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Sewer smell - what is this extra line under bathroom sink?

ClickClack
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AG
My house was newly built less than 2 years ago and recently 2 of 4 sinks have started having a terrible sewer smell. I first thought they just needed cleaning/drain-o (also I thought of p-trap), but I noticed a common denominator. The 2 sinks that smell have a configuration like this (notice the extra black line):



The other 2 that don't smell don't have that black line, just the hot, cold, and drain lines. What is that black line and is this smell attributed to it? How do I get rid of this smell?
agcivengineer
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AG
That is your primary air conditioning drain line. If you pay attention, it probably only stinks when the ac is blowing. How to stop it, or what causes it? I have no idea, I'm sure someone else on here knows that. One thing I know is that every so often, you should go in your attic and pour a mix of water and bleach down the spout that sticks up in order to keep it clean.
ClickClack
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quote:
One thing I know is that every so often, you should go in your attic and pour a mix of water and bleach down the spout that sticks up in order to keep it clean.

Appreciate the info on what it is. I've heard that about the pipe in the attic, although admittedly I haven't done it yet and need to. Are they connected?
ClickClack
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Just poured a cup of bleach down each spout in the attic. Right now the sink that was the worst smells completely like bleach, so that's a good sign so far.
agcivengineer
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If that ever gets clogged for any reason, you will see water dropping out of your secondary ac line. That is located outside just underneath your roof line, usually by a window. That helps you know there is a clog and you need to clean it. Since you smelt the bleach, you know it's not clogged. I had this same problem in a house we moved from a few months ago, I never really solved it. I was hoping someone else will see this and help answer it. Good luck.
ClickClack
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Thank you sir. I did actually know where the secondary line comes out of the house. Had no idea the primary went into our sink drains.
ClickClack
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So I can smell bleach in one of the sinks but not the other. I poured another cup in the other and still can't smell it, but it goes down fine. Not sure if I should be concerned.
Absolute
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As to the smell. Couple theories/questions that might help.

Do you use this sink regularly? If not, the trap could dry out and allow some sewer gas to come up and be noticed.

When you were pouring the bleach in the condensate line in the attic, was there a trap in that pipe? Some installers do not put the trap in the condensate line. When it is missing you get airflow from the HVAC unit through the condensate drain pipe into the sink. Sometimes you can hear the airflow hissing, if the unit is close enough to the sink. This can exaggerate the smell problem if the trap at the sink has dried out.

It is normal to smell the bleach at the sink when you pour it in the condensate drain. Which is a good practice. The condensate drain in and by itself does not have anything to do with a sewer or a sewer connection.
ClickClack
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No p trap in the condensate line. I coukd only smell the bleach in one of the sinks. Hopefully there's not a blockage in the other but the bleach went down from the small area of the pipe I could see.
Seven
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Remove the line at the sink that didn't get the bleach smell. Rig it up to your shop vac and go to town. See if that does clears anything. You may need to cap the other line to get good suction.
bco2003
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Why would the line be going to two sinks? Are there two attic units?
ClickClack
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Yes, two units.
agcivengineer
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If there are 2 units, then there should be two pipes you'll need to pour the bleach down. One unit goes to each bathroom sink.

The smell is what I don't understand. When I had the problem, it was the sink we used all the time. I wonder if it has to do with the trap not being in the line and air coming from the air conditioner. When the ac was on, we would have air coming out of our sink. Is that the case for you?
ClickClack
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I poured the bleach down both yesterday. I can hear the air running into the sink when the AC is on, just barely.
ClickClack
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Took the p trap off the sink wgere I couldnt smell the bleach and put a measuring cup under it. Poured a cup down the pipe in the attic and got a cup back so no blockages.
Mom Class of '03,'05 and '09
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IF, you don't have or know about the primary AC drain line, don't know or have a secondary drain outside under the roof line or near a window so that it serves as a visible clue you WILL HAVE a wet puddle on your new carpet and the new wood floor in the entry hall will warp.

Guess how I know all that?
agracer
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If you don't have a secondary drain you'll have a pan under the unit with a float switch.

it's always fun when you go in the attic and bump that switch by accident and can't figure out why it's so hot in the house a few hours later.
The Fife
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Maybe it's bad luck but I'm 0-3 for houses where the float switch actually shut off the A/C.
Kenneth_2003
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Did the bleach clear up the odor at all? If there is a small amount of algae or other crud growing in the drain line, keep in mind it enters above the P-trap so any vapor in that line will be common to the room.
ClickClack
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The bleach definitely helped. I don't smell it in the other sink and the sink that smelled the worst (mine, in the master bathroom) smelled great for a week and now the smell is faintly there again. I'll probably do a little more bleach.
Aggie1
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To avoid an odor problem - for whatever reason - the A/C drain line should have been connected past the P-trap on the horizontal into the wall.

Where it is located on the vertical above the P-trap is fine for drainage - but not for escaping odors from the A/C unit/pan, etc.
Gary79Ag
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quote:
To avoid an odor problem - for whatever reason - the A/C drain line should have been connected past the P-trap on the horizontal into the wall.

Where it is located on the vertical above the P-trap is fine for drainage - but not for escaping odors from the A/C unit/pan, etc
This method would eliminate any odors in the bathroom area, however having the drain beyond the p-trap would allow sewer gases to enter the attic area so that in of itself can lead to a potential hazard.
Aggie1
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Maybe a bit - but rather in the attic than in the occupied house - also, that's why the plumbing stack is vented. Either way a P-trap in the A/C drain should resolve the odor issue.
Absolute
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The primary condensate MUST be connected after the trap. It is against code and unsafe to have them connected with before the trap or directly into a vent in the attic.

I suspect the smell may just be drain gunk smell being blown up because the condensate drain is not trapped up at the unit to reduce air flow toward the sink. Might try cleaning out the trap/pop-up area at the sink. They can get pretty yuck, especially the girl's side with hair and products.
Ted Logan
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That p-trap should rarely be dry, unless it's winter and you don't use the sink. A/c systems put out tons of water so that trap should stay full all the time. (Side note...someone should figure a way to harvest the water from a/c units to use for irrigation, etc... In the summer)

Could the smell be from the water itself? What about a potential vent that is behind the wall that isn't vented correctly?
ClickClack
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I think it's just mold/mildew/algae? in the condensate line. It got a lot better after the bleach, even after a week and a half. I need to do another round.
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