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Be sure to tend to sewer vent pipes during chilly season

It's a good idea to insulate or extend sewer vent pipes -- if you haven't already -- so they don't get covered with snow or freeze shut during the winter season.

Sewer vent pipes are on the roof of homes and other buildings. The diameter of the vent pipe varies from 1 1/2 to 6 inches. Typically, the short, smaller diameter vent pipes (less than 4 inches) can freeze shut. Sewer vent pipes on the north side of the roof also may be more prone to frost plugging than those on the south side of the roof.

Sewer pipes have to be vented to allow air into the pipe for proper flow. The sewer pipe and vent pipes are connected so wastewater flows downward and sewer gases rise. During extended periods of very cold weather, sewer gas containing water vapor will condense, then freeze to form a frost layer in the exposed uninsulated end of the vent pipe. This frost layer can seal the pipe partially or fully, forming a plug. Hard, drifted snow also can plug the end of the vent pipes.

If the end of the vent pipe is plugged, proper draining will be hampered and air will be pulled into the piping, which will drain the traps of water fixtures. This allows sewer gas into the house. The gas, primarily composed of methane and carbon monoxide, will have an obvious “sewer” smell; however, some harmful gases are odorless.

Frozen sewer vents have been a problem in many newer homes because the plastic pipes in those homes do not conduct heat from inside the house as well as the cast iron vent pipes in older homes. 

A common method of preventing freezing of sewer vents is to attach an insulated sleeve over the outside end of the vent pipe. These sleeves not only insulate the vent pipe but also extend it higher into the air. The intent is to keep the water vapor and gases above the freezing point until they exit the end of the vent pipe. 

Many hardware and home supply stores carry pre-made insulated units that you can attach to the vent pipe easily, and some are made to slip on without tools. However, for these to work properly, you must know the diameter of the vent before you purchase the insulating sleeve. This will require getting on the roof to measure the vents first.

Even sewer vent pipes with insulated sleeves have been known to freeze shut, which may be due to exposed bare pipe in the attic. If you can get access to the attic from inside the house, wrap a batting of fiberglass insulation around the vent pipe in the attic. Another option is to wrap heat tape around the exposed sewer vent pipe in the attic and then insulate over the tape.

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