Table 1. The Causes and Effects of Poor Airflow

How To Traverse the Duct

After familiarizing oneself with the problems of airflow, it is time to define what is meant by “poor” airflow, particularly across the evaporator. The industry rule of thumb is 400 CFM per ton. This means that for a 5-ton air conditioner, the fan should move roughly 2,000 cubic feet of air per minute to remove heat at the rate for which the equipment is designed. Whatever restrictions the fan has to overcome will cause less air to be moved. There are several ways to measure airflow, but a common tool that fits in a service technician’s tool bag is the hot wire anemometer. The anemometer uses a telescopic probe to directly measure the average air velocity at multiple points in a two-dimensional space of the ductwork, which is converted to cubic feet per minute after the technician measures the square footage of said space. The technician will need to drill holes in the side of the duct, downstream of the fan, and far enough away from bends and sudden changes in the ductwork.

Traversing ductwork, the process of taking relatively even-spaced measurements, is important because air does not travel down ductwork in a smooth, uniform column. Rather, it tumbles around and even creates pockets of no air movement, thus only an average of many measurements will yield an accurate measurement of how much air is moving. Figure 1 demonstrates an example of how to traverse using an anemometer to measure CFM. Comparing the CFM to the rated tonnage of the equipment will determine whether there is sufficient air movement for cooling.

FIGURE 1: Consult instructions for detailed guidelines, but the general concept is to record measurements at evenly spaced points throughout a cross-section of ductwork, preferably at a location far enough downstream of the fan to allow airflow to come to a uniform distribution. (Courtesy of Lianna Schwalenberg)

How To Measure Static Pressure

A metric often associated with airflow is static pressure, commonly measured in inches of water column, whose concept is similar to that of blood pressure in the human body. Static pressure does not directly measure air flow; rather, it measures how much pressure the fan has to push and pull against to move air through the duct system. All fans have a rated maximum external static pressure above which they will not be able to move the required amount of air. In this sense, static pressure can indicate whether and roughly where air is struggling to flow. The industry rule of thumb for total external static pressure is a value close to or greater than 1.00 inch of water column indicates a problem. The problem can be isolated to specific locations within the duct system—such as the filter, coil, supply ductwork, or return ductwork—based on which area has abnormally high static pressure.

The test instrument required for static pressure measurement is the manometer—which can either be analog, such as a Magnahelic, or digital. The technician will also need tubing and static pressure tips. Similar to traversing a duct, the technician will need a drill, but not as many holes are needed for measuring static. The static pressure tips should be pointed against the direction of airflow for the most reliable measurements. Table 2 outlines the common places to take static pressure measurements as well as “budgets” recommended by the National Comfort Institute for the percentage of what each component of the system can account for in the total external static pressure.