That’s a small variance. It really won’t matter. That’s a standard size flue tile typically referred to as 8x13 or 9x13 depending who you ask. There can be variances like that depending on the manufacturer and even from the same manufacturer. It’s terra cotta clay tiles so none of them are perfect. You will be absolutely fine using the ones that are available to you. I’ve rebuilt hundreds of chimneys and have done it many times in the past. If your existing flue tiles are really old you may never find an exact match. If you want to know the actual math a rectangular flue should be a 1:10 ratio to the opening of the firebox. So if the firebox is 100sq/in the interior flue tile should be 10sq/in. That’s in NFPA 211 code book. Just because that is how it’s done doesn’t mean your chimney has the properly sized flue. Many chimneys are constructed with undersized flues so yours may not be correct as it is, if it drafts ok in the past then don’t worry about it.
4
u/chief_erl 🔥Hearth Industry Professional 🔥 2d ago
That’s a small variance. It really won’t matter. That’s a standard size flue tile typically referred to as 8x13 or 9x13 depending who you ask. There can be variances like that depending on the manufacturer and even from the same manufacturer. It’s terra cotta clay tiles so none of them are perfect. You will be absolutely fine using the ones that are available to you. I’ve rebuilt hundreds of chimneys and have done it many times in the past. If your existing flue tiles are really old you may never find an exact match. If you want to know the actual math a rectangular flue should be a 1:10 ratio to the opening of the firebox. So if the firebox is 100sq/in the interior flue tile should be 10sq/in. That’s in NFPA 211 code book. Just because that is how it’s done doesn’t mean your chimney has the properly sized flue. Many chimneys are constructed with undersized flues so yours may not be correct as it is, if it drafts ok in the past then don’t worry about it.