Venting a power-vent gas water heater with a 5 ft run

Notes

• "Can I vent it with PVC" depends entirely on the category, and getting it wrong is not a code technicality. power-vent gas (Category III) vents with AL29-4C stainless, or PVC only where the manufacturer lists it. Plastic on a Category I heater softens and fails at exhaust temperatures it was never rated for; metal on a condensing heater is eaten by acidic condensate within a few years. The appliance's own manual overrides every general rule, including this page.
• 5 ft is comfortably inside the limit for this category, but measure it as equivalent length: each 90° elbow adds roughly 5 ft and each 45° about half that. A run that measures 5 ft on the tape with four elbows is closer to 25 ft to the appliance.
• Combustion air is the part that gets skipped. still drawn from the room — the fan moves exhaust out, not air in. A confined space — a closet, a small utility room — needs either 50 ft³ of volume per 1,000 BTU/hr or two permanent openings, one high and one low, each 1 in² per 1,000 BTU/hr. Sealing up a draughty basement is a common way to make a heater that worked for twenty years start backdrafting.
• A drain pan with a line to somewhere is worth its cost the first time. Tanks fail by leaking, not by bursting, and they do it while nobody is home. Where the heater sits above finished space, most codes now require the pan and the drain; where they do not, install it anyway.
• Have the draft checked after any change to the house. A new bathroom fan, a kitchen hood, or a tighter set of windows can pull enough air to reverse a Category I vent. A smoke test at the draft hood takes a minute and is the difference between a working flue and carbon monoxide in the hallway.