This is one of those problems that are not dealt with seriously enough by the building officials in really cold regions. I have opened a blog space at the bottom of this article for you to add in your case history so we can demonstrate that this is a large and continuing problem. Take a look. At least you will feel less alone.
Plumbing traps, like the loop you can see under the sink, are designed to stay full of water and block those odours from coming back into the house from the sewage system while letting the plumbing waste through to the sewer system. At the end of use, water stays behind, keeping the trap full. They don't always work and you can get annoying to nasty smells in the house when the water does not close the trap. The good news is that although these odours are very disagreeable, health authorities tell me that this nasty smell is not really a health risk as the gasses are not biologically active (backed up sewage water is dangerous for the health but not the gasses that come off that water) and you would have to be a city maintenance man working in the sewer pipe itself before the concentrations of these gasses would be a real problem. That probably explains why there is not much plumbing code work done on making sure that plumbing traps work 100% of the time.
There are two different mechanisms that can dry out these traps and let the smells into the house:
-- Evaporation: If a drain sitting in a dry atmosphere is not used for a long time, the water simply dries out -- something very common in basement floor drains -- or in houses or individual drains not used for a long time. That is part of the genius of the system, the more you use it, the more reliable it is. Sometimes your problem can be solved simply by occasionally pouring a cup of water into otherwise smelly drains.
-- Blockage of the plumbing stack: All of the water drains in the house are connected to a plumbing vent pipe which goes through the attic and sticks out on the roof. The drainage vent system is shown in the graphic in Red. This vent piping allows air to flow in the drain system and prevents water flowing down the drain from trying to draw air through the drain traps. If you didn't have such a vent -- or if it is blocked off -- things would drain very slowly indeed, and probably gurgle in every sink in the house when you drained the bathtub. If it gurgles enough it will suck the water right out of the trap, letting sewer gasses into the house until you fill the trap the next time you run a little water into it. This is why all toilets are designed to trickle a little water into the bowl after the flush is completed -- the flush is designed to completely empty the bowl with a syphon action and then the trickle is designed to add fresh water up high enough in the bowl to block the odours in the sewage piping.
Evaporation problems, especially in basement drains can be dealt with in several ways.
-- Pour a little Mineral Oil in the drain to float on the water and slow down the evaporation. Mineral Oil, found in drug stores, is bio degradable but will not turn rancid. This works for a long time, but not forever.
-- Run a water line to the drain connected to a special drain filler valve put in the line to the washing machine. Every time the washing machine kicks off, the valve squirts a little water into the drain. This is a great permanent solution, but often requires digging up concrete to get that pipe to the drain.
-- Add a dry valve to the floor trap. This is a little inexpensive gadget that will let water into the drain but will not let gas out, so it doesn't matter if there is water in the trap or not -- the best known of these is called Dranger. Follow this link for details on difficult floor traps.
Vent Stack blockage
Although a bird's nest in the spring can cause trouble, the most common source is ice build up at the top of the stack, called Ice Capping, which literally closes the vent pipe with a block of ice. The ice cap can often be seen from the ground with the zoom on a camera or a pair of binoculars. The traditional solution is to climb up on the roof and pour boiling water down the pipe -- and then relax the rest of the winter with a broken leg from falling off the roof. There are two mechanisms that can cause this icing to happen, and they can occasionally work together.
Wet blowing snow with just the right temperature conditions, as is common in Winnipeg, can deposit the ice on the top.
Hot water running down the drain gives off steam, particularly with a hot shower. This steam finds its way up the plumbing vent and out the top of the house. If you have heavily insulated your attic and live in an extremely cold climate, the upper portion of that vent pipe is much colder than it was before you insulated -- and so the steam freezes to the top of the vent pipe before it escapes out the top.
The building code was modified to require that old 1-1/2 inch pipes used for plumbing stacks be increased to 3" when they go out through the roof. The larger pipe prevents ice from totally closing off the top in many parts of Canada but I am getting more and more reports that even this is not working.
At one point CMHC responded to this "Northern problem", which I am discovering is more prevalent that we think even in areas like northern Ontario, with a great little free publication on the problem. Unfortunately the Harper years saw CMHC research and publications shut down and the documents dissapeared. Essentially this dicument said, make the vent stack short and keep it as warm as possible -- and they gave a number of options.
There has been a lot of success by putting a 3" to 4" transition in the attic just before the roof -- then going out onto the roof with a 4" pipe. The larger pipe tends to frost over less.
The Insulation Solutions
Occasional ice capping can be driven away by simply wrapping a good quantity of fiberglass insulation (R-20) around the pipe in the attic space right up to the underside of the roof to keep the steam hotter longer. This is now relatively standard for new construction in cold climates -- and should probably the first and least expensive thing to try.
You could build an insulated box around the vent stack on the roof. Far easier is to use an insulated flashing -- special vent stack flashings that have a foam insulation liner -- like an insulated Stack Jack Flashing from Thaler Metal Industries.
A plumber in the cold American prairies took on the task of inventing an insulated termination for the plumbing stack. He started with the idea of a vacuum thermos as a total vacuum is one of the best insulators we can have. But that proved to be very difficult and expensive to produce. So he tested out a double PVC pipe with a simple air space – as you can see in the photo. The combination of a dead air space, warm gas coming up from the house and black paint on the outside to absorb heat from the sun gives tested temperatures that are almost magic: from minus 28 deg C outdoors to 4 deg C inside the dead air space and 9 deg C in the vent stack. Woodring Plumbing seems to be getting great results in a prairie climate for only $50 US. FrostFreeSewerVent.com
For those difficult cases where insulation isn't sufficient, buy some thermostatically controlled electric heating cables designed for cold-water pipes. Be careful on how you use these cables, they can cause fires. They come in 1-metre lengths for the smallest sizes and are approved for application to metal pipe (no insulation can be added over the heating cable). If your vent pipe is metal, the top foot inside the attic could be wrapped with this cable and the rest with insulation. The electric cable will keep the end of the vent pipe warm when the air outside is below 3 degrees Celsius. If the vent pipe is plastic, you could replace the upper portion with metal or go outside and add a foot of metal pipe which could then be wrapped in cable. With a bit more wiring, a cable could be installed without a thermostat and activated by a switch only when ice capping occurred. (Get a switch that has an "on" light to remind you to turn it off.) Never put such a cable inside the vent stack. These cables are not made to be explosion proof and there is methane gas inside that pipe.
The absolute solution is an ArcticVent, a product out of Ontario but which has proven its worth in Alaska and the Yukon. This is a total replacement for the top of the plumbing stack. It starts inside the attic, is connected to electricity inside the attic so there is no problem of running wires, has an explosion proof cable to deal with the presence of methane gas in the sewer gasses. Being in the business, they have run into and dealt with one problem that no-one else has dealt with -- if you have a large block of ice in this pipe and suddenly heat it -- the ice block breaks free from the pipe and slams down to the basement, doing considerable damage when it lands. So they have a retention system to hold the block there as it melts slowly. It is expensive until they get to mass distribution but it is the one system that works in every environment all the time.
Help from Minnesota
The blog below has provided a most interesting solution from Mark in winter cold Minnesota:
"I live in Minnesota and used to have periodic problems with a frozen vent stack until my friend suggested taking a 1/2" diameter piece of PVC pipe (in my case about 9' long, but as long as you can make it before you run into your first elbow) with a PVC tee glued to one end of the pipe with short pieces of PVC pipe glued in each end of the tee to prevent it from falling down the vent stack. Drop this assembly down the stack (the tee prevents it from falling out of sight) and that's it. The warm air coming up the small pipe from inside the house keeps the vent stack from freezing over."
Now there's a creative solution, although I imagine it only works if there is no elbow inside the cold attic, allowing your pipe to get down to the heat of the house. Thanks Mark.
Help me to document the extent of this problem
I have added a blog to the bottom of this entry to provide a forum for people with this problem. I want to encourage anyone who has experienced these sewage odours in the winter to make a quick blog entry -- especially if it is associated with ice capping. If there are enough of you we might even get some building code support in building houses to avoid these problems in the first place. Local health departments tell me they get "some" complaints every year but I am sure that most people just don't know that this is something that is well known and has solutions.
If you try one of the above solutions, please let us all know where you live and how well it worked in your climate.