P-Trap Cunundrum

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Hi hope that someone from this excellent group can help with this one. It's sure got me scratching my head.
We have a main-floor laundry room with a sink in our two story home. The washer drains into a pipe that joins the sink drain prior to the p- trap. Every week or so we've noticed an awful smell in the laudry room, which I suspect is a sewage odour, as if the p-trap has lost it's prime. If I run water down the drain the smell eventually goes away.
I've checked under the p-trap but can't find any water there and don't think the joint is leaking. It's as if something is sucking the water out of the trap, but have never heard of this before.
The house is located in the greater Toronto area in Canada. The street has a gentle slope to the south. We have an apartment in the basement of the house, with a full kitchen and bathroom. The second floor has 2 bathrooms, but nothing directly over the laundry room.
We've taken to just leaving the stopper in the sink as a temporary measure, but obviously would like to resolve this problem.
Thanks for all replies.
Peter H
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Was it all working ok previously? Need to know that to determine if you have a functional problem or a design problem. You could check to make sure nearby vents are ok. Easiest way to do that is to get on the roof with the garden hose.
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We just bought the house 8 months ago. It's an intermittant problem, but I think it's been there since we moved in.
There is a little bit more to the story, but left it out 'cause I didn't think it relevant.
When we inspected the house the home inspector turned the tap on (Hot water side) and instead of water coming out it sucked air. We looked at it and realized that there was a small hose connected to the faucet, on the hot water side. The hose was of the size that you would expect to see supplying a humidifier on a furnace. The hose was bleeding hot water continually, even if the tap was turned off. The owner, a single woman, seemed oblivious to the problem, but said that she'd have the faucet replaced before we took over. She did do that, but I found that the little hose had just been hacked off down at the base and wasn't connected to anything. I assumed, since the basement was finished, that this was a primer for the p-trap in the basement floor drain. I reinstalled it to the cold water supply, but put a valve on it. I open the valve and run the water periodically to prime the basement drain. This action has not coincided with the prime problem we have in the laundry sink, so I didn't think it relevant.
Thanks for your help !!
Peter H
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Do you have any durabond , flour, or cornstarch around , cover the area under the trap and pipe, any leak should be easily proved or dissproved. A trap should stop smelling with a cup of water so if you have to run a hose for awhile maybe its something else.
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I'm sure that I would have noticed water down there if the trap was leaking. I've checked it a number of times and nada.
Peter H
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Peter H wrote:

A common question on plumbing license tests. http://www.managemylife.com/mmh/questions/111392-traps-lose-seal
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On Jul 22, 10:41am, FatterDumber& Happier Moe

Thanks Moe. That link was very helpful.
I don't think that there is anything else draining into the line that this laundry sink drains to. All the other water lines are on the far side of the house... and the sink drains well, so I suspect the vent is ok. I'll have a look though. There is too much water running from the laundry there ( 6 of us living in this house and using that one clothes washer) for the trap to ever dry out.
Peter H
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Peter H wrote:

tests.http://www.managemylife.com/mmh/questions/111392-traps-lose-seal- Hide quoted text -

If there are strings hanging over the weir of the the trap (from the washing machine) the water will capillary out. Leave a wet towel hanging over a full sink and you will see how it works. Also self siphoning which is how a toilet works, but toilets have a refill tube that tops them off after every flush. It's hard to do but a trap can flush its self almost dry if the velocity of the water is enough. Google drain waste vent system and you will find lots of information. What it all amounts to is getting water to stay in the trap, I suspect you might need a ventless vent and it has to be installed correctly to work. https://www.centraltrailer.com/cart/index.php?main_page=product_info&cPath 9_134&products_id26 My old boss used to say there are only three things a person needs to know to be a plumber, 1. The boss is a SOB 2. Sh*t runs downhill 3. Payday is Friday at five
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https://www.centraltrailer.com/cart/index.php?main_page=product_info&cPath 9_134&products_id26
LOL, #4. Don't chew your fingernails.
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I always heard #1 as "Hot's on the left"
--
The best argument against democracy is a five-minute conversation
with the average voter. (Winston Churchill)
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Larry W wrote:

And I've heard #3 as "Sh*t won't run uphill."
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
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wrote:

Not usually, but with the right sump pit and sewage ejector pump many modern miracles are possible...
~~ Evan
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FatterDumber& Happier Moe posted for all of us...

tests.http://www.managemylife.com/mmh/questions/111392-traps-lose-seal- Hide quoted text -

https://www.centraltrailer.com/cart/index.php?main_page=product_info&cPath 9_134&products_id26
If it's green it may not be a pickle.
--
Tekkie Don't bother to thank me, I do this as a public service.

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On Jul 22, 11:49am, FatterDumber& Happier Moe

Your old Boss left out a couple of items Hot on the left, Cold on the right. Keep your fingers out of your mouth. -- Tom Horne
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wrote:

Thanks Moe. That link was very helpful.
I don't think that there is anything else draining into the line that this laundry sink drains to. All the other water lines are on the far side of the house... and the sink drains well, so I suspect the vent is ok. I'll have a look though. There is too much water running from the laundry there ( 6 of us living in this house and using that one clothes washer) for the trap to ever dry out.
Peter H
It would be of value to know where your vent is in relation to the sink, age of the previous piping, size and type of pipe.
If your sink is an 1.5 inch drain and you install a washer drain up to that line, you will have siphon issues. You should have a 2 inch line to drain your washer into, preferably a 36 inch stand pipe with a p trap at the bottom. You also need to have proper venting in line to maintain your water in your traps. If you have older cast iron pipe, there is a chance that corrosion may have reduced your pipe diameter and if the vent is not close to the traps, you may have exceeded your minimum critical distance.
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Peter H wrote:

As another poster suggested, it may be the vent.
Just guessing, here, but:
If (several, all?) the vents are clogged (or missing completely), stuff moving down ANOTHER drain could generate suction on the entire sanitary system. This suction COULD evacuate the p-trap.
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On 7/22/2010 8:10 AM, Peter H wrote:

it sounds like your flow from your washer is making the p-trap siphon and suck dry. You need a vent or an air admittance valve AFTER the trap. OR you could just plumb the washer in after the trap, but it still may suck it. The air inlet is the best bet.
--
Steve Barker
remove the "not" from my address to email
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+1 to Steve's answer...
The pressure from the washing machine's pump is draining the p-trap shared by the sink and washing machine...
Sounds like an avid DIY'er did a weekend plumbing project and really should have asked for some help...
~~ Evan
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Evan wrote:

I don't know, all my washing machines pumped like all hell but then the last 1/2 gallon or so goes pretty slow, especially pumping during the spin cycle. That should be plenty to fill the trap again. Maybe this sink is draining fast enough to do it? Either way, I'd install one of those ventless vents (air admittance valve). Although it wouldbe best to take it apart first and check for string and stuff that would empty it by capillary action.
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Check for a blocked vent pipe. Water from another source exits and creates a negative pressure in the sanitary line if the vent is blocked or inadquate.
Nonny

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