Modine PDP200 Natural gas heater with power exhaust. 6" galvanized, approx 15' exhaust through roof. Condensation/rust after two years. Replacing exhaust duct. Question if condensation drain/trap required or recommended? Double wall or single wall OK?

Garage unit heater? Should be double wall within 1 foot of ceiling and all through attic and out through roof. Can use single wall off unit up to the one foot before penetrating ceiling. Revert to owners manual(installation instructions} for trap requirements, but I've never seen one that is required or recommended. Also make sure you have a belmont top on top of vent to keep rain from running inside pipe.