If you go to a Refrigeration & Air Conditioning supply house you can get some small tablets called "Pan Tabs". Pan tabs are small tablets of a chlorine product that are primarily used in air conditioner condensate drain pans. Drop a couple of the tablets in the pan about once a month and they slow dissolve chlorine into the water to kill bacteria growth and help prevent odor or mold build up. They are also good to prevent the formation of bio-film which is a clear gelatin like mass of bacterial growth that has been linked to "Legionaires Disease". Although the tablets are primarily designed for AC condensate drain pans they also work in Dehumidifier pans. I have seen them on the shelf in some Lowes and Home Depot stores in the section where they have HVAC parts and supplies. The tablets are relatively inexpensive. I would expect under $5 for a year supply of them. Although not quite as effective as the slow dissolving pan tabs, you can help prevent the bacterial growth by occasionally pouring about an ounce of ordinary laundry bleach in the pan.

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.)

hi coolvt, our contractor said that all of our drains have ptraps. so far, the shower and all of our other sinks, etc, drain quickly. we had our contractor come back and he cleaned out the shower drain and flushed it out with water as well. we checked all of the cleanouts around our house and they seem ok, so i don't think there is still a clog anywhere anymore. i also scrubbed the floor and everything with bleach and soap and doused it with boiling water, so i don't think the smell is from residual sewage. but in the hot afternoon, the entire bathroom still fills up with an overpowering sewer smell or gas. when it is cooler outside, the smell lessens to some degree. do you think it is from a leak in the ptrap or a venting problem? how do we figure out which it is? thanks so much for your help!

Could I have a clogged roof vent? Possibly Backed up septic (pumped 3 years ago, live alone)? Probably not since it drains fine. Bad valves? Possibly If you have roof vents why do you need the AAVs? I get the feeling the AAVs are sticking open. Either bad design or need cleaned and lubed or something. This is one reason to avoid AAVs they require maintenance and are another thing to break or not work.