It will happen if the two pipes for the waste are joined and the flow from the basin is very good. Is this not why in older houses each item is to a separate place? My toilet goes straight into the side of the stench pipe, but the basin and shower/bath goes to a pipe that goes into a hopper from a pipe further along the sewer than the stench pip, otherwise the um stench would surely come up the basin on every flush, surely? Brian
If every water-filled container has a P-trap in line to the stench pipe, that's what prevents sewer vapour from coming up.
If you don't have a P-trap, yes, it'll stink.
If you don't use a basin for five years, the P-trap dries out and stench comes up in the basin. Water traps only work if there is water in them.
The bowl of your toilet, contains water at all times. When you flush, some of the recharge water is used to refill the bowl. The bowl then, is the P-trap for that device. The water in your toilet bowl, prevents stench from coming up from the sewer.
The letter "P" is oriented, such that the loop on the letter P is pointing downwards. Grab a letter P by its bottom, and rotate 90 degrees clockwise. The loop of the P is the part that holds the water, that blocks the sewer gasses. Your wash basin in the bathroom, should have a section of pipe imitating the letter P turned as described.
Paul
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