Chimney Questions

[cjsamuels]

Hello all!

I've been building an oven using Alan Scott's Bread Builders, and ready to set my chimney. I plan to use an 8" square clay flue and surround it with common red brick. Here's the question: after setting the flue pipe with refractory mortar, can I use standard type S mortar modified with some fireclay for the chimney brick work around the flue pipe? I plan to leave a 1/2" air gap between the flue line and the brickwork. Also, would it be out of the question to use structural bricks (with holes) for the chimney brick as opposed to solid reds? Thanks for the help! I am trying to avoid buying more of the expensive refractory mortar.

Thanks!

[rsandler]

Yes, standard mortar should do just fine for you. The flue gasses from a wood fired oven don't get nearly hot enough to damage portland cement. If you're worried and don't want to buy refractory, the homebrew mortar recommended on this forum (3-1-1-1 sand-portland cement-lime-fireclay) also works great, though for a chimney the lime and fireclay are probably unnecessary.

Structural bricks should work fine. The usual downside of a brick chimney (as opposed to cast or a stainless flue) is that they are very heavy, and potentially require buttressing of the entry arch to support. Using lighter-weight bricks would help with that.

[jhonson]

I’ve done something similar with my oven build. Using type S mortar mixed with fireclay around the flue should work fine. Just make sure the mix is good for high heat. The 1/2" gap you’re planning is smart—it helps with heat expansion. As for the structural bricks with holes, they should be okay as long as they’re heat-resistant and well-sealed. I used some similar bricks for my chimney, and they’ve held up well.