Random sewer smell in basement

[bcreilly]

We moved into an old house (1940s) in the summer of 2013. Since moving in, we have noticed a fairly random but persistent sewer smell in the basement. The only patterns that we have noticed are:
a) The smell seems to be worse in the colder months.
b) The smell seems to occur most often when we do the laundry.
My initial thought was that there was a problem with the drain line. We had plumbers come in in March of this year and replace the clay drain line with ABS. Unfortunately, that didn’t seem to fix the problem.

In the basement we have:
a) Washing machine dumping into a laundry sink.
b) A bathroom with a sink and toilet
c) A floor drain (the ac/furnace drains into here)

Can anyone suggest ways to troubleshoot where the smell is coming from?
My current thought is that the laundry sink is not vented properly so when we run the washer the water in the laundry sink trap is getting siphoned out. Is there any way to check this? Pictures attached show how the laundry sink drains and is vented.

In the picture you can see the laundry sink (left pipe) and washroom sink (right pipe) connecting to a single drain line. Both sinks have p-traps. The top pipe I assume is the vent, but to be honest I lose it once it goes into the wall. That said, the main stack is about five feet to the left of this picture and a) I can't see how this pipe would make its way back to the stack b) there are no other holes in the roof.

I should also note that there isn't any other real evidence of an issue with venting... the sinks seem to drain OK and I haven't noticed any bubbling in the toilet etc.

Thanks,
Brian

 

[frodo]

when was the last time your ac ran? that p trap for the floor drain, with the ac draininging into it. pour a 1/2 galon or water in there
during the winter mths. your heater does not make condensate. bet that trap is dry

 

[phishfood]

We moved into an old house (1940s) in the summer of 2013. Since moving in, we have noticed a fairly random but persistent sewer smell in the basement. The only patterns that we have noticed are:
a) The smell seems to be worse in the colder months.
b) The smell seems to occur most often when we do the laundry.
My initial thought was that there was a problem with the drain line. We had plumbers come in in March of this year and replace the clay drain line with ABS. Unfortunately, that didn’t seem to fix the problem.

In the basement we have:
a) Washing machine dumping into a laundry sink.
b) A bathroom with a sink and toilet
c) A floor drain (the ac/furnace drains into here)

Can anyone suggest ways to troubleshoot where the smell is coming from?
My current thought is that the laundry sink is not vented properly so when we run the washer the water in the laundry sink trap is getting siphoned out. Is there any way to check this? Pictures attached show how the laundry sink drains and is vented.

In the picture you can see the laundry sink (left pipe) and washroom sink (right pipe) connecting to a single drain line. Both sinks have p-traps. The top pipe I assume is the vent, but to be honest I lose it once it goes into the wall. That said, the main stack is about five feet to the left of this picture and a) I can't see how this pipe would make its way back to the stack b) there are no other holes in the roof.

I should also note that there isn't any other real evidence of an issue with venting... the sinks seem to drain OK and I haven't noticed any bubbling in the toilet etc.

Thanks,
Brian

The laundry sink is S trapped, probably not a problem when you run the faucet on the laundry sink, but very likely to siphon the trap with the high volume discharge of the washing machine.

 

[bcreilly]

Thanks for your comments both.

Frodo: That was one thing I considered (definitely makes sense given what time of year the smell generally occurs). That said, I have checked the floor drain and it seems to be at a good level. I will continue to monitor that though and add water periodically.


phishfood: Please see the attached photo which shows the trap for the laundry sink. Isn't that a P trap?

 

[phishfood]

Yes, but the manner in which it connect to the vent stack is incorrect. It should connect with a sanitary tee, not a wye and 45. Same thing with the sink on the other side of the wall.

 

[bcreilly]

phishfood: I think I understand, you need to use a sanitary t because the air from the vent basically can't go up and under the wye... is that correct?

If I were to redo the plumbing would it be OK to have one sanitary t (from the bathroom sink drain) above the other (from the laundry sink drain)?

Thanks again for your response and have a great thanksgiving.

 

[Mr_David]

phishfood: I think I understand, you need to use a sanitary t because the air from the vent basically can't go up and under the wye... is that correct?

If I were to redo the plumbing would it be OK to have one sanitary t (from the bathroom sink drain) above the other (from the laundry sink drain)?

Thanks again for your response and have a great thanksgiving.

stacked sanitary tees would be wet venting the lower santee.
What is happening on the other side of the wall at he bathroom sink?
The upper wye seems high for a sink.
If the 90 is a vent, the horizontal section should be above the flood rim of both sinks.

aside from fixing the horizontal part of the vent, you can replace the 90 with a santee and extend a 2nd vent to the laundry trap arm just before it connects to the wye. The laundry will no longer be wet vented.

Or if it doesn't matter how low the p-trap is under the other sink you can use a double fixture tee

 

[bcreilly]

Mr David, thanks for your response and putting the effort into the pictures.

I don't have a picture of what's happening on the bathroom side, but it is connected the same way as the laundry sink. It is a relatively small and shallow sink so I think that is why it is relatively high. I will probably go with your first suggestion.

Can you explain why the horizontal section of the vent should be above the flood rim of both sinks? I don't think its higher than the top of either sink. From what I can tell, shortly after entering the wall, the vent turns back to vertical.

 

[Mr_David]

Horizontal vent above the flood rim in the event the sink backs up it will not foul the vent. Say for example your main sewer backed up. Floating debris can foul the vent and remain there when the fluids drain out. Anything greater than a 45 degrees from vertical is often considered to be horizontal. It will work the way it is, just the potential for other problems they may never ever happen.

 

[Mr_David]

P.S. Between my previous 2 post ( I'm on call ) I had to run out and pop a pooper.

I was out the door and back in less than 1 hour.


Have a Happy Thanksgiving.

 

[bcreilly]

That makes sense. I don't think I will address that issue now given I would have to take down the bathroom wall to fix it, but thanks for explaining.

 

[phishfood]

In my area, it is allowable to wet vent a sink in that manner, & we do it regularly without problems. The flat part of the vent does not meet code, but as Mr. David said, the only time it might become a problem is if the entire drain line backs up.

 

[Matt30]

pop a pooper.

Brilliant terminology

 

[bcreilly]

I've followed phishfood's and Mr David's advice and installed sanitary tees in place of the wye connections (as shown in the picture). I did a lot of laundry over the weekend and no smell so far so I'm crossing my fingers that the problem is solved! Thanks everyone for the help.
Note, it looks like the horizontal pipe from the sink is going up, but that is just because is is angled toward the wall.

 

[Matt30]

Your top tee is backwards, but no big deal good work

 

[Matt30]

Just curious....the middle Sanitary tee that's picking up the other sink in the bathroom, is that straight from the p-trap? If so, you cannot connect it like that if it hasent already been vented.

 

[bcreilly]

I was about to ask you why, but then realized Have to do over that part.

 

[Matt30]

Learning curve man! Post pics when finished!

 

[bcreilly]

Two quick questions, both sinks have 1.5in traps which then reduce to 1.25in and connect to the drain pipe which is 1.25in.

1) Is there an issue with having the drain pipes reduce from 1.5in to 1.25in after the trap?
2) Is there an issue generally with having 1.25 drain pipes? Seems pretty narrow.

 

[phishfood]

In you original picture of under the laundry tub, I am not seeing the 1 1/4" pipe?