That sewer smell and why you need a P trap

Imagine your home and its beautiful bathroom, but the minute you set foot into it, you cringe your nose. A sewer smell in your house could be the result of a dried-out P-trap or drain with improper ventilation.

It’s not uncommon for a customer to ask for our help finding the source of a bad sewer gas smell. This plumbing problem is tough enough to diagnose and locate, but it happens quite often. We usually start with a few pointers over the phone to see if they can solve the problem without our help.

Tracking the smell of sewer gas

First, we ask whether they’re experiencing a real sewer gas smell or something else, perhaps a dead animal smell? The difference is fairly easy to discern. Usually, a dead animal smell has a strong, sickeningly sweet odor that will almost always bring about a gag response in your throat and often makes your stomach churn.

A regular sewer gas smell is just a bad stench with a definite odor of feces. Sometimes it includes a rotten egg (hydrogen sulfide) smell and possibly a moldy mustiness.

We also ask what has been happening in the home or business lately in terms of remodeling, pest control, etc., to get clues. Finally, we talk to them about P-traps and the possibility of some fixture’s P-trap “drying out.” An empty or dried-out P-trap is the most common cause of sewer gas smells.

Sewer gas and P-traps

Every plumbing fixture and plumbing drain has a P-trap on the drain if it is installed according to norms. These P-traps protect us from sewer gas. If a P-trap has become empty of water or dried out for any reason, it will allow sewer gas inside the waste pipes to pour into where we live and work.

Every P-trap on every drain needs to have a vent that protects it. Plumbing vent stacks are what all of those pipes are that stick out of our roofs.

P-traps require good ventilation

Every time a drain is used, the water and waste rushing down our pipes displace a lot of air, and it makes our plumbing systems breathe, in a way, both in and out.  If that drainpipe can’t pull and push air down the drain, then it will try to pull and push air right through our P-traps, sometimes making the sewer gases go into the room.

That’s why sometimes, when one drain is draining we’ll start to hear a “glug, glug, glug” sound from a different drain. It’s because air and sewer gas are being sucked or pushed through a P-trap. And if the water in that P-trap gets sucked or pushed enough, it gets so low that the seal is lost and no longer protects us from the sewer gases.

Unused or improperly vented P-traps pose problems

Some drains have visible P-traps, such as those under a bathroom or kitchen sink, and some drains have P-traps that are hard to see or hidden, such as on a floor drain, shower or toilet. However, every drain must have a properly vented P-trap, and every P-trap must have a regular supply of water going through it to keep the “trap seal” working properly. Sometimes just running water for a couple of minutes in the bathroom sends the smell back.

Nine times out of ten, a sewer gas smell is coming from a P-trap that has lost its water for some reason. Maybe the trap is just drying out from not being used; maybe it’s leaking, such as a clump of long hair that’s wicking the water down enough to break the water seal.

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