Venting a atmospheric gas water heater with a 15 ft run

Notes

• "Can I vent it with PVC" depends entirely on the category, and getting it wrong is not a code technicality. atmospheric gas (Category I) vents with Type B double-wall vent, or a lined masonry chimney. 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.
• A Category I vent runs on buoyancy, so height is the constraint, not length. 15 ft of horizontal needs at least 20 ft of vertical above it, every horizontal section must rise at least ¼ inch per foot, and the whole thing has to terminate above the roofline with the right clearance. A vent that runs level, or that drops even slightly, spills exhaust into the room.
• Combustion air is the part that gets skipped. drawn from the room — a closed closet needs two vents, each 1 in² per 1,000 BTU/hr. 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.