FC FC Ductwork & Air Distribution 1 — Questions and Answers
Question 1: What is the purpose of a static pressure test on a forced-air furnace system?
- To check gas pressure
- To measure airflow resistance in the duct system and verify proper system operation (Correct answer)
- To test the heat exchanger for leaks
- To measure combustion air volume
Correct answer: To measure airflow resistance in the duct system and verify proper system operation
Static pressure testing reveals duct restrictions, undersized ductwork, or dirty filters that reduce airflow and system efficiency.
Question 2: What is the maximum total external static pressure (TESP) typically allowed for a residential furnace?
- 0.1 inches W.C.
- 0.5 inches W.C. (Correct answer)
- 1.0 inches W.C.
- 2.0 inches W.C.
Correct answer: 0.5 inches W.C.
Most residential furnaces are rated for a maximum TESP of 0.5 inches W.C.; exceeding this restricts airflow and causes overheating.
Question 3: What is the primary cause of high static pressure in a residential duct system?
- Oversized ductwork
- Restrictive air filter, undersized ducts, or excessive elbows (Correct answer)
- A dirty heat exchanger
- Low gas pressure
Correct answer: Restrictive air filter, undersized ducts, or excessive elbows
Clogged filters, undersized supply or return ducts, and numerous sharp elbows are the most common causes of excessive static pressure.
Question 4: Which type of duct fitting creates the least resistance to airflow?
- Sharp 90-degree elbow
- Square elbow with turning vanes
- Long-radius swept 90-degree elbow (Correct answer)
- Tee fitting
Correct answer: Long-radius swept 90-degree elbow
A long-radius swept elbow allows air to change direction gradually, creating far less turbulence and pressure drop than sharp elbows.
Question 5: Return air duct sizing should be based on which primary factor?
- Furnace BTU input
- System airflow (CFM) requirements of the equipment (Correct answer)
- Number of supply registers
- Duct material type
Correct answer: System airflow (CFM) requirements of the equipment
Return ducts must be sized to carry the full system CFM with acceptable velocity and pressure drop to ensure adequate airflow to the blower.
Question 6: What is the recommended maximum air velocity in residential supply duct main trunks to minimize noise?
- 400 FPM
- 900 FPM
- 700–900 FPM (Correct answer)
- 1,500 FPM
Correct answer: 700–900 FPM
Residential supply trunk velocities of 700–900 FPM are recommended to balance duct size with acceptable noise levels.
FC FC Ductwork & Air Distribution 1
What is the purpose of a static pressure test on a forced-air furnace system?