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Last Updated: Thursday, October 7th, 2010, Created: Friday, October 15th, 1999
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.
The problem
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 second graphic above 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.
The Solutions
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.
CMHC has 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. Go to CMHC.ca and type "How to Prevent Plumbing and Heating Vent Stack Freeze-Up" in their search field. Essentially they say, make the vent stack short and keep it as warm as possible -- and they give a number of options.
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.
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.
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 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.
Keywords: Attic, Basement, Bathtub, Codes, Drains, Flashings, Foam, Freezing, Gas, Health, Ice, Insulation, Maintenance, Methane, Odours, Pipes, Plumbing, Problems, Roof, Shower, Sink, Smell, Steam, System, Trap, Valves, Vent Stack
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We are having this Ice Blockage of the Vent stack problem presently in our house in Calgary. This is the worst it has ever become because of the long cold weather.I will be adding an Insulation Wrap of R12 around the Plastic Pipe in the Attic. Thanks for your web site and all of the help full hints.
Here is my temporary solution till the outdoor temperature warms up. Since the odors emanate from my bathtub, I (a) open the windows after a shower to vent the bathroom, (b) plug the drain with a stopper, and then (c) leave a little water in the tub. This acts as seal to prevent the odors.
Hello Mike, Hot steam in the right outdoor conditions might open it up but in very cold conditions would just add more ice. As for that wire you want to put up there, when I began to research that option I ran into the problem that none of the common de-icing cables made for roofs are "explosion proof" and there is usually methane gas in the plumbing stack. Boom! Steep roofs tend to have more problems because the stack is often sticking out in the cold higher to get up and away from the roof in a horizontal measurement. More exposure, colder stack. This is where the Artic Vent would be a real solution.
I have an older house in Western Manitoba. Every Winter the vent stack frosts up, and a lovely rotten egg smell develops in the basement. It's happening right now...arrrr. Problem is my roof is very high, and very steep, so jumping up there and pouring hot water down is out of the question. I need to create some kind of 'remote control heat ring' that sits on top of the stack... so when it frosts up i go outside, hit a button, and the ring heats up for an hour, removing the frost.
Do you think running a hot shower for a while will help to remove the frost? or help it just get thicker.
Cheers,
Frozen & smelly
We purchased a home in Winnipeg (70s construction) and now that it's cold we are having this problem. My father in law thinks it's ice capping. I will post a follow up here when I determine exactly what the issue is. Thanks for this tip page by the way, the main text and the comments are extremely helpful.
I have seen the smell problem show up in some new homes. As a builder and responsible for some large scale renovations, all work has to be inspected. In the case of a new home where everything was to code, the inside of the vent stack developed a thick layer of whore frost just at the roof line - possibly from infrequent use of the system. An electric solution would help to fix the problem but by spraying some grease down the inside of the pipe would stop the moisture build-up. As a builder I would go to the 4in vent stack. I do like the idea of an animal guard.
I had this problem recurring in a new house built in 1975. It involved the basement floor drain, puzzling the builder and several plumbers who were consulted. It was finally solved by placing a fiberoptic camera in the main sewer line under the basement floor and watching the opening to the floor drain as the upstairs toilets were flushed. Water was seen flowing from the sewer line into the line from the floor drain (the pipe from the floor drain P-trap had been incorrectly installed, sloping upward from the bottom of the trap to the sewer line). I wonder how often this happens?
If you experience a blockage, don't over look the possibility of an animal falling down the vent pipe and dying at a piping junction. Happened to me. Now have 1/4 hardware cloth over the vent stack end.
I only had ice clogging the vent stack once. After hours of snaking my drains I eventually smartened up and checked your web site. I cleared up the problem quickly once I knew the cause.
As to the odor problem - my floor drain in the laundry room is right under the washing machine near the laundry tubs. I installed a permanent hose hookup to refill the floor trap whenever odors occur (manual to the faucet). I really appreciate your info & miss your tv program.
I worked at a body shop in Lloydminster for years. Every so often each year the vents would start to gurggle almost sounds like a sploosh or sloshing. Then it would smell like sewer so bad we had to open the front doors or periodicly head to the back shop or outside because it was very strong some days.
The contractor who built the structure was back a few times to try to fix the problem but it never worked, I guess the owner just gave up asking because it is still going on after at least 12 years.