Can A Mechanical Valve Replace A P-Trap?

Some things just work. No matter how much someone tries to improve on a device, the standard method just works best. This statement applies fully to P-traps. If you don’t know what a P-trap is, this Boston plumbing tip is for you!

A P-trap is a small piece of drain plumbing that fits under your sink. It has a unique curved shape to it, and it is designed to prevent sewer gases from escaping into your living space. (That’s good because aside from the obvious “fresh-air” benefit, sewer gases can build up and explode.) Traps also prevent harmful bacteria from being introduced into the living space and serve to catch little items (like rings, earrings, Lego blocks) that accidentally make their way down the drain.

Over time, nasty debris – like biofilms, decayed material, hair, and soap films – also build up in the trap. Generally, traps are easy to remove and can be cleaned out mechanically without too much effort and without the need for special tools.

Traps work because after each use, the trap fills with water. The water in the trap acts like a plug and prevents the gases from escaping out of a drain hole. All plumbing fixtures – toilets, basins, tubs, etc., – must have a trap of some sort that prevents sewer gases from getting back up the drainpipe.

Traps have been in use almost since plumbing moved indoors. Newer plumbing codes restrict or prohibit the use of certain types of traps (S-traps, specifically) but P-traps are the gold standard of traps. They’re simple and they work better than anything else on the market.

Enter the mechanical P-trap. The mechanical P-trap is a straight piece of pipe that includes a membrane that is supposed to perform the same function as the “water plug” I discussed earlier. The biggest potential problem I see with a mechanical P-trap is malfunction.

When a traditional P-trap gets clogged with debris, biological buildup or a mechanical object, it still performs its basic function – blocking the movement of sewer gases back up the drainpipe. It can’t’ fail at this job because as long as there’s water (or sludge, debris, biofilm, etc) occluding the trap, the sewer gas isn’t going anywhere.

When you interfere with the open-and-close mechanics of a mechanical valve, the valve can potentially be held open. The design of a straight pipe – by its nature- will allow the offending gas to escape the pipe. In other words, when the valve fails, you can’t be sure that you’re protected from the dangers that unchecked sewer gases and bacteria present.

The traditional P-trap works even when it clogs because the design of the pipe itself provides the protection you really need. There are no moving parts to worry about so mechanical failure is impossible. P-traps can dry out – the water that forms the seal can evaporate over time if the fixture isn’t used, and deterioration of the pipe or the joints can force replacement of the trap. But under most circumstances, it’s hard to conceive of an improvement to the P-trap that’s worth taking a chance on.

If you have questions about your plumbing, or smell sewer gases when you use your plumbing fixtures, contact Boston Standard Plumbing & Heating at (617) 288-2911 for an inspection. We’ll find the source of the odor and recommend a solution!

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