CSLB C-20 HVAC ContractorHeating SystemsMedium
A 90% AFUE non-condensing furnace uses an induced-draft fan to create a slight positive pressure in the vent while flue gas temperatures remain above the dew point, avoiding condensation. This describes which venting category?
- ACategory III
- BCategory II
- CCategory I
- DCategory IV
Show answer & explanationAnswer & explanation
Correct answer: A. Category III
Category III venting is defined by positive vent pressure with flue gas temperatures high enough to prevent condensation within the vent. This requires special positive-pressure-rated venting materials such as AL29-4C stainless steel, unlike Category I which operates under negative pressure.
Why the other options are wrong
- B. Category II is negative pressure with condensing flue gas, rarely used in residential furnaces.
- C. Category I uses negative vent pressure and non-condensing flue gas, typical of standard atmospheric furnaces.
- D. Category IV is positive pressure with condensing flue gas, used in 90%+ condensing furnaces with PVC venting.
Venting Category III
Venting classification for appliances with positive vent pressure and flue gas temperatures above the dew point, preventing condensation.
- Requires positive-pressure rated vent material like stainless steel
- Common on some induced-draft 80-90% furnaces
- Different from Category IV which also condenses
Memory trick: Positive & dry = III; Positive & wet = IV.