Journeyman Plumber ExamGas PipingMedium
A plumber discovers a gas appliance that draws its combustion air from the living space and also has a draft hood. This appliance is exhibiting flue gas spillage at the draft hood. Which of the following is the MOST likely cause of this spillage?
- AAn oversized gas supply line to the appliance.
- BThe appliance's thermocouple is faulty.
- CInsufficient combustion air supply to the building.
- DA high-efficiency direct-vent exhaust fan operating nearby.
Show answer & explanationAnswer & explanation
Correct answer: C. Insufficient combustion air supply to the building.
Flue gas spillage at a draft hood indicates a problem with the draft, often caused by negative pressure in the building or insufficient combustion air. If the appliance cannot draw enough combustion air, it struggles to vent effectively, leading to spillage. This is particularly critical in tightly sealed homes.
Why the other options are wrong
- A. An oversized gas line would not directly cause spillage; it might lead to over-firing if not properly regulated, but spillage is primarily a venting/draft issue.
- B. A faulty thermocouple would cause the pilot or main burner to shut off, not cause flue gas spillage during operation.
- D. A high-efficiency direct-vent exhaust fan operates independently and does not typically interfere with the draft of a natural draft appliance, unless it's a powerful exhaust fan creating negative pressure in the entire building.
Flue Gas Spillage
The condition where flue gases from an appliance fail to vent properly and spill into the occupied space, often indicated at a draft hood.
- Indicates a venting or combustion air problem.
- Can lead to carbon monoxide poisoning.
- Commonly caused by negative building pressure or blocked vents.
Memory trick: Spillage means trouble, air or vent is the rubble!