John Tooley said, “Air is like crooked rivers, crooked people, teenagers, and cheap labor. It always seeks the path of least resistance.” He didn’t say that Angry air is Noisy air. Air doesn’t like being forced through corrugated, flexible ducting, pushed around corners, and made to force open dampers. It resists being made to perform in a way that it doesn’t want to. It takes more and more force as the resistance increases. Air is just fine when you just let it move at will. It can become amazingly strong as any building that has met a hurricane or tornado can attest to. And as objects like asteroids and space capsules hurtle through the atmosphere they burn up!
ASHRAE 62.2 requires bathroom fans to make no more noise than a quiet refrigerator in a quiet kitchen: 1 sone or less. And if you put an Energy Star bathroom fan on the bench and plug it in, you can barely hear it. It’s amazingly quiet. “Is it running?” people ask. And it is. So how come once you install the fan in the ceiling it gets uncomfortably loud?
Fan manufacturers not only made these fans quiet, they put DC motors in them that are extremely tolerant ofchanges in pressure. As the pressure increases in the installation, the fan motor compensates by using more power to increase the speed of the spinning wheel that is pushing the air. (Notice the curve on this graph that starts on on the left side and then drops off the cliff at about 75 cfm. It has about the same airflow from 0.45 iwg as it does at 0.0 iwg!) That’s a wonderful thing because people can install the fans horribly and step on the duct and lots of other nasty things and still come out with the same airflow . . . but not the same sound level. What was really, really quiet is now uncomfortably loud. And as houses get tighter they get quieter and a noisy fan is annoying which is why so much effort was made to get them quiet so they could run all the time without bothering anyone!
I have found that builders get aggravated because these quiet and expensive fans that they have been compelled to install really aren’t all that quiet. And they should be quiet. They have been designed to be quiet. Tested to be quiet. And if you disconnect them from the installation, they are quiet.
So here’s a simple way to determine if the fan is working right: listen to it. If the air is angry, it will be noisy and noisy DC fans equal bad installation. The air is yelling at you. I have found ducts filled with the foam that was sprayed on the house for insulation. Backdraft dampers remain taped closed. Ducts terminated against a wall or floor in the attic and don’t actually get to the outside. If a bathroom fan that is rated to be < 0.3 sones is noisy, its a bad installation. Period. Fix it. It may still be moving enough air to meet the ventilation requirements, but if it is noisy the homeowner will find a way to turn it off and stuff it full of socks. Then the air in the house will get bad and people will get sick. And the occupants will get angrier than the air! And the really dumb thing is that all these codes and standards and mathematical computations and formulas to size the fan correctly mean absolutely nothing if the fan is turned off.