Chimney question

[jrgreene1968]

Howdy from east tx, I've got an offset smoker I bought off craigslist, seems pretty decently built, all 1/4", good welds etc. except.. I think they messed up when they figured the chimney size. The chimney measures 2.75" od pipe, and it's right at 20" long.
My question is, when using the online calculators, are they figuring the id of the chimney pipe or od? I just randomly typed in 2.5" figuring the id, and the calculator says for that size pipe it would need to be 53" long, not 20". I'm having a heck of a time maintaining a fire, I know the intake is to small, so I'm going to go by the calculations and fix that, but didn't know about the id, or od on chimney. I'm thinking I'll just cut out the existing pipe and use 4" pipe.
The smoker is.. Cooking chamber 20 x 40, and the firebox is 16 x 29 . Reckon 4" pipe would work ok?

[k.a.m.]

Welcome to the forum jrgreene1968. You are on the correct path to be sure of. The calculator is based on volume and area so it is using the ID of the pipe. By increasing the intake size and going to 4" pipe 24" long you will balance your cooker a lot better. The next opening to check is the one between the firebox and cooking chamber to see if you have adequate flow there.
I hope this helps.

[jrgreene1968]

Welcome to the forum jrgreene1968. You are on the correct path to be sure of. The calculator is based on volume and area so it is using the ID of the pipe. By increasing the intake size and going to 4" pipe 24" long you will balance your cooker a lot better. The next opening to check is the one between the firebox and cooking chamber to see if you have adequate flow there.
I hope this helps.

Thanks buddy, I'm gonna head to the shop later and see if I have some 4" pipe , if not I'll have to locate some locally. Hopefully I can get this thing to cook