CSLB C-20 HVAC ContractorHeating SystemsEasy
A new 96% AFUE condensing furnace is installed. Which venting material is code-approved for its Category IV flue gas venting?
- AUninsulated aluminum flex duct
- BSingle-wall galvanized steel
- CType B double-wall vent
- DSchedule 40 PVC
Show answer & explanationAnswer & explanation
Correct answer: D. Schedule 40 PVC
Category IV condensing furnaces produce acidic, cool flue gas condensate under positive pressure, requiring corrosion-resistant plastic venting such as PVC, CPVC, or polypropylene as listed by the manufacturer.
Why the other options are wrong
- A. Aluminum flex duct is for air distribution, not flue gas venting.
- B. Galvanized steel corrodes quickly from acidic condensate.
- C. Type B vent is for Category I atmospheric appliances, not positive-pressure condensing units.
Category IV Venting
Positive-pressure, condensing appliance venting requiring corrosion-resistant plastic pipe (PVC/CPVC/polypropylene) per manufacturer's listing.
- Condensate is acidic and will corrode metal vents
- Vent operates under positive pressure, requiring sealed joints
- Common in 90%+ AFUE furnaces
Memory trick: Acidic sweat needs plastic coat — PVC keeps the vent afloat.