There is a foul smell coming from the floor drain, why?

An unpleasant odour may be due to one of the following reasons: the removable water trap is not in place, the drain is dirty or there is no water in the drain. If the floor drain is located in a space where only little water is used, it is possible to install a special water trap in the drain to prevent sewer odours from spreading into the room. The side inlet pipe may also be dirty (e.g. the pipe may have sagged during casting and now collects water -> dirt -> bad odour from the floor drain).