Q: At a northern lakeshore home used intermittently during the winter months, some of the plumbing vents were reported to be dry. This allowed septic tank gases to seep back into the house. This condition was detected when the house was opened for a chilly weekend visit. Why did it happen?

A: My friend and colleague, Jim Anderson, who recently retired from the University of Minnesota, reported this problem to me. After retiring, Jim and his wife, Chris, spent much of their summer building a new year-round home by a lake in northern Wisconsin. The home required a septic system, so Jim designed a series of drainfield trenches using drop box or sequential distribution for the gravity flow of effluent from the septic tank. (Obviously the result of good training!)

To have gravity flow to the drainfield, the septic tank had to be higher than the drainfield. The basement sewage wastes flow into a sump, which contains a sewage ejector pump. This pump delivers the basement wastes into the outlet sewer flowing into the septic tank.

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The new furnace uses LP gas as the energy source. It is a condensing furnace and does not need an exhaust pipe. The condensed liquid wastes from the furnace in the basement flow into the sump whenever the furnace runs to keep the house warm. When enough liquid wastes discharge to the sump, the float mechanism triggers the pump into action and the wastes are discharged to the septic tank.

Vacuum the Culprit

Why was there a problem with this? The weather had been cold and moisture vapor escaping from the house system up the plumbing vent began to freeze when it came into contact with the cold metal of the plumbing vent on the roof.

As you know, the plumbing vent on the roof is needed to supply air to the plumbing system so the use of one plumbing fixture does not pull air from other plumbing traps inside the house, and possibly suck them dry.

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The cold winter temperatures of northern Wisconsin caused more and more water vapor to be frozen on the inside and near the top of the rooftop vent. The vent finally was frozen shut. But why should this be a problem if the system isn’t being used?

The sewage system was being used even when the Andersons were not there. When the sump pump kicked in and removed the liquid wastes from the sump, this created a vacuum condition in the sump. Under normal operation, the air to relieve that vacuum would come from air flowing down through the roof vent.

But now the roof vent was frozen and could not supply the air needed to eliminate the vacuum in the sump. As we know from basic physics, nature abhors a vacuum, so the air supply for the vacuum in the sump had to come from somewhere else.

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Where, but through the other plumbing traps located in the house. And as air was sucked through those plumbing traps, they were left open and without a liquid seal.

Gases generated in the septic tank would normally exit through the plumbing vent on the roof. But now that vent was frozen shut.

As the septic tank gases built a slight pressure in the plumbing system, they could escape into the house through the plumbing traps, which were now open. This was not a good situation to be greeted with when opening the home for a winter vacation.

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Solving the Problem

The solution to the problem is obvious. The roof vent must remain open under all conditions. Perhaps an insulated or double-insulated roof vent is the answer. Perhaps the use of thermostatically controlled heating tape is the answer. In any event, the roof vent must remain open under all weather and use conditions.

After Jim brought this problem to my attention, I recalled earlier reports of non-insulated roof vents causing similar problems on homes that were occupied full time. Apparently severely cold weather can cause this issue on a non-insulated roof vent even when the house is being used.

It is always better to prevent a problem than to take steps to solve it. Particularly if the outside temperature is below zero!

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