G3 Combustion and Venting Principles 2 — Questions and Answers
Question 1: What is the primary purpose of a draft hood on a gas appliance?
- To increase efficiency
- To provide a safety break preventing excessive draft and backdraft (Correct answer)
- To filter combustion air
- To regulate gas pressure
Correct answer: To provide a safety break preventing excessive draft and backdraft
A draft hood prevents excessive updraft and protects against downdraft conditions.
During downdraft conditions, it allows flue gases to spill into the room rather than forcing them back through the burner.
Question 2: What is a Category I venting system?
- Positive pressure with condensing flue gases
- Negative pressure with non-condensing flue gases (Correct answer)
- A direct-vent sealed system
- A common vent for multiple appliances
Correct answer: Negative pressure with non-condensing flue gases
Category I operates under negative pressure with flue gas above the dew point.
Natural draft creates the airflow. Type B vent is the standard material used.
Question 3: Why must vent connectors slope upward from appliance to chimney?
- To reduce noise
- To ensure proper draft and prevent condensate pooling (Correct answer)
- To increase gas flow
- To allow thermal expansion
Correct answer: To ensure proper draft and prevent condensate pooling
Upward slope (minimum 1/4 inch per foot) maintains draft and prevents pooling.
CSA B149 requires this minimum slope for all vent connectors.
Question 4: What happens when flue gases cool below the dew point in a non-condensing vent?
- Appliance shuts down automatically
- Condensation forms, potentially causing corrosion and failure (Correct answer)
- Draft increases significantly
- Gas pressure drops
Correct answer: Condensation forms, potentially causing corrosion and failure
Acidic condensation corrodes vent material and can cause premature failure.
Oversized vents, long runs, and exterior chimneys increase this risk.
Question 5: What is the purpose of combustion air openings in a furnace room?
- To provide breathing air
- To supply oxygen for proper combustion and draft (Correct answer)
- To cool the furnace room
- To allow maintenance access
Correct answer: To supply oxygen for proper combustion and draft
Combustion air openings provide oxygen for combustion and draft operation.
CSA B149 specifies sizing based on total BTU input of all appliances in the space.
Question 6: What is spillage in a gas appliance vent system?
- Gas leaking from piping
- Flue gases escaping from the draft hood into the room (Correct answer)
- Water from a condensing furnace
- Excess gas past the valve
Correct answer: Flue gases escaping from the draft hood into the room
Spillage occurs when flue gases escape into the room instead of flowing up the vent.
Causes include inadequate combustion air, negative building pressure, and blocked venting.
G3 Combustion and Venting Principles 2
What is the primary purpose of a draft hood on a gas appliance?