Sewer Gas Smell Through Furnace Ducts

Vent Pipe

• Aside from dry traps, cracks in drain lines and vent pipes are the other possible sites for a sewer leak. Leaks in drain lines are usually accompanied by a water leak, so they are easy to spot and thus unlikely to be the culprit. Vent pipes are intakes that allow air from the roof to replace air forced down into the sewer system by wastewater. Locating a leaking vent pipe can be a somewhat trickier challenge.

Leaks

• If the malfunctioning vent pipe is located close to a return air duct, the furnace system may suck up the sewer gas and distribute it throughout the house. The result is not only stinky but potentially dangerous, because sewer gas contains methane, which is a highly flammable gas. The hydrogen sulfide in sewer gas can also become toxic if allowed to reach very high concentrations. If the sewer gas smell is only present when the furnace is operating, it's a telltale sign that could point to just this kind of problem.

Location

• Vent pipes are usually located on the roof, so finding the leak isn't easy. Plumbers can tackle this problem with the aid of a machine that creates artificial smoke. First, the plumber caps off vent pipes and the drain pipe, then he'll hook up the machine and force smoke into the plumbing system. By watching to see where the smoke emerges, the plumber can often locate the source of the leak.

Considerations

• Vent pipes that are clogged, rather than leaking, can cause other problems instead. If leaves and other debris are allowed to make their way into the vent pipe and accumulate, air can no longer find its way through the blocked vent pipe, and water may be sucked out of the traps, allowing sewer gas to enter the house. Rust scale flaking off the inside of the pipe can cause a similar problem.