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Question DetailsAsked on 12/25/2017

we have a sewer smell coming from our sump pump. This only happens if the temp outside is way below 0. What can w

Our temp. outside is about -25. Whenever it gets below zero we can smell sewer gas coming from the sump pump. What can be done about this? We keep adding water in the shower and toilet but this is not helping.

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Tough one to track down many times. Below are some common causes of that problem, more or less in more common to less likely order. However, while you may certainly be right about it coming from the sump, that would be far from common - normally would come out of a floor drain or a DWV (drain, waste, vent) stack pipe that is open in the house. Diagnostic key of course is the cold temp being the only time it is happening.

1) probably by far the most common when happening only in cold weather - in very cold weather the vent pipe on the roof can frost up solid over a period of days as the escaping moist air condenses in the cold exposed part of the stack above the roofline, thereby preventing the sewer gases from escaping through it. Hence, as sewer gases build up pressure in the line or maybe just accumulate from the public line into your line as happens in our case, the sewer gas can slowly bubble through the water in a trap. It will of course pick the trap with the least depth of water in it, or any cracked or unsealed joint. So might seem to be coming from the sump, but actually be coming from nearby drain in the same level or room.

In our case, can happen due to vent frosting solid - also from vent largely frosted up but still venting a bit, but the snow/ice in the street icing over the vent holes in the manholes, allowing the sewer gas in the street sewer mains to back up into out lines, which serve as the high-point vent for the branch we are on because we are at the high point and also at the end of the sewer branch. Will not pressure up normally, but if a trap is low (usually floor drain which has been forgotten and not filled with water for awhile) then the gas may push through it into the house.

Can also happen, at "warmer" temps sometimes, if your vent pipe does not stick up aboe the accumulated snow level and the moist air fills in the voids in the snow with frost because the cold air above is keeping the snow cold enough to not melt a vent hole through it.

Run hot water in a sink for several minutes - if you do not see a vapor cloud / "steam plume" coming out of the vent stack(s) on the roof while the water is running hot into the drain lines it is likely blocked with frost - usually but not always (especially with metal pipes which can cool enough below the outlet to frost up) will first have a pretty visible frost cap around the top of the pipe, which in very cold weather can frost solid.

Rarely the drain lines can freeze up from very cold air flowing "wrong way" from the public sewers or manholes up through your vent pipe, freezing up the lines inside the house - though of course freezing up more than just the vent pipe or even freezing up the vent line inside the house could only occur if they are running in a very cold part of the house - uninsulated walls or exterior walls or unheated crawlspace or basement, for instance. Running hot water in the most "upstream" fixture should thaw out any frozen drain lines real quick unless frozen solid (so it backs up) - but thaewing frozen vent line without directly applying heat into the line can take 15-30 minutes or more of running hot water, as the heated air rising into the vent pipe does not circulate very much until it open an opening through the blockage, so take a lot longer.

This in-house freeze usually only happens in homes with VERY low water usage (typically single elderly person), or a house with heat shut off in part of the house or grossly underheated (which many houses are at 25 below - many will start freezing clear through walls and floors above unheated spaces at those temps). Course, increasing heat to the house can help thaw the pipes back out - direct heat on drain lines is generally not a great idea.

While pouring hot water into the top of the vent pipe (be sure you are on the sewer vent, not a bathroom fan vent - the drain stack pipe vent should be an open pipe with no cap, and usually 1-1/2"-3" black ABS or HDPE but occasionally 3-4" castiron or heavy steel, but NOT light gauge ducting metal. Obviously clears it the fastest with direct heating at the outlet, but you may not be able to safely get on the roof, so sometimes running full hot water into the pipe in the house and letting the warm air rise in the vent stack and melt the plug is the way to go - maybe half faucet flow full hot in a bathroom basin for instance, will thaw it in 1/2-1 hour commonly. Of course, if your house has multiple vent stacks through the roof, you have to figure out which drains are on the drain stack served by that vent stack which is frozen up.

2) when it is very cold the outside air pressure is higher, which can damp down the sewer gas escaping through the vent pipe on the roof so it "weights down" the air in the pipe, causing it to leak out a crack or unsealed joint or afloor drain or laundry drain or such without a good water seal in the trap. This will not make it through a full trap, but if your cold air is combined with high winds (like blizzard conditions for instance) then the wind can pull down the water level in the traps (like high wind causes surges in the toilet, lowering the water level in the bowl). So - check around for ANY drain which might not have a good full water seal - floor drain, laundry tub, long unused washer drain stand pipe, basement shower or sink, etc.

3) sump should NOT be connected to the city sewer - illegal in most areas, and in those where it is legal to run it into a sewer line the sump pipe is required to have a trap in it (which is generally a bad idea for sump pumps because they commonly pump some solids too, which can fill the trap) - but if connected to the sewer, then sewer gas can back into that line and down through the pump into an empty sump. Would normally happen only in case 1) or 2) above - but if the sump is basically empty (pumped down to the pump inlet level) then gases could come out there. Solution, until the discharge into the sewer line can be remedied, would be (assuming sump is deep enough for this) to change the sump setting switch so the pump turns off while there is still a few inches of water submerging the water intake for it.

3b) if the sump pump discharge line IS connected to the sewer line, then if it is the lowest sewer pipe on the system in the house, any backed of the sewer line (say due to freezing in the ground or where DWV line goes near or through exterior wall) would go into the lowest elevation line and come out there - which could be at the sump pump. Shoud have a backflow prevention valve in it unless it has an air vent at the high point (usually right at top of discharge line stack coming up from the pump) to prevent backflow - but if that vlave cruds or freezes up backflow of liquid or gases can occur. Also, if it does have an air intake vent at the high point to let air in to prevent backflow, if that air vent also lets air out sewer gas backing up in the system for any of the reason in this listing could be coming out there - above the sump pump, an dropping straight down 9because it is cold) to the sump pump, making it smell like it is coming from the sump when maybeit is not.

4) in very cold weather, ground may be freezing up, causing a septic system leach field backup, forcing leachate out into the ground, which then makes its way to the basement drain system and hence to the sump. If this is happening, would persist for typically at least a few days AFTER the cold snap ends and the ground starts warming up (though not necessarily above freezing - remember the leachate coming from the septic tank is warm so will thaw out a frozen leach field over time.

5) if your sewer pipe is freezing up, causing a backup in the sewer line outside, then gases can build up and back up. usually will go up the vent stack, but if your line has a low point in it with a DWV line entering in the low zone, can build up pressure there similar to 2) above and force its way out of a trap like in 2) above.

6) if your sump pump commonly runs a fair amount, may be it is always pumping a bit of sewage effluent from a leaking or overflowing sewer line or septic system, but when it gets very cold for an extended period the groundwater flow trickles to very little, so the sewage concentration in the sump increases and the pump is pumping less often, letting gases leak out of the sump

7) rarely, an underdrain system (especially in organic soils common in swampy or floodplain areas) will go septic as the natural oganics in the soil biodegrade, and if the incoming flow drops off in the cold like in 6) above, could start putting gases out into the house. Also, if yours tend to do that (would normally but not always have black slimy sediment in the sump) if the outlet of the underdrain pipes are exposed in the sump rather than always submerged, then gases generated in the underdrain pipes or the soil under the slab can enter the open sump.

8) quite rare but happens - gases from the soil/rock formations under the house can accumulate under the house and "drain" into the underdrain pipes and hence to the sump - more commonly carbon dioxide or natural gas, sometimes (especially in oil field areas) hydrogen sulfide. In coal areas methane, carbon dioxide, and sometimes carbon monoxide and the commonly stinky nitrogen compounds can do this too - but natural gases from the soil would rarely smell like "sewage".


In checking out the above, pouring a bottle of vanilla extract into a drain (preferably most upflow one in the house) can sometimes let you trace it. Best to do in kitchen or bathroom and pour quickly and cleanly (with enough water to make sure it will flow all the way down the pipes - like a couple of quarts) down a drain you can then cover with a drain cover AND run fan in that room thereafter to prevent air transfer of the smell.

Then move around the house (maybe one roamer, one concentrating in the area with the sump pump) to smell where the smell first pops up. If it appears with a couple of minutes (both as gas coming from the drain and, if coming up through a drain/sump, as a concentrated vanilla smell in the drain itself) your problem is almost certainly a leaky DWV pipe, or it is backing up through the drain the smell appears in. If it takes a long time (hour to days) to pop up, then a backing up outdoor sewer pipe or septic system is a lot more likely the cause - especially if it takes a long time to pop up and then does in the sump pump sump. Course, if vent pipe is frozen when doing this test you are defeating your purpose other than isolating which place it is appearing - but in that case does not mean the problem is really at that drain, so eliminate the vent pipe freezing up as a source before doing this.

If the above does not allow you to track it down, then a Plumber or Sewer and Drain contractor would be your Search the List category to get someone to diagnose it, and they can do a smoke test on the DWV stack and vent pipe to see if the smoke (cold chemical smoke, should be no fire hazard involved) pops up somewhere. Sometimes the smoke test does not identify it, but usually does if inside the house.

If they are not able to solve it, then rarely a civil/environmental engineering firm has to investigate it, at very significantly greater cost - but almost all cases can be solved by above checklist or vanilla/smoke test.

Good Luck

Answered 2 years ago by LCD

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