FOLLOWUP! In July 2012- I had sewer gas leaking into my bedroom, bathroom, and then up to the upstairs level of the house for five months. It was awful. I have an 18 inch crawlspace and a raised floor. Under the house is dirt and the pipes. I had FOUR plumbers and TWO handymen check it out. They found NOTHING! They were too big to fit around the pipes!!! We had put in a new toilet, and I had that pulled up and put down again, just in case. but that was fine. We checked all the traps, and now keep them full of water, so they are fine. The plumbers gave me estimates of $4500 for a sewer cleanout installation in front of the house, and $6800 to repipe the bad pipes, if they found any.... I did not have the money, and they really had no plan, just guesses. Last August 2012, I luckily found a neighbor friend who was able to crawl around under the house. He was a little guy, and knew how to do plumbing and other repairs!! He found leaking and large cracks in the main drain sewer pipe at the ground level, around the back of the pipe, hidden from general view and he used epoxy to repair it. This pipe was attached to other pipes, and if it needs to be replaced it will be a difficult job. That pipe was directly below the downstairs bathroom. That reduced about 70 % of the smell and seems to be holding tight with no more leaking there. But sewer gases were still leaking from somewhere else, but not as bad, or as often. it still kept coming back about every three days.They centered in my bedroom, and seemed to come out of the floor. Oddly, from the beginning, the gases only happened around 1am until 5am, then stopped all day long, so no one could smell them, unless they stayed all night. From the first repair, the gasses only occurred about every three days or so, and not as severe, but still stinky. My friend came back into town in November, and went back under the house, and found stress marks on the larger pipe that attached to the MAIN drain sewer pipe, and ran towards the front of the house. This pipe traveled directly under my BED, and the cracks were basically under my pillow area. (What LUCK!) He had to replace that whole pipe, and to get it inside we opened the ground vents and slid it through, and the old pipe out. After he replaced that and all the fittings, the gases were gone! THANK YOU GOD!!! It cost me $800 just to investigate all this, and get nothing for the repairing of it from that! Plumbers were just ready to get paid, with no idea what was actually wrong!!! My friend only charged me $550 plus parts for this! I suffered through it for five months, and hope the sewer gases didn't harm me in any way! I sure appreciate the clean smell now!!! By the way, it was an acrid stinky burning smell. It did NOT smell like METHANE, or anything described as general sewage smells. It was just hard to breathe it, and would cause eyes, throat and nose to burn. I had a runny nose right up to the day the last pipe was replaced, and it all stopped. GOOD LUCK TO ANYONE STILL HAVING THIS PROBLEM. I learned far more about plumbing than I ever thought I would need to know!

I see one of four potential problems here: 1.Pour about 2 quarts of water in the drain then use a flashlight and look down the drain to see if there is any standing water in the pipe? If you see standing water down in the pipe that is your trap. Hexus mentioned pouring distilled water in the drains to prevent evaporation. I can assure you that will not work because distilled water would evaporate at approximately the same rate as tap water. On the other hand, if you pour about 2oz of vegetable cooking oil in the drain it will float on top of the water and retard evaporation. 2.Examine the floor drain. If it is cast iron and appears to have a center pipe extending up to approximately 1/2" below the floor level and an outer section surrounding that pipe about 2" wide that holds water there is no trap under the drain opening. For those type of floor drains the lid has a pipe section that hangs down into that outer channel. With the lid in place water enters through an outer ring of holes into the outer ring, flows under the section of pipe extending down from the lid, then up and over the center standpipe to enter the drain. If the cap is missing or lost there is no trapping action. (In most jurisdictions we are no longer permitted to install that type of floor drains), 3. Check your furnace to see if you have ducted return air or is the air ducted into the basement then is drawn from the basement into your furnace return air duct. If this is the case you may not have sufficient return air ducting and while the furnace is running it is creating a slight negative air pressure in the basement and the sewer gas is being sucked up through the water in the traps. The solution here is to have dedicated return air ducts installed from the living space above directly to the furnace air return. 4. If you pour water in the traps and top it off with the cooking oil but you are still smelling the sewer gases your basement may be too tight to permit an adequate supply of combustion air for your furnace, water heater and laundry dryer. The code requires that we must find the manufacturers BTU rating on each appliance and add all the BTU values together to get a total BTU/hr rating for all combustion appliances. Code defines a "Confined space" as any space that has less than 50 cubic feet for each 1,000 BTU's. By example furnaces typically range 50-200Kbtu/hr. For the purpose of illustration let us assume the furnace is 100Kbtu plus 35Kbtu for the water heater and another 35Kbtu for a laundry dryer. The total combined BTU/hr rating would then be 170Kbtu per hour. We are required 50cu.ft for each 1kbtu so it would be 50cu.ft x 170 = 8500cu.ft. Now assuming the basement has an 8ft ceiling we have 8500cu.ft / 8'= 1062.5sq.ft. In this example if the room in the basement where the furnace, water heater and dryer are located has less than 1062.5sq.ft it is a "Confined space". The end result of a confined space is that the appliances will produce a negative air pressure, resulting in improper combustion which produces high levels of carbon monoxide. In addition the slight drop in air pressure will result in sucking air down from the living space and both air and sewer gases up through the floor drains. Whenever we determine that the gas appliances are in a "Confined space" we are required to install ducting to allow additional outside air into the machinery space. (The size and placement of the air ducts is also very closely regulated by code.)

Residue can accumulate in the top of the drain above the trap, esp. where the overflow meets with the drain. Occasionally, I pour a tiny splash of bleach down the drain, then let it sit for a bit. That takes care of the smell for a couple months, then we repeat. Another method would be to remove the pop-up, then scrub the drain walls with a bottle brush...but that can be messy if you remove the pop-up lever to do it,

If your not using that drain you should pour about a quart of water in it monthly to insure the trap stays full. After you pour the water in you can also pour a couple teaspoons of cooking oil in the drain. The oil will form a film over the water and retard evaporation.