By Eamonn Ryan
The following is derived from a Cape Town TechTalk on 10 June by Theo van der Linde, Pr. Tech, Head of Project at SFI Group, on the topic ‘Basics of Airflow Commissioning’. This is Part 8 of a 10-part article.
Thorough documentation and effective training are critical for the long-term success and maintainability of any HVAC system. Freepik
Real-world installations often present complexities that require adaptive strategies and diligent troubleshooting.
- Ideal measurement points: Accurate airflow measurement requires laminar flow, which is typically found in straight sections of ductwork, away from turbulent areas caused by elbows, transitions, or branch take-offs. As a general rule:
Avoid taking measurements immediately after elbows, dampers, or flexible ducts, as the airflow will be highly turbulent and readings unreliable. Flexible ducts, in particular, should never be used for final airflow measurements due to their inherent tendency to create turbulent and inconsistent flow.
Seek straight duct sections, ideally at least 3-5 duct diameters downstream and 1-2 duct diameters upstream of any fittings.
When measuring in ducts, follow established guidelines for taking multiple points across the duct’s cross-section (e.g., using a grid pattern for rectangular ducts or concentric circles for round ducts) to obtain a reliable average velocity.
- Permanent access plugs: After taking measurements in ductwork, it is crucial to seal the access holes properly. Using a dedicated duct plug is far superior to simply applying silver tape. Plugs ensure a robust seal, prevent air leakage, and, crucially, make the measurement points easily identifiable for future reference or re-commissioning, ensuring continuity of data.
- Troubleshooting low airflow: a case study: Consider a complex installation, such as a kitchen fresh air makeup system, which also serves a canopy. In one case, significant challenges arose due to a design that incorporated numerous short-radius elbows and flexible ducts in close proximity. This configuration inherently promotes turbulent airflow, making accurate measurement extremely difficult and often leading to low actual airflow compared to design.
When faced with low airflow, the troubleshooting process should begin systematically:
- Initial design review: Has the design been thoroughly vetted? Was the ductwork sized appropriately for the required airflow and static pressure, considering all fittings and their associated pressure drops?
- Filter/coil inspection: Check the condition of filters and coils. Are they clogged or severely loaded? As previously mentioned, high pressure drops across these components are a common cause of reduced airflow. In the example cited, a weather louvre with filter media was identified as a potential source of substantial pressure drop.
- Fan selection and performance: Was the correct fan selected for the duty point? In the specific case, the client had downsized the fan due to space constraints, impacting its ability to deliver the designed airflow against the system’s resistance. Even if a fan is operating, it may not be capable of overcoming the actual static pressure losses in a poorly designed or installed system.
- Ductwork integrity: Are there significant leaks in the ductwork? Airflow measurements at the fan outlet and at terminal units should be compared. If the sum of the airflow at the terminals is significantly less than the fan’s discharge, it indicates substantial leakage in the ductwork.
- Obstructions/blockages: Visually inspect accessible duct sections for internal obstructions or collapsed flexible ducts.
