The bottom line on the proper and efficient operation of any HVAC system is that the air flow through the duct system has to be correct. If it’s not, then any other segments of the equipment operation such as the refrigeration system, fuel-burning system, or resistance-heat system, simply cannot perform as they are designed. The end result of this imbalance of operation between the equipment segments is poor performance that wastes energy and money, and can even become a safety and health concern for the building occupants.
What this means is that specific measurements must be accomplished to ensure proper operation of the equipment, and one way to approach this task is to use an in-duct hot wire anemometer and a traverse measurement system in the main supply plenum. A digital device such as the one shown in Figure One that is capable of monitoring air temperature as well as measuring volume and velocity, allows you to enter duct sizing information, and has the capability to average so you can accurately calculate the total air flow, will tell you if there is sufficient air flow overall through the equipment. In a comfort cooling system, this accomplished simply through the process of accurate measurement in a duct, and then comparing the information you obtain to the manufacturer’s standard recommendation of 400 CFM per ton of equipment capacity.
Determining possible traverse locations for measurement applies to both rectangular ducts and round ducts, a sample of each of which is shown in Figures Two and Three.
With a fundamental understanding of traverse locations, a technician can use available tables and other information to consider the proper procedure for determining traverse air flow testing in a specific situation. As an example, consider the equipment shown in Figure Four where the supply plenum is shown positioned on the air handler cabinet.
For our purposes, we’ll determine that the duct assembly has a width of 24” and, because of that factor, we have chosen five traverse locations in a position on the plenum that is far enough away from the air handler. These five points are the locations for drilling a 3/8” hole in the duct. To accomplish the traverse measurement process, the probe of our anemometer will be inserted slowly through each opening until the tip of the probe touches the inside of the duct on the opposite side from our measurement position. During each insertion, the anemometer will be set to take a measurement, and those measurements will be averaged to tell the technician what the total air flow measurement is in CFM.
In the event that the air flow is found to be insufficient, steps can be taken to modify the duct system or adjust the fan speed in order to ensure a proper balance between the air flow and refrigeration systems in the equipment.