CSLB C-20 HVAC ContractorHeating SystemsEasy
Which venting category describes a furnace that operates with a negative vent pressure and produces non-condensing flue gases, typical of an older standard-efficiency atmospheric furnace?
- ACategory II
- BCategory III
- CCategory I
- DCategory IV
Show answer & explanationAnswer & explanation
Correct answer: C. Category I
Category I appliances operate with negative vent static pressure and non-condensing flue gas temperatures, matching standard atmospheric gas furnaces vented through a natural draft metal vent.
Why the other options are wrong
- A. Category II is negative pressure but condensing, rare in furnaces.
- B. Category III is positive pressure, non-condensing (power vented).
- D. Category IV is positive pressure, condensing (high-efficiency sealed combustion).
Venting Categories I-IV
NFGC classification of gas appliance venting based on vent pressure (positive/negative) and flue gas condition (condensing/non-condensing).
- Category I: negative pressure, non-condensing (standard furnace)
- Category III: positive pressure, non-condensing (power vented)
- Category IV: positive pressure, condensing (high-efficiency PVC vent)
Memory trick: I is old and lazy (negative draft, dry gas); IV is new and wet (positive, condensing).