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Absolutley can be double wall B vent. What size is heater? BTUS will determine requires diameter
I see now it is 45,000, Let me look this up right quick
The 3" TYpe B double wall is fine.
Note from manual the 3" is large enough for up to 75,000 BTU.
"A3. All heaters come with a factory installed vent adapter for attaching the vent pipe to the heater (3” for model sizes 30-75, 4” for model sizes 100-125). Attach the vent pipe to the adapter with 3 non-corrosive screws. (Drill pilot holes through the vent pipe and adapter prior to screwing in place)."