The sink in my master bathroom emits a horrible sewer gas smell when the water is first turned on and runs down the drain. After a short time the smell goes away (I guess the gas that has built up dissapates). If the sink sits for half a day or so without being used, then the gas smell is back when water is run down the drain again. Any suggestion?
What this sounds like is classic capillary attraction where some hair in the trap is partically hanging into the drain pipe acting like a wick much like a cigarette lighter sucking the fluid ( water out) thus causing a trap seal loss.
The trap is only designed to prevent foul air from eentering the fixture and the water in the trap is the seal. Good luck
A mix of soap and hair (shaving) build up in the trap, the smell is those two rotting. Pull the trap out and flush it fully. Or use a mild sulphuric acid to break down the rotting tissue. ANY ACID WILL BURN …..USE GLOVES GLASSES AND ANY OTHER PROTECTION THAT IS SUGGESTED ON THE BOX. SULPHURIC ACID IN WATER WILL BOIL, THIS MIX IN A CERAMIC TOILET WILL CAUSE THE BOWL TO CRACK. BE BLOODY CAREFUL !!!!!!!!!!