Vent Probably Frozen

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by FMC1959, Dec 20, 2013.

  1. FMC1959

    FMC1959 New Member

    Messages:
    2
    Location:
    Montreal, Canada
    I live in the great white north, plenty of snow and cold weather. I bought my house new construction 10 years ago and last year during a very cold spell had sewer smell in the house. When temperature got milder, smell went away.

    First cold snap this year and same issue. The smell is coming from upstairs bathroom. Main floor has kitchen, second bathroom, garage drain and basement drains, none of these ever smell. When I flush upstairs toilet during these cold spells, I hear the distinct gurgling sound right after the flush, like the water being siphoned and resonatin sound as it goes out, and this sound goes away with milder weather. Quite convinced this vent is freezing up.

    I checked attic last year and there are at least 2 vent pipes that go out to the roof. Because of where upstairs bathroom is located, quite sure it does not share venting with other plumbing. Attic is very big, 20 x 28 and very high pitch roof, 20 foot apex in the attic, very well ventilated.

    Two options I am considering for my problem as fixes till next summer. First thought is the vent pipes run close together before exiting the roof, putting a “T†on each pipe and connecting them together, allowing the upstairs bathroom to use the other vent when it freezes, or using its own when thawed.

    Option 2 is to put an AAV on the vent pipe for the upstairs bathroom, installed in the attic. Would either solution be against code, but more importantly, would either solution pose health hazards, or other plumbing issues?

    In the summer I could see about having a larger diameter vent pipe (have read that a 4" should deal with any cold), that hopefully would not freeze, put through the roof. Not an option I would like in the middle of our winters and with the high pitch roof I have.

    Thanks
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,834
    Location:
    New England
    For an AAV to work, it can't be the only vent in the system, so if your roof vent is frozen closed, it probably wouldn't help. As long as you can slope a connecting pipe between the two vents so it can drain, and you have the room to do it, you could connect them together if you wished. It may also be the height or location of the vent that causes the smell.
  3. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,239
    Location:
    Land of Cheese
    The attic should be close to the same temperature as the outdoors. Frost closure only happens on a small vent termination. Most areas here in the great white north do not allow a stack through the roof which is less than 3" diameter pipe for that reason.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,315
    Location:
    Cave Creek, Arizona
    It used to be at least 1" larger than the pipe inside the building AND a minimum of 4".
  5. Caduceus

    Caduceus Master Plumber

    Messages:
    99
    Location:
    Pittsburgh, PA
    I can't imagine that a vent would freeze unless something dramatic has changed in the past ten years that you haven't noticed. A sudden change in performance of the drainage/vent system usually indicates that there is a clog in the drain and the vent stacks are not the problem.
    I have seen ice clogs in drains from slowly leaking toilets with water trickling down the drain when not properly insulated. Typically a flush will clear the drain in a few seconds before getting to the sewer beneath the house and not have time to freeze. A slow flow of water could build up over days and bottleneck the drain causing a loss of trap seal and sewer odor as well as the gurgling that you described.
    Quite the theory, eh?
    It happens here in Pittsburgh when we get deep freezes that last a week (20 deg. F. or below).
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,834
    Location:
    New England
    Hoar frost in the part through the roof is not uncommon where it is quite cold...and is the main reason why codes call for the pipe diameter to be larger so it is much less likely.
  7. FMC1959

    FMC1959 New Member

    Messages:
    2
    Location:
    Montreal, Canada
    cacher_chick, you are correct, my attic is pretty close to the outside temperature, my thought on using the AAV was in hoping that it being sheltered from the out ice rain/snow and wind, it would not freeze up. But from other responses, not an option being it would be the only vent on the line.

    Caduceus, you bring up an interesting point, for the past 8 years never a problem, what has changed in the past 2 years? Off hand, nothing I can think of. When it happened last year, I had asked a buddy of mine’s son, who is a plumber. He thought it most likely was ice buildup in the vent, or clogged because of an animal or debris that had fallen in. When the weather got milder, it went away, it was thought that it could be a onetime occurrence, no need to do anything right now.

    Now that it has happened again, and this is not happening at 20F, much colder like below 0F/or -20 Celsius. Also, not just from an overnight cold hitting these temperatures, we get these temperatures frequently overnight. It needs to be at least 2-3 days or longer where the daytime high is 0-5 F and colder overnight. In this instance is where the smell comes gradually and gets worse as the sub zero cold remains. When milder weather comes around, right around freezing or a couple of degrees below, it gradually goes away.

    A couple of other “symptoms†to add, Caduceus, again to what you say, not just in the winter, but year round, this upstairs toilet gets blocked much easier than the one downstairs. Too much toilet paper/waste and pretty good chance I need to get the plunger, definitely blocks easier.

    Also, this being the upstairs bathroom and the only bathroom we have for bath & shower, we use the shower daily multiple times, there is going to be a fair amount of moisture going up this vent that could freeze.

    My thought on connecting the 2 vents with a couple of “Tâ€s and a pipe was it is a simple and inexpensive fix, and 1) it could solve the problem.

    OR 2) it could cause the other stack to freeze and the downstairs toilet and other drains would also smell. This would confirm a definite vent freeze problem? I am guessing for bathroom or kitchen sinks to smell, they would need to be filled and then unplugged to create the siphon effect necessary to empty the P/S trap.

    Possibility 3) is that the upstairs bathroom would continue to have these sewer smells when it gets very cold, but the smell does not emanate from the downstairs drains, this would mean that whatever the problem is, it is not a frozen vent, right? Maybe something like what Caduceus is saying about freezing in the drain itself.

    Thanks for any thoughts or ideas.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,834
    Location:
    New England
    If things aren't plumbed properly, you could be siphoning a vent dry. When this happens again, take a bucket of water ans slowly pour some into the other drains in the bathroom (tub/shower/vanity) and see if the smell stops. If it does, they are not vented properly and are getting siphoned dry. If that bathroom isn't used often, say the shower doesn't get used on a regular basis, it's trap may be dry...adding some water may stop the problem. Also, if the wax seal on the toilet is broken, and this is likely the case if the toilet can rock at all, you need to address that with some shims and a new wax ring.
  9. kcodyjr

    kcodyjr New Member

    Messages:
    103
    Location:
    Chelmsford, MA
    A few days of deep cold before the problem manifests? Could there be a thermal mass at work here? Perhaps the trouble point is just far enough inside the house that it takes time to get cold?

    If there's anything to that hypothesis, then we're talking about a material shrinking when it gets cold to allow a gap, then expanding when it warms to seal that gap. A loose or perhaps unglued joint, or a hairline crack in a pipe.

    Very likely such a problem would be hidden in the wall, but we can at least deduce that it has to be on the downstream side of any trap.

    Have you tried following your nose to see exactly where it's strongest? I know it's foul, but...

    IMO the likeliest would be the sink. It's the closest to the top of the vent stack. Is its trap arm all plastic, or part metal?
  10. Caduceus

    Caduceus Master Plumber

    Messages:
    99
    Location:
    Pittsburgh, PA
    The gurgling after the flush and the fact that this is a new problem in 10 years since the house was built would still push me into thinking that here is an ice clog as I've mentioned before. Look for a slow leak or drip from a fixture that would allow icing in the drain. Running hot water for a while in a drain then testing for the flush-gurgle noise would confirm this unless the toilet is at the end of a branch and hot water can't be directly introduced.
    The symptoms sound identical to what I've seen many times in the past.
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