Have you ever wondered what that "U” shaped pipe under your kitchen sink does, besides getting clogged with gunk? It’s called a P-trap. It got its name because if you hold it straight up and down it somewhat resembles the letter "P”. I personally would have went with U-trap, but I presently have no influence in the plumbing nomenclature department. So for the purpose of this article, I’ll stick with just "trap.”
Whether or not you can see them, every drain in your home has one. Traps are designed to "trap” water after each use of the drain. This trapped water acts as a barrier which prevents foul smelling sewer gasses from wafting back up into your home. In order for a trap to work it needs to be filled with water. A dry trap does not keep the offensive smells at bay. Kitchen sinks, bath sinks, showers and tubs usually get used enough to keep their drain traps filled with water. Floor drains and washing machine disaster pans are another story.
Most houses have at least one floor drain somewhere on the lowest level. Often times it can be found in the utility room where the furnace is located. Floor drain traps are buried under the concrete, and if they don’t get filled with water, in all likelihood, your nose will know. Most furnaces, air conditioning systems and water softeners use a floor drain during the course of their normal operations, so chances are, the floor drain trap in your utility room is being replenished on a regular basis.
But you may have a floor drain in another location that isn’t being used often enough to keep the trap filled. We have come across situations where an uninformed prior homeowner lays carpet right over a floor drain without capping it off. Also, some builders install a floor drain where the washing machine is located. This is good planning, but again, homeowners need to be aware that every trap needs to be replenished every now and then.
If your washing machine is not on the lowest level of the building, it may be sitting in a fiberglass disaster pan. These pans are designed to act like a floor drain and catch the water in case the washing machine springs a leak, hopefully saving thousands of dollars in water damage repairs in the process. Disaster pans are normally plumbed differently than a floor drain; it’s called an indirect drain.
Plumbers know that disaster pans are used infrequently or not at all, so they typically do not tie the pan drains "directly” into the house’s plumbing system, but rather have a line that runs to a floor drain somewhere else in the house. Consequently, disaster pans plumbed indirectly do not have a trap that could run dry from infrequent use. This isn’t always the case though. We have encountered disaster pans that are plumbed the same as a sink, which means, every so often they need to be filled with water.
Filling a trap is as easy as pouring water down the drain. A quart of water should be sufficient. If you know that a particular drain is not going to get used anytime in the near future, after pouring in the water, add a couple tablespoons of vegetable oil. The oil will sit on top of the water and minimize the evaporation process. Even without the oil, it could take a year or more for a trap to dry out.
If you do have a dry trap, you may not experience a consistent odor. It may be an intermittent problem. It all depends on where your home is located relative to the other homes in the neighborhood. You may have a dry trap and never smell a thing, but if you do occasionally catch a whiff of some unpleasant odor, stop blaming it on your poor cat and check your traps.