Every one of your plumbing fixtures is a delivery point located at the end of your water supply system, with faucets and valves to let you control when the water you want comes in. Used water leaves through your drain openings, which are the entry points to your sewer/septic system; each includes a trap to keep noxious gases within your sewer/septic system from backing up into your living space.

The traps for your sinks are the u-shaped bends in the drain piping you see underneath. Traps for toilets are built in to them, while traps for tubs and showers usually are concealed in floors or in the basement.

Because sewer gasses can’t pass through water, your traps are designed to insert a column of water in your drain lines as the means for blocking these gasses from rising into your living space. That is, they trap some water in your drain lines to seal the drain path to your sewer/septic system so that no gasses can back up into your home.

To work properly, vents are required on the sewer side of your traps to balance outside air pressure against the air on the house side. Occasionally, atmospheric conditions can cause correctly vented traps to lose their seal. This may be due to incorrect design of vents, to an absence of vents or to evaporation of water in the traps. To guard against evaporation of water in traps under fixtures that are used infrequently, they should be filled every so often by running water through them for a short time.

If your traps are filled with water and you detect the smell of any sewer gas in your home, call your licensed professional plumber right away. Sewer gasses definitely are neither pleasant nor healthy.