We built our house in 1995. We have a basement floor drain with a metal screen cap. We are told by our plumber that it is a "P" trap or drain. We are supposed to pour some water into this drain every one or two weeks. If the drain runs dry an odour emanates from the drain. The smell is sewer-like and I would like to stop this odour from coming into our basement. Presently I pour some water with a little Javex into the drain to remove the odour. The odour returns in a matter of hours or in just a few days. What can I do to correct this problem?? Is there someting I can add to the drain that will not evaporate and stop the smell from escaping into our basement?? I hesitate to use vegetable oil for fear that it may turn rancid, thus causing another problem.