The traps in floor drains — or for that matter, any drains that aren't used often — will eventually dry out. This may sound harmless enough, but a dry trap can cause a room to fill with potentially harmful sewer gas from the septic tank or the city sewer system. Eliminate this problem by adding about a quart of fresh water topped with a couple of tablespoons of cooking oil. The oil floats on top of the water and seals it against evaporation. Your drain will hold water in the trap much longer.
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