I just bought a new home that was pre-wired for ceiling fans. When I went to install the first one, I found an odd looking plastic junction box. It has no threaded holes for the hanger screws. The cover plate was held on by what look like wood screws that had been screwed directly into the box. Where there would normally be threaded slots for screws, there are slots but no threads. The box has half of the diameter solid on bottom and the other half has wires. Is this a special ceiling fan box, or some temp box that needs to be replaced?

Thanks

If it is the one I'm thinking of, they were/are meant for fans. You screw through them and into Wood Joist? I always use 4X4 Pressure treated wood and Fan Approve Boxes, Fans don't hang by box screws.

Thanks, that's what I suspected but it's good to get advice from the experts. Any advice on how long of a wood screw to use for a normal installation?

I've never actually used these for fan installation, I only use metal boxes, but I've seen them.

My advice would be to go to a hardware store and inspect the same type of box in person. They usually come with screws, you can match up the screws you buy to those.

2 2" #10 Panhead, washers, locking will not hurt. There may be a spot in the middle for a lag bolt.