FC FC Venting & Exhaust Systems 2 — Questions and Answers
Question 1: What is the primary danger of a blocked or restricted furnace flue vent?
- The furnace will short cycle on the thermostat
- Combustion gases including carbon monoxide will spill into the living space (Correct answer)
- The heat exchanger will overheat and crack
- The gas valve will fail to open
Correct answer: Combustion gases including carbon monoxide will spill into the living space
A blocked flue prevents combustion gases from exhausting, causing them to spill into the living space and creating a potentially fatal CO hazard.
Question 2: When two or more gas appliances share a common vent, which appliance should be connected closest to the chimney?
- The larger appliance
- The appliance with the highest BTU input (Correct answer)
- The appliance located highest on the vent
- The appliance with the smallest vent connector
Correct answer: The appliance with the highest BTU input
The appliance with the highest BTU input (most flue gas volume) should connect nearest the chimney to minimize back-pressure on smaller appliances.
Question 3: Which condition causes condensation and flue gas spillage in an oversized masonry chimney venting a high-efficiency furnace?
- The chimney is too short
- The large chimney volume cools the small volume of low-temperature flue gas before it can draft properly (Correct answer)
- The furnace BTU output is too low
- The chimney liner is too smooth
Correct answer: The large chimney volume cools the small volume of low-temperature flue gas before it can draft properly
An oversized chimney has too much volume relative to the small flue gas output of a high-efficiency furnace, causing cooling, condensation, and loss of draft.
Question 4: What is the minimum upward slope required for horizontal sections of Category I single-wall vent connector pipe?
- Level is acceptable
- 1/4 inch per foot rise toward the chimney or vertical vent (Correct answer)
- 1/2 inch per foot rise
- No slope; vertical sections only
Correct answer: 1/4 inch per foot rise toward the chimney or vertical vent
Horizontal vent connectors must rise at least 1/4 inch per foot toward the chimney to help flue gases rise by natural draft.
Question 5: What causes a furnace vent to backdraft when exhaust fans operate in the home?
- The exhaust fans increase flue gas temperature
- Exhaust fans depressurize the home, creating negative pressure that reverses natural draft in the flue (Correct answer)
- The furnace increases BTU output to compensate
- The gas valve opens wider to increase draft
Correct answer: Exhaust fans depressurize the home, creating negative pressure that reverses natural draft in the flue
Large exhaust fans can depressurize a home enough to overcome chimney draft, pulling flue gases back down the vent and into the living space.
Question 6: PVC vent pipe for high-efficiency furnaces must be listed to which standard for use in the US?
- ASTM D1785 (Correct answer)
- ASTM D2665
- ANSI LC-1
- UL 651
Correct answer: ASTM D1785
Schedule 40 PVC pipe meeting ASTM D1785 is the standard listed material for condensing furnace venting in most US installations.
FC FC Venting & Exhaust Systems 2
What is the primary danger of a blocked or restricted furnace flue vent?