People who work on houses are a creative lot.In addition to the bathroom fan cover, another important place to check is the housing and motor for dust buildup. These five problems aren’t the only ways that your bath fan’s air flow might be compromised, of course. There are plenty of nice timer switches available for this purpose. Don’t just turn the fan off when you leave because it probably hasn’t done enough yet. What this means is that you need to run the bath fan with the door open for a while to get a decent amount of air exchange. When those cubic feet get hard to pull in, they aren’t going out as easily either. Every cubic foot (or cubic meter for those of you outside the US) of air that the fan removes from the bathroom must be made up by a cubic foot (or cubic meter) of air coming in. Unless your bathroom has a large gap between the bottom of the door and the floor or some other way for air to get into the bathroom, the bath fan will quickly be pulling against a pressure that makes it difficult to move much air. (But, hey, they have a radiant barrier!) If the duct is supposed to be going one way, the installer needs to install the fan housing with the duct opening facing that way. They’ll be lucky to get half the rated flow out of that fan. Not only does it have to make that turn, which reduces air flow, but the duct isn’t pulled tight, which reduces it even further. A bath fan with really bad ducting like this will not move enough airĪs stated before, flex duct has a high friction rate, and it’s easy for air not to have the oomph to make it through imperfect installations. I don’t know why this happens so frequently, but it seems that nearly half of the bath fans I see roughed-in at predrywall inspections are mounted so that the duct immediately makes a 180 degree turn on its way to the outside. The wire spiral that gives flex duct its round shape also gives it extra friction, especially when it’s not pulled tight, as you see here, so the air flow just peters out. …just as we get tired trying to find out how far this thing goes, the air inside gets too tired to make it all the way to the end. Here’s another fan with a duct that starts off OK. Right? Oh, wait that duct was installed decades ago. I’m sure the installer just took his lunch break and will be back any minute to finish this up. That duct needs to go all the way to the outside to do its job. (Even with a bigger gap, though, you don’t want to vent your bath fans into the attic because you need to get the moisture out of the house completely.) There might be a quarter inch gap between the duct and the joist, not enough to let much air out. Then it goes into a piece of rigid duct that goes right up to another ceiling joist. First, it goes over a ceiling joist in the attic. Duct terminates at obstructionīut let’s follow that duct and see where it takes the air. You can see from the dark spot on the fiberglass that some air does escape, but it won’t be much here. Here, instead of blowing into an open duct or open space, the air hits compressed fiberglass insulation. Except when it doesn’t, of course, as in in this case. The air doesn’t go where you’d like it to, but it flows easily. As you can see in the photo above, there’s no duct attached to this fan.
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