This month’s troubleshooting situation centers around a very recently installed heating and cooling system, and a customer who has called to complain that certain rooms in the house “just don’t seem to be getting enough air.”

(Before we begin with the specific troubleshooting information on this situation, we’ll establish a couple of points… First, the problem isn’t related to the refrigeration system in any way, and second, there are no electrical problems contributing to the complaint. In this situation, we’re dealing with an airflow problem. Note that Figure 1 shows the design of the supply duct system exactly as it appears in the attic crawl space.)

When you arrive at the customer’s home, you find that the two rooms the customer is complaining about are closest to the air-handling unit in this duct system that employs one central return and an extended plenum system on the supply duct assembly. In assessing the situation, you determine that the airflow from each supply register in these two rooms should be approximately 120 cfm, but when you measure the airflow with a digital velocity meter, you find the airflow to be approximately 90 cfm. As your next step, you move on to the crawl space to check the supply ductwork and find no leakage or crushed duct, but you do find, as we mentioned above, the supply duct system in Figure 1.

Your troubleshooting problem: What design flaw is responsible for the specific airflow problem we’ve described in this situation?

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Publication date: 01/05/2009