I realize this is an ancient topic, but it is a classic problem. Ceiling exhaust fans have to be cleaned or they will spin and spin without moving air. I decided to tackle mine today. We leave it running 24/7 because the bedroom suite it is in has no other source of fresh air. The fan brings in air from the rest of the house. I found the brand name (Hampton Bay) on the grille and Googled it. I know it came from Home Depot and was bigger than I needed. I'm pretty sure it was the 140 CFM model for our 100 square foot bathroom. Found the installation instruction manual online and found that I only needed to remove 3 screws to lower the fan/motor out of the housing. I was about to remove the housing and all, so glad I found the instructions. To clean the fan I used a stiff bristled brush, wooden shishkabob skewer, a serrated PLASTIC knife, and of course a vacuum cleaner. Using metal scrapers or a wire brush would damage the plastic blades. Here's a better picture of the job half done. You can see the partially cleaned blades on the left and the gunk on the bottom. On the right side you can see what I was up against. This fan was installed a year ago. The people contributing to this topic who suggested using the brush attachment on a vacuum, moist cloth, or compressed air clearly have never cleaned a squirrel cage type exhaust fan. The moist air combined with dust forms a solid plaque on the fan edges that has to be cracked and then scraped off. The serrated parts of the plastic knife worked great for breaking the solidified muck off the blades. Unfortunately it seems like hundreds of blades to clean. I didn't count them, but you can see there are a lot. Every blade needs individual cleaning. I broke the surface muck off and then used the stiff brush to help scrape off the rest. That didn't work completely, so I had to hit it with both the pointed and blunt ends of the wooden stick. In the end I went over each blade about 3 times from the front and back sides. Here is the after picture. It is not perfectly clean, but getting the dirt off around the top is especially hard. That is the bottom of the blade when installed but the top when cleaning it. The whole project took 3 hours and was very tedious. Removing the squirrel cage from the motor and soaking it first might have made it easier to clean. Might try that next time. Another idea is to run just the squirrel cage through dishwasher on the top rack.

I would guess you can, but one big fan will work more efficiently than 2 small ones. I can't say for certain without seeing in person.

"...there are only 2 black wires, and 1 copper wire (ground). The instructions say i have to hook up the white wire to a set of white wires in the junction box. but there are no white wires in the junction box." I'm a bit confused by this description. Did you mean that there are 2 black wires, 1 copper wire and 1 white wire?. That's what would make sense. Assuming that's the case, here's the deal: There are a couple of ways switches can be wired. One way is that the power feed first enters the switch box with both a hot (black) and neutral (white). The neutral is simply passed on to the fixture (i.e., the incoming white is connected to the outgoing white). The incoming hot connects to the switch and an outgoing hot goes to the fixture. (So, two wires in, two wire out, right?) A second way is that the power feed goes first to the fixture. The white (neutral) connects directly to the fixture and the incoming hot goes to switch along with what we'll call a "return" wire. This is known as a "switch loop" and it has no neutral. One wire is "always hot" (the feed) and the other is the "switched hot" because it only gets power when the switch is on. [There are a couple of other ways to wire a switch, but these two are the most common.] A simple switch does not consume power itself. It only connects the two wires then it is closed ("on") and disconnects the two when it is open ("off"). The problem arises when a more complicated switch needs power for something. For example, a switch with a pilot light or night light consumes power itself and so it needs a neutral. Ditto with some timers. You cannot wire a switch that is itself a power consumer to a simple switch loop, which is apparently what you have. So, there are two possible solutions. First is to do some rewiring to bring a neutral into the box. One way to do this is replace the switch loop cable (two wires) with a three-wire cable, one of which will connect to the neutral at the fixture. Since you're not familiar with wiring, I won't elaborate on how to do this, or other ways this can be accomplished. You'd probably need an electrician. A second possible solution is to find an old-fashion mechanical timer that doesn't use electrical power itself. (It's basically a switch with a "wind up" spring.) This assumes, of course, that your intended purpose is just to let the fan run for a while and then switch off by itself. If your objectives are more sophisticated than that, I'm afraid you're SOL. Good luck.

The fan you selected has a built-in back draft damper. You should not have to install a special vent cap. A 4 inch wall vent with a damper should work well. As tempting as it is, I would not join the duct work with a wye. I think it is going to cause problems. I am not familiar with the Delta fans. Are they made by Panasonic? None of my fans have a humidity sensor. I installed an electronic timer switches. I start the shower and the timer at the same time. Both fans are inside the shower stall. It has worked well for the past 10 years.