Flow hoods are used to measure supply and return grille airflows in HVAC systems. Depending on hood accuracy, these measurements can be used to determine if individual rooms receive adequate airflow for heating and cooling, to determine flow imbalances between different building spaces, to estimate total air handler flow and supply/return imbalances, and to assess duct air leakage.
Accuracy requirements are based on the flowhood application. A poorly calibrated flow hood and sensitivity to grill flow variances, result in an over or underestimation of flow by 25-35%. Also, one must consider original design specifications of the HVAC system and if any room changes have been made. A great original duct design and a new HVAC system originally did everything it was intended to do, however, poor HVAC maintenance, room modifications without design re-work, and changes in source conditions, will result in building related issues. Flowhoods are useful and are sometimes limited to the application such as duct breakage, room comfort, and duct leakage. The change in airflow pattern between calibration and application creates large flow measurement errors.
Commentary regarding flow hood accuracy issues are discussed in ASHRAE Standard 111 (ASHRAE 1988. Current testing procedures do not account for approach errors and that most diffuser measurements are not reliable. They recommend a pitot tube traverse to field calibrate the flow to account for these problems. The ideal situation would be to have laboratory calibration account for actual field conditions. CIH Equipment’s flowhood laboratory calibration station is configured to mimic actual field conditions with a supply and a return side and using pitot tube traverse measurements.
Why calibrate your flowhood?
- Your flow hood is compared against our reference standard under the most accurate and realistic conditions, with both a supply and return register.
- We achieve this by means of taking the required velocity measurements both horizontally and vertically, average the readings, and compare to our NIST traceable reference.
- Our procedures are designed to maximize the accuracy, repeatability, and reproducibility of your velocity measuring instruments.
- Our standards are NIST traceable and our lab is ISO 17025 accredited.