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Question DetailsAsked on 2/5/2015

the downstair shower backs up and floods the basement only when it's below 0 and I'm not home.

I also noticed that there is no water in the toilet bowl which is right next to shower. This is the lowest part of the basement. The floor gets flooded but the time I get back there is only residual left on the shower floor. This only happens when I go away for 2 nights and it's below 0.

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Wow - this one would have made a NASTY test question for a construction apprentice program I once taught at - but I am not THAT mean, contrary to some opinions. Tis response is going to be a little long, because there are a lot of potential causes.


I am going to presume you are not letting the inside of the house actually freeze - because if you do that, there are several ways water could escape into the drains or basement as water pipes freeze and squirt water out faucet seals under high pressure.


IF there are two or more units and the neighbor(s) do not run much water, especially hot water, I can see this happening if your sewer line is freezing up, so the water can't go down the sewer and backs up into your shower/basement - assuming yours is the lowest unit. A partial blockage from rime ice freezing on the inside perimeter of the pipe is the easiest to imagine - so it only overflows into your basement when the neighboring unit uses a large quantity of water in a short time - commonly occurs with clothes washer or emptying tub as those have 10 or more gallons of water emptying in a short time commonly. Any smaller use, including the 2-5 gallons from a toilet flush, might back up partway but the volume of the pipe upstream of the blockage might store it adequately till it drains away, without backing up into your drains. This same sort of backup is VERY common with pipes with partial blockages, and would have been my first guess except you said ONLY does it below zero and while you are gone, which does not fit a partial blockage in the pipe from soap scum/fibers/grease, etc - which would back up under the same conditions every time, commonly starting with clothes washer emptying since generally has the greatest discharge flow rate in a residence.


If single family unit, then a bit harder to imagine - I can only see it reasonably happening in one of five (now six - no, now seven - nope - eight) ways. I started off with 2, then 3, then 4 - opps, just thought of another - then another and another, so there could well be others I am missing. All obviously involve the sewer pipe freezing up, the crux of your problem, on the assumption from above that this truly is happening ONLY when very cold.


1) if you have a very flat lying or sagging section of pipe right after the shower that stays filled with water, then as the pipe freezes (would have to be starting at the outward end of the low spot) the first spot to freeze blocks any outflow. Then as it freezes the water it expands and pushes the unfrozen water progressively up the pipe, coming out at your lowest drains - floor drains or shower/tub, typically. This could happen with sewer pipes suspended in a crawlspace, for instance - particularly if you are turning the heat down to a lower temp when you go away for a few days.


2) sewer pipe is freezing up, possibly from too shallow a burial or exposed pipe in wall or crawlspace freezing, or maybe cold air blowing from the street sewers through the sewer pipe and up out your roof vent. Then forthis scenario you need a water source - constantly running toilet, a laundry room trap with a continuous flow trap fill tube, dehumidifier with direct dumping to drain, gutters feeding rain water to sewers, groundwater leak into sewer pipe, sump pump discharging into sewer line, etc. This water would freeze in sewer pipe, then start backing up as above once it plugs off. This happened to me one winter where my sewer froze solid in normal use (with only one person in house for a few weeks) so only relatively small amounts of hot water going down drain - but occurred at far below zero air temps for over a week, not zero for a day or two.


3) cold air blowing through, or ground freezing up and causing freezing at the sewer main at the street, resulting in backup in your sewer per above from street up through your sewer pipe


4) freezing in the sewer mains themselves so liquid backup could be from other users. A very rare thing, but can happen in very shallow burial areas and in the few areas where they still use ventilated lids or grates on sewers, which can then freeze when they get inordinately cold weather for some days. Can also freeze up in combined sewers - where sanitary sewer is connected to storm sewers, so cold air pours through storm drains into the sewer. Takes a unique set of circumstances to freeze up enough to back up into individual sewers, because usually the pipes are buried deep enough to avoid this, and usually there is enough warm water coming through to keep it thawed out and flowing. But I have seen it in serious winter conditions in lightly-used feeder branches with only a few houses on the line, freezing in from glaciering of groundwater infiltration. And of course can easily happen in permafrost areas, which pretty much limits that cause to high mountain areas and Alaska/Canada in North America. Could glacier up and back up overnight when flow in sewer is very low, then thaw back out in the morning from domestic hot water uses, draining the backup.


5) if on septic - unusually cold weather for your area so septic tank manifold/leach lines are freezing up with no warm water running through them, then as septic tank starts to freeze the expanding ice is forcing sewage back through your drain pipes to the lowest level drains


6) late entry which requires a really nasty turn of coincidence worthy of a murder mystery novel. Freezing pipe scenario as above, but the water is coming from the automati flush cycle on your salt-based water softener, so the flushing cycle water goes into the pipe and freezes (or is already frozen at a sag) so backs up, but because it is quite salty water then thaws itself out and drains the water.


7) Could also work with an automatic flush cycle on a boiler, or an overpressure/ overtemp valve going off on hot water heater or boiler because they are overheating while you are gone and gradually building up pressure which would normally be depleted with use when you are home. Ditto for a pressure relief valve on household water system discharging, or a leaking water heater discharging from a catchpan piped to the sewer. Of course, would have to dump to sewer pipe for that scenario to work.


8) scenarios like ones above except #6 & 7 - and someone is coming into your house and flushing toilet or otherwise running water, causing backup of frozen pipe. Do you have anyone wathcing/checking the house or feeding animals, maybe ? Ditto if your unit's sewer pipe is freezing but gets the wash water from a shared laundry room, for instance.


The complexity of the above scenarios is in it being drained away when you get home - BEFORE you do anything that could thaw the pipe out, presumably. This would work if there is a partial blockage backing up from a neighbor's water use, but if caused by just your house unless your house is going cold for a period (intermittent furnace, thermostat, or power failure) then heating back up to thaw it out and let it drain, I don't see how it can reasonably be working except with #4 and #6.


In all these scenarios, the presumption is that it freezes up only below zero and when you are gone for 2 days or more is that when weather is warmer or you are there, your normal hot water use is enough to keep the sewer pipe warm enough to stay thawed from day to day.


As to the toilet going dry - this implies to me that you had a blockage that either directly, or due to backup of water, blocked the sewer air vent pipe to the roof so air cannot get to/from the sewer pipes. Then, when the water ran down the shower/floor drain back into the pipe, there would be no makeup air from the vent to fill the pipe behind the receding water, so it would cause a partial vacuum and suck the water out of the shower and toilet traps till it was sucking air through one or both. Presumably, as the water went down in the shower (causing the partial vacuum) it could not suck air through there because it still had standing water in the trap, so it would suck the air from the next easiest source - basin or toilet, and basin trap is generally deeper so toilet is easiest place to pull the water through the trap to get air to relieve the partial vacuum in the pipes.


If above thoughts don't solve it for you, sounds to me like you need a Sewer and Drain contractor to do an inspection and maybe a camera run with locater head on it to trace the line and dig down a few places to see if it is buried below frost depth - or maybe you have a pipe in a wall freezing up, or uncapped inspection/cleanout access pipe(s) outside your foundation that are letting cold air in and freezing the pipe. Sounds like if you solve the freezing up, you might solve the backup issue.


If you don't have a neighbor on the same sewer pipes, figuring out how it thaws back out to let the water drain out before you come home is a puzzlement unless #4 or #6 works.


Good luck - this may be a tough one to track down. If you have any other thoughts which might clarify matters, or want to bounce some other thoughts off the contributors here, you can post them using the Answer This Question button right below your question.

Answered 5 years ago by LCD




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