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Hey Ed, How Does Manual D Deal with Variable Speed Systems?

In this edition of "Hey Ed," Ed talks about Manual D Deal and variable speed systems.

Hey Ed, how does Manual D deal with variable speed systems? It doesn’t. This is a problem that I have seen quite a bit in the last few years. And if you look at Manual D, it specifically states it’s for a constant volume system. Meaning the guidance in Manual D in conjunction with Manual T is looking at a specific volume of air coming out of a supply outlet that doesn’t vary. When we use systems that vary the volume coming out of supplies, then if you want that throw and spread to be consistent, we would also have to vary the opening, which makes up that register. I guess that could be a thing, but it’s presently not. So keep in mind, if you have a system that moves various amounts of airflow, your manual D duct design was not ever designed for that. And if you must do it, there’s always zoning, and that’s the way I see it.

Ed Janowiak is the Manager of HVAC Design Education at ACCA.

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