Photo by Pat Lenius

Probably no HVAC part is more misapplied than airflow diffusers. I have spoken about this subject many times before RSES, ACCA, and ASHRAE groups as we sat in meeting rooms and auditoriums.  I usually say to the group (to prove my point), “Just look up!” And almost invariably, I can point out at least 10 airflow mistakes in any engineered design, for they are so common.

The fact is, most airflow diffusers are selected more on the basis of low cost versus performance, and most are installed with no consideration to proper flow and distribution. Location is almost always placed secondary (almost as an afterthought) to light fixtures, columns, walls and decorative features.

Is there a place in your office or home right now where you don’t care to sit because it is either too drafty or too stuffy? There probably is, and the culprit is likely poor diffuser selection and/or application. But such problems usually go undiagnosed because people in general know so little about proper air distribution.

The ideal location and application for air diffusers in an air-conditioning application is a ceiling mount (because cold air drops) where every part of the inhabitable lower portion of the room is equally bathed in conditioned air blowing at about 50 fpm. The diffusers shouldn’t blow into each other, into a return or into light fixtures or walls. The conditioned air should drop evenly into the room from the ceiling.

The best location for heating vents is in the floor, because heat rises. And contrary to the favored design parameters of cooling vents, noticeable air movement is less desirable in heating applications, since it has a chilling effect (the problem with heat pumps). But regardless of the best location and optimal air movement, most jobs actually go in using the same diffusers.

It is important to have constant airflow in commercial buildings due to the typically higher ceilings. When all airflow stops (as in cycling), the room becomes stuffy and uncomfortable. However, we usually put up with this in our homes in an effort to lower system operating costs.

But when speaking to engineers and contractors about the need for proper air distribution, the most common thing I hear is, “I just want to get the air down and do it at a reasonable price.” So all those design factors and tests that diffuser manufacturers are so concerned with don’t make a lot of difference in reality.

I can testify to the fact that here in Florida most residential air systems are never balanced after installation, and diffusers are selected on rough room-size estimates and price. People buy ceiling fans to turn uncomfortable rooms into a better design.

Another rather recent problem is the rising popularity of variable-speed evaporator fans. Although they lower operating costs, they effectively negate all design considerations, since constant duct pressure is required to maintain proper “throw” and diffusion.

So what am I suggesting? If your company sells air diffusers and you offer training, classes on proper diffuser selection and application are sorely needed.