CCP HVAC TAB & Functional Testing 2 — Questions and Answers
Question 1: During TAB, an air handler's measured total airflow is 10% below design. What is the FIRST corrective action to investigate?
- Replace the supply fan motor
- Check and adjust variable frequency drive speed settings (Correct answer)
- Increase duct insulation thickness
- Add supplemental exhaust fans
Correct answer: Check and adjust variable frequency drive speed settings
VFD speed settings are the first adjustment point when measured airflow is below design, as they directly control fan RPM without physical modification.
Question 2: What does a negative static pressure reading at the supply plenum of an air handler indicate?
- Excessive filter loading
- Fan operating in correct positive pressure zone
- Potential fan rotation issue or severe restriction upstream (Correct answer)
- Normal operation for draw-through units
Correct answer: Potential fan rotation issue or severe restriction upstream
Negative static pressure at the supply plenum suggests fan reverse rotation, severe upstream restriction, or significant duct leakage pulling air back.
Question 3: Which instrument is most appropriate for measuring airflow velocity in a large rectangular duct during TAB?
- Pitot tube traverse with manometer (Correct answer)
- Vane anemometer at duct face
- Thermal anemometer at single centerpoint
- Rotating vane at outlet only
Correct answer: Pitot tube traverse with manometer
A Pitot tube traverse across a grid of points in the duct provides the most accurate average velocity measurement for large rectangular ducts.
Question 4: During functional testing of a VAV system, what parameter must be verified at minimum airflow setpoint?
- Maximum cooling capacity
- Adequate ventilation per ASHRAE 62.1 requirements (Correct answer)
- Full heating coil operation
- Return fan tracking at 100%
Correct answer: Adequate ventilation per ASHRAE 62.1 requirements
At minimum airflow setpoint, each VAV box must still deliver sufficient outside air to meet ASHRAE 62.1 ventilation requirements for occupants.
Question 5: A chilled water coil leaving air temperature is 5°F above design during TAB. The entering water temperature and flow are correct. What is the most likely cause?
- Coil bypassing due to fouling or air entrainment (Correct answer)
- Excessive supply airflow across the coil
- Undersized expansion valve
- Incorrect thermostat calibration
Correct answer: Coil bypassing due to fouling or air entrainment
Coil fouling, scaling, or air binding reduces heat transfer surface contact, causing leaving air temperature to exceed design despite correct water conditions.
Question 6: What is the purpose of the traverse method when measuring duct airflow with a Pitot tube?
- To measure static pressure at multiple duct locations
- To account for non-uniform velocity distribution across the duct cross-section (Correct answer)
- To determine duct leakage class
- To calculate duct friction losses
Correct answer: To account for non-uniform velocity distribution across the duct cross-section
The traverse method uses multiple measurement points across the duct cross-section to capture velocity variations and calculate accurate average airflow.
Question 7: During TAB documentation, what must be recorded when a system cannot be balanced to within acceptable tolerances?
- Only the final measured values
- Deficiency report identifying cause and recommended corrective action (Correct answer)
- Approval from the building owner only
- A note that balancing was attempted
Correct answer: Deficiency report identifying cause and recommended corrective action
NEBB and AABC standards require formal deficiency reports that identify the cause of unacceptable results and recommend specific corrective actions.
CCP HVAC TAB & Functional Testing 2
During TAB, an air handler's measured total airflow is 10% below design.
What is the FIRST corrective action to investigate?