HVAC Ductwork 4 — Questions and Answers
Question 1: What is 'total external static pressure' (TESP) in an HVAC system?
- Static pressure inside the refrigerant circuit
- Combined pressure drop across all external components the blower must overcome (Correct answer)
- Barometric pressure at the installation site
- Velocity pressure at the supply registers
Correct answer: Combined pressure drop across all external components the blower must overcome
TESP is the sum of all static pressure drops external to the air handler — including ductwork, filters, coils, and grilles — that the blower must overcome to move air.
Question 2: A duct system has a design static pressure of 0.08 in. w.g. per 100 feet. What design method is being used?
- Velocity reduction method
- Equal friction method (Correct answer)
- Static regain method
- T-method
Correct answer: Equal friction method
The equal friction method designs all duct sections to have the same friction rate (pressure drop per 100 feet), making it straightforward to size ducts from tables.
Question 3: Fire dampers are required by code to be installed where ducts:
- Change from round to rectangular shape
- Exceed 12 inches in diameter
- Penetrate fire-rated walls or floor-ceiling assemblies (Correct answer)
- Connect to return air plenums
Correct answer: Penetrate fire-rated walls or floor-ceiling assemblies
Fire dampers must be installed wherever ducts penetrate fire-rated construction (walls, floors, ceilings) to prevent fire and smoke from spreading through the duct system.
Question 4: What is the function of a duct sound attenuator (silencer)?
- Reduce velocity pressure to static pressure
- Absorb sound energy to reduce noise transmitted through ductwork (Correct answer)
- Prevent vibration from transferring from the AHU to ducts
- Filter airborne particulates from supply air
Correct answer: Absorb sound energy to reduce noise transmitted through ductwork
Sound attenuators contain sound-absorbing material (typically fiberglass) that dissipates acoustical energy from fan and airflow noise before it reaches occupied spaces.
Question 5: In a duct system, what causes the 'stack effect' to influence duct airflow in tall buildings?
- Excessive duct insulation reducing heat transfer
- Temperature difference between inside and outside air creating buoyancy-driven pressure differences (Correct answer)
- Over-pressurized supply plenums on upper floors
- Condensation inside vertical duct risers
Correct answer: Temperature difference between inside and outside air creating buoyancy-driven pressure differences
The stack effect occurs when indoor-outdoor temperature differences create buoyancy forces that drive air up or down through vertical shafts, affecting duct pressure and airflow distribution.
Question 6: A technician measures 350 CFM at a supply register designed for 400 CFM. After checking the duct system, they find a collapsed section of flex duct. What is the most likely effect on system static pressure?
- Static pressure decreases upstream of the restriction
- Static pressure increases upstream of the restriction (Correct answer)
- Static pressure remains unchanged throughout the system
- Static pressure drops to zero at the register
Correct answer: Static pressure increases upstream of the restriction
A restriction (collapsed flex duct) increases resistance, causing static pressure to build up upstream of the blockage while reducing airflow downstream.
Question 7: What is the primary advantage of a radial duct system over an extended plenum system?
- Lower material cost for large commercial buildings
- Each branch run is roughly equal length, improving airflow balance (Correct answer)
- Easier to install fire dampers throughout the system
- Allows higher air velocities without noise problems
Correct answer: Each branch run is roughly equal length, improving airflow balance
In a radial (spider) system, individual ducts run directly from a central plenum to each register, creating equal-length runs that naturally balance airflow without extensive damper adjustment.
HVAC Ductwork 4
What is 'total external static pressure' (TESP) in an HVAC system?