FC FC Venting & Exhaust Systems 1 — Questions and Answers
Question 1: Which furnace venting category uses PVC plastic pipe for the flue because flue gas temperatures are low?
- Category I
- Category II
- Category III
- Category IV (Correct answer)
Correct answer: Category IV
Category IV furnaces (high-efficiency condensing) have low flue temperatures and positive vent pressure, requiring corrosion-resistant plastic pipe like PVC or CPVC.
Question 2: What is the minimum slope per foot required for horizontal PVC vent pipe on a condensing furnace to allow condensate to drain back to the furnace?
- 1/16 inch per foot
- 1/4 inch per foot toward the furnace (Correct answer)
- 1/2 inch per foot away from the furnace
- No slope required
Correct answer: 1/4 inch per foot toward the furnace
Horizontal PVC vent sections must slope at least 1/4 inch per foot back toward the furnace so condensate drains to the trap rather than pooling in the pipe.
Question 3: How far must a direct-vent (concentric) termination kit terminate above the ground in a US snow region?
- 6 inches
- 12 inches
- 18 inches (Correct answer)
- 24 inches
Correct answer: 18 inches
In areas subject to snow accumulation, direct-vent terminations must be at least 18 inches above grade to prevent obstruction by snow.
Question 4: What is the purpose of the concentric vent system on a direct-vent condensing furnace?
- To save duct space by combining supply and return
- To use one pipe for exhaust and a surrounding outer pipe for combustion air intake (Correct answer)
- To increase combustion efficiency with preheated air only
- To allow longer vent runs
Correct answer: To use one pipe for exhaust and a surrounding outer pipe for combustion air intake
Concentric venting combines the exhaust flue (inner pipe) and combustion air intake (outer annulus) in one penetration through the wall or roof.
Question 5: A Category I naturally venting furnace must be vented with which type of material?
- Schedule 40 PVC pipe
- Type B double-wall metal vent pipe (Correct answer)
- Single-wall galvanized connector only
- Stainless steel class IIb liner
Correct answer: Type B double-wall metal vent pipe
Category I appliances require Type B double-wall vent pipe for the vertical section because flue temperatures are high enough to require insulated metal venting.
Question 6: What is the maximum length of single-wall metal connector pipe allowed between a Category I furnace and a Type B vent or masonry chimney?
- 6 feet (Correct answer)
- 10 feet
- No limit if slope is maintained
- 3 feet
Correct answer: 6 feet
Single-wall connector pipe is limited to 6 feet between the furnace outlet and the Type B vent or chimney connector per most codes.
FC FC Venting & Exhaust Systems 1
Which furnace venting category uses PVC plastic pipe for the flue because flue gas temperatures are low?