An unused bathroom drain can begin to smell after a certain amount of time. This smell most often comes from tubs and sinks due to an unused plumbing trap, or P-trap. Plumbing traps are the U-shaped dips in the pipe underneath the bathroom drain. These traps are in place to prevent your bathroom and home from being filled with sewer odors. With normal use of a bathroom, these traps remained filled with water, which provides a seal that prevents sewer smells from coming up through the drain.
However, in the case of an unused bathroom drain, the water in the traps gradually evaporates after a certain amount of time, at which point the seal that was keeping the sewer odors out is eliminated. Bathroom cleaners and liquid drainage products can temporarily mask the smell, but neither prevents evaporation, therefore neither can prevent the sewer odors from permeating.
Luckily, the solution is simple. If the problem is a lack of water in the P-traps, the answer is to put water back in these P-traps. You can do this by running the water in these bathroom drains for a few minutes, weekly or monthly, depending on the severity of the smell.
If running water isn’t solving the problem, you can try pouring some ingredients down the drain every so often. First, pour a small amount of vinegar (a cup should do) down the drain, followed by a quarter cup of baking soda. Let that sit for about 15 minutes, then run hot water run down the drain, rinsing out the solution for about 3 minutes. Rinsing is critical. After refilling drain traps with water, you can pour a couple of tablespoons of vegetable oil down the drain. The oil will float on top of the water in the trap to prevent the water from evaporating too quickly.
If the smell is still pervasive after following these steps, call a professional.