I bought a tankless water heater that allows me to vent using category III. I've seen that wall thimbles are included in many category III termination kits, but am wondering if they are really necessary as the flue doesn't seem to get very hot.

  • Are wall thimbles necessary for side venting category III flue though combustible materials like plywood??

  • How about for category IV?


In accordance with NFPA 37 and NFPA 1 Fire Code: "An approved thimble shall be used where exhaust piping passes through combustible walls or partitions."

This code would apply to all venting categories.

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