Despite having central air, we'd be lost here without our box fans. With the exception of nights over 70 degrees, we put a box fan in the masterbedroom window every night of the year. I'm not kidding. Ten degrees outside? We've got a fan in the window bringing in that sweet, cold air!
In the summer, we run two box fans plus a ceiling fan in the kitchen (which is an amazing fan. With the exception of days we are gone away on vacation, that ceiling fan has run 24 hours a day, 7 days a week, for 24 years!). The more you can keep air circulating, the better your central air will run, and the better you will feel.
But fans attract dust. If you don't clean that dust, it attracts more dust, and then mold. If the dust gets caked onto the motor of your fan, your fan will stop working. And if you don't clean off that dust and mold, your fan is blowing dirty, unhealthy air around your house.
I take on the task of cleaning my two box fans twice a year. It takes me about an hour to do both fans. It's actually quite easy, especially with a battery-operated screwdriver or electric drill with a screwdriver bit.
If this is your first time doing something like this, take a pencil and make a little mark at the top of the fan cover, front and back, so when you go to put the covers back on, you can tell which is the top. Also, get out some kind of container to place all your screws (we used a paper plate). It's nothing but a waste of time to have to hunt around the floor for mis-laid screws!
First, I remove the six screws on the front cover, take off the plastic cover, flip the fan over, and remove the six screws on back, and take off that cover. (Obviously, you want to make sure your fan is unplugged before you do this.)
Next I fill my kitchen sink with hot water and Pine-Sol cleaner. I bring the fan covers to the sink and wash them well, front and back of each cover. If your fan is really dust-caked, you may need to use some kind of brush between the grate of the fan. Keep at it until it's clean! If you're going through all the work of taking apart the fan, you don't want to put it back together if it's as clean as you can get it.
Then I get out my vacuum and put the small brush attachment on, and vac as much of the blades and motor and inside case of the fan as I can reach.
Then I bring a damp cloth (not dripping wet!) to the fan, and wipe down the front and back of each blade. Do NOT let water drip into the motor section of the fan. I also wipe down the inside of the fan case, the outside, and the on/off switch and handle.
I dry off the covers, bring them back to the fan, and put the screws back in (look for your pencil mark at the top of the panel if you made one), making sure to get the front cover and back cover placed correctly.
That's it! Free of dust and mold now, and no dust clogging up the back of the motor, your have just extended the life of your box fan, and improved the quality of air in your home.
And look -- I had a helper! Mr. B's first time with power tools. He did a great job.