306A Gas Piping Systems 3 — Questions and Answers
Question 1: What are the BTU requirements for sizing a gas pipe serving multiple appliances?
- Use the rating of the largest appliance only
- Add the BTU ratings of all appliances served by the pipe section and size accordingly using gas pipe sizing tables (Correct answer)
- Divide the total BTU by the number of appliances
- All gas pipes are the same size regardless of load
Correct answer: Add the BTU ratings of all appliances served by the pipe section and size accordingly using gas pipe sizing tables
Gas pipe sizing requires calculating the total BTU demand of all appliances downstream of each pipe section. The pipe must be large enough to deliver the combined gas volume at the required pressure, using sizing tables that factor in pipe length and allowed pressure drop.
Question 2: What is the typical BTU rating range for a residential gas furnace?
- 10,000 to 20,000 BTU/hr
- 40,000 to 120,000 BTU/hr (Correct answer)
- 200,000 to 400,000 BTU/hr
- 500,000 BTU/hr and above
Correct answer: 40,000 to 120,000 BTU/hr
Residential gas furnaces typically range from 40,000 to 120,000 BTU/hr input rating, depending on the home's heating load, climate zone, and the furnace's efficiency rating.
Question 3: According to Canadian gas codes, what is the minimum height above grade for a gas vent terminal on an exterior wall?
- 150 mm (6 inches)
- 300 mm (12 inches) (Correct answer)
- 450 mm (18 inches)
- 600 mm (24 inches)
Correct answer: 300 mm (12 inches)
Gas vent terminals on exterior walls must be at least 300 mm (12 inches) above grade or the expected snow line to prevent snow blockage and ensure proper exhaust dispersal.
Question 4: What is the danger of using an oversized gas orifice in a burner?
- The flame will be too small
- Too much gas will flow, causing a large, unstable flame, incomplete combustion, carbon monoxide production, and potential sooting (Correct answer)
- The appliance will consume less gas
- No danger — larger is always safer
Correct answer: Too much gas will flow, causing a large, unstable flame, incomplete combustion, carbon monoxide production, and potential sooting
An oversized orifice delivers too much gas to the burner, resulting in an excessively large flame, incomplete combustion, and the production of carbon monoxide. It also causes sooting and can overheat the appliance, creating a fire hazard.
Question 5: What is the difference between natural gas and propane gas orifices?
- There is no difference
- Propane orifices are smaller because propane has a higher BTU content per cubic foot and requires less volume for the same heat output (Correct answer)
- Propane orifices are larger because propane is a heavier gas
- Natural gas orifices are always red; propane are always blue
Correct answer: Propane orifices are smaller because propane has a higher BTU content per cubic foot and requires less volume for the same heat output
Propane has approximately 2.5 times the BTU content per cubic foot compared to natural gas (2,500 vs. 1,000 BTU/ft³). Propane orifices are therefore smaller to restrict the flow and deliver the correct BTU output for the burner.
Question 6: What must be done before purging air from a new gas piping installation?
- Light a pilot flame to burn off the air
- Ensure all appliance shut-off valves are closed, confirm the pipe has passed the pressure test, and purge the air to a safe outdoor location (Correct answer)
- Open all windows in the building
- Nothing special — just turn on the gas
Correct answer: Ensure all appliance shut-off valves are closed, confirm the pipe has passed the pressure test, and purge the air to a safe outdoor location
Before purging, the system must have passed the pressure test, all appliance valves must be closed, and the purge must be directed to a safe outdoor location to prevent any gas-air mixture from accumulating indoors where it could ignite.
306A Gas Piping Systems 3
What are the BTU requirements for sizing a gas pipe serving multiple appliances?