Sewer smell from basement drain

I'm finishing my basement and just had a bathroom installed down there as well. Nothing (new pipes, laundry) has been in use for a few weeks. Last night, I started to get a sewer smell coming from the drain in the middle of the floor.

I'm having carpet installed this week and am scared that something might happen. The carpet is expensive!!!

Does anyone know what could be causing this? Will it go away when the pipes start being used again? Is it safe to be carpeting over the floor drain?

Floor drains should never be carpeted over.

#1 pour water into it and the smell should go away

#2 If you carpet over it the moisture will cause mold and mildue to grow in the pad and carpet.

#3 once the trap drys out under the carpet you can't get water into it to prevent the odor.

The reason you are getting that smell is the trap in the drain has evaporated. As Cass notes, you should never carpet over a floor drain. Another thread recently had a similar problem. One solution was to pour mineral oil in the trap because it wouldn't evaporate. The problem with the oil is that if there ever was a sewer backup, your basement and carpet would be flooded. My suggestion was to totally seal the drain by filling the trap with Fixall so that water couldn't backup into the basement.

This is probably (certainly ) not to code but you can get test plugs that fit into standard 2", 3" or 4" drains. Put it down into your floor drain, tighten the wingnut and the rubber gasket will seal the drain. This would eliminate the risk of odor and backup, but would allow you to remove it in the future should you want to resurrect the drain. I'd still provide access to the plug so it can be periodically inspected easily.