CSLB C-20 HVAC ContractorHeating SystemsEasy
A homeowner installs a new 96% AFUE condensing furnace. During inspection, a technician checks the condensate drain system. Which component is required to prevent flue gases from escaping through the condensate drain line?
- AA barometric damper
- BA condensate trap
- CA backflow preventer
- DA check valve
Show answer & explanationAnswer & explanation
Correct answer: B. A condensate trap
Condensing furnaces produce acidic condensate that must drain through a P-trap style condensate trap. This trap holds a water seal that blocks flue gases (which are under slight positive pressure from the induced-draft blower) from escaping through the drain line into the living space.
Why the other options are wrong
- A. A barometric damper is used on Category I natural-draft venting, not condensing furnace condensate.
- C. A backflow preventer is a plumbing device for potable water systems, not condensate drainage.
- D. A check valve prevents reverse flow in a pipe but does not provide the water seal needed to block gas.
Condensate Trap
A water-seal trap installed in the condensate drain line of condensing furnaces to prevent positive-pressure flue gases from venting through the drain.
- Required on all Category IV (condensing) furnaces
- Must be primed with water before startup
- Failure to install can allow flue gas leakage or drain blockage from soot
Memory trick: No trap, gas escapes — think sink P-trap.