I know that the riser pipes at the top of a water heater should be rigid/sem-rigid (min. 18" I believe?), especially on a gas water heater near the draft hood. Anyway, can anyone show me any documentation/wording/pictures etc. to support this? I’m having trouble finding any information.
They just have to be installed according to manufacturers instructions. If you google the make and model you might be able to find a digital copy of them, and confirm that your instance is according to those…