Replacing traditional masonry chimney with metal chimney pipe
I live in 1950s ranch in Chicago. We’ve invested a lot into improving the efficiency of the home: 95% eff radiant heat, indirect DHW, minisplit for AC, lots of airsealing and the new low carbon CC sprayfoam at our roofdeck. We are now looking to replace our roof (keeping asphalt). We’ve hired a roofer but he won’t start until we rebuild the chimney. The brick is crumbling from when the mechanicals used to pump moisture through it (they no longer do).
My initial thought is to tear the chimney down to the roofline and replace with two metal chimney pipes. My thinking is A) it would be cheaper and B) we’d replace 15 sqft of brick thermal bridging with two 8″ diameter pipes. The rest would be re-framed as roof. We’d CC spray foam the empty cavity left over and we’d have a much tighter & efficient home.
My question is can this be done from building code perspective? Main concern is we’d have a flue made of two different materials that you could probably never fully smoke/air seal. We do have chimney liners connected to fireplace inserts that are already installed. If I kept these then could that serve as the continuous flue? Those liners only work for our inserts. If someone ever removed our inserts I’m worried the chimney would be a fire hazard.
I know from a cost, mantime and energy efficiency perspective that my solution is better. I just don’t know if its allowed. I’ve kind of talked myself out of this in writing my question as I don’t see how this could be engineered to have a flue that is air/smoke tight. I would love if someone could talk me back into it by explaining how I could make this work.
To be clear, whether I rebuild or replace the chimney has to be torn down to the roofline one way or the other.