Sewer Smell

[Steve H]

Need HELP. I have a home that is 30 years old. It's built on a slap. anytime we use the dishwasher or the washer I have a sewer smell for 30 min. Nothing looks to be leaking, Note hot or cold water same smell? Smell just started a month ago

[LazyPup]

Drain pipes are sized so that the normal flow will fill the pipe approximate 1/2 full. That allows sufficient volume and force of flow so that the liquids will also carry away any solids that may be present in the waste, while still leaving the top half of the pipe open to permit vent air to move.

Normally the drains flow by gravity force, however, washing machines and dishwashers have pumps which makes them a very high volume discharge when compared to a sink draining by gravity flow. As the waste is moving through the pipe it is displacing the air in front of it as it goes. Normally that displaced air would pass over the liquid and occupy the space behind the liquid, keeping everything in equilibrium.

If the drain lines happen to have a partial clog, or more likely a heavy buildup of scum on the pipe internal walls that will reduce the effective diameter of the pipe, thus raising the water level and reducing the amount of vent air that can move in the pipe. As the liquid continues moving downstream, it continues displacing the air in the pipe. The air now having no place to go, is forced up another branch line or waste arm and will eventually bubble up through a trap somewhere, which causes the foul odor you are smelling.

You may have some success in using a liquid drain cleaner to dissolve the excess scum, but usually that is a futile effort at best.

The permanent solution would be to have the line snaked with a power auger that has a cutter sized to match the diameter of the pipe to insure it will cut the scum off the pipe walls.

Normally what you are experiencing now is just a precursor to a much more serious clog that will occur in a short time unless the above described precaution is taken.