part of a boiler room addition to my shop. I have a Yellow Flame boiler that I plan on using but may switch to an EFM 520 sometime down the road if the Yellow Flame would become difficult to get parts for, etc. Both require 8" flue pipe. After reading all the posts I could find concerning this the consensus seems to be I want an 8" x 8" flue liner with an 8" thimble. After talking to my mason he said it is doable, but a bit funky. He said normaly an 8" thimble needs to go into a 8" x 12" flue liner. I have read many posts stating to large a chimney can cause draft issues in certain instances. This is brand new and I want to get it right and not have any draft issues if at all possible. Chimney height will be approximately 21'. Any help would be greatly appreciated!
Thanks!

I believe the 8"x8" tiles are 8" OD....so anything more than a 7" thimble won't cleanly go through the side of tile. My recommendation is to use 8" round tiles with an 8" thimble, all work well together and the draft is excellent due to reduced turbulence in the round flue. If the round flue tiles aren't available in your area, step up to an 8"x12" flue...which is really a 9"x13" OD tile.

My chimney is constructed with 9"x13" tiles and is about 32' high, it drafts well and allows me to run my EFM through the hottest weather without issue.

Dragonslayer wrote:After reading all the posts I could find concerning this the consensus seems to be I want an 8" x 8" flue liner with an 8" thimble. After talking to my mason he said it is doable, but a bit funky.

Seems funky to me too. From what I have seen, the flue is always 2" or more larger than the thimble.

Then would not the 8X8 actually be 9X9-----LOL go to a place that sells them with your tape measurer, OR, trust your mason knows what he's doing. I mean, you & him are actually there!

Rob R. wrote:I believe the 8"x8" tiles are 8" OD....so anything more than a 7" thimble won't cleanly go through the side of tile. My recommendation is to use 8" round tiles with an 8" thimble, all work well together and the draft is excellent due to reduced turbulence in the round flue. If the round flue tiles aren't available in your area, step up to an 8"x12" flue...which is really a 9"x13" OD tile.

My chimney is constructed with 9"x13" tiles and is about 32' high, it drafts well and allows me to run my EFM through the hottest weather without issue.

go with an 8/12 modular tile in a precast block designed for 9/13, fill the resulting gap with vermiculite, put an 8" round thimble in and be done.



(rob, they have 9/13 standard and 8/12 modular tiles... of course bear in mind that quality control even from the best mfgr's such as superior and logan is something to be desired, they're rarely cut square and the actual dimensions may vary 3/4 of an inch)

You can get parts for your Yellow Flame boiler from Marks Supply in Shanendoah Pa. As far as I know he is the only one with parts.