Venting About a Frozen Vent Problem

Septic Answer Men past and present give their advice on keeping roof plumbing vents from freezing during cold winters

Interested in Systems/ATUs?

Get Systems/ATUs articles, news and videos right in your inbox! Sign up now.

Systems/ATUs + Get Alerts

Editor’s note: This article features a Septic System Answer Man column that first ran in Pumper magazine in March 2011. It includes the original column with tips from Roger Machmeier and has been updated to include the solution used by current “Answer Man” Jim Anderson. 

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, 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.

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 is the culprit
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.

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.

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.

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

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


An Update on the Frozen Vent Issue

By: Jim Anderson

The above was an article describing a problem that I had with the roof vent at my lake home. The description of the situation and what happened remains accurate. This winter (a typical northern Wisconsin winter) has led to questions and similar reports of noninsulated roof vents freezing even in occupied residences.

When this issue arose, I was asking people attending one of my classes on septic system operation how they dealt with similar problems.

Two comments from attendees stood out as having merit. One related to the height and location of the vent. The other was about using a length of copper tubing in the vent. This idea came from some people that live in Alaska.

When the pitch on the roof is steep and a typical vent pipe is brought through the roof, it may extend only a foot or two above the roof at that point, while the peak of the roof is several feet in elevation above the end of the pipe. This can create the potential for downdrafts and interference with proper venting depending on wind direction, speed, etc. As a side note, this condition will result in sewer odors outside the home in other seasons because the vapor and accompanying odors are directed downward instead of upward. The solution is to extend the vent pipe so it ends above the elevation of the roof peak. There still may be problems if the pipe is in an area where there are hills or tall trees that alter airflow around the house.

Using a length of copper tubing that extends downward into the warmer air space as the vent pipe passes through the house can keep an air passage through the vent open because copper is a good conductor of heat; so at least the air can pass through the tubing, which is kept open.

Both of these suggestions have merit and can be tried; and in fact, I tried them at my lake home. In both instances, things were better; but during extended periods of cold where the temperature did not get above zero degrees F for several days or a week at a time, the vent still froze. Admittedly, depending on your location and the number and length of cold-weather events, these may be viable solutions for your customers.

The route I ended up taking was using controlled heat tape. I did not try a double insulated roof vent, though as mentioned this may be a very good long-term solution, depending on location.

If heat tape is used, it should be thermostatically controlled when the residence is not occupied continuously — so when there are midwinter thaws or the weather begins to warm in the spring, the heat tape is not operating. It is important to have the heat tape shut off when the temperature rises above 32 degrees F to avoid overheating and to conserve electricity.  


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.