Septic smells are caused by sewer gas, a mixture of poisonous and nonpoisonous gases including ammonia, methane, carbon dioxide, hydrogen sulfide and sulfur dioxide. These gases are produced when waste materials decompose in your septic system. If your system is working properly, you shouldn’t smell gases in your basement or anyplace else. Smells that only appear during cold weather can be due to one or more problems.
Septic systems can freeze in very cold weather, especially if you’re not using the system very much. However, in addition to smelling sewer gas, you’ll see sewage backing up in the toilets or sinks. Inspect your system outside. If you see snowmelt over your septic tank, your system’s probably not frozen.
A partial blockage of your system can cause odors in the basement, but the odors would probably coincide with an increase in use, not with cold weather. The exception would be if air leaving the vent stack sinks back toward the ground instead of rising up and away from the house. This could happen during cool weather.
A vent stack is a pipe installed to vent sewer gases to the outside of the house, usually through the roof. It also creates an air supply that lets drains and toilets flush properly. In cold weather, the vent stack can become blocked by snow or ice, forcing sewer gas back into the house. Ice forms when water vapor condenses out of the sewer gas and freezes inside the opening of the stack. To check your vent stack, look for a 4-inch pipe sticking out of the roof. Fix the frozen vent stack by knocking the ice off or pouring hot water into the stack. If you have repeated problems with your vent stack, you can try insulating it.
Downdrafts can cause sewer gases to be blown back into your yard or through an open window or air-conditioning unit. Cool, seasonal winds can also move gases to areas where they aren’t usually detected. Evening temperatures are often cooler than daytime temperatures, and wind velocities are usually less, too. Odors might be more noticeable at that time.
Another problem can occur when a large amount of water emptying from a dishwasher, bathtub or washing machine causes sewer gas to back-vent. This can indicate a sluggish drainfield, a slow drain or a problem with your plumbing vent. The problem may be more noticeable when it’s cold because the cooled sewer gases are falling toward the ground instead of rising away from the house.