I had a hot water tank replaced and they flushed the tank into the floor drain. The top of the drain sits about 5-6 inches down in a trench in some fairly new concrete. Well the top of the floor drain was covered with sand, which was scraped clean to drain that hot water tank. Since then I have had sewer gas smell leaking into the house, being pulled into the whole house by the furnaces nearby. Some days are worse than others. I have tried dumping anywhere from a couple glasses to a gallon of water into the trap in hopes of stopping the smell. I am not sure if this is a modern style trap, the house is 121 years old.