How a Plumbing Trap Can Lose Water [VIDEO]


So, Richard, this email from Frank in North Carolina, you got it, right?

I did. He puts in a brand new washing machine and the first time he starts to use it when it drains, it actually pulls the water out of the toilet bowl in the half bath next to it.

That’s right. You know what’s going on? I think I do and plumbing is really hard to understand so maybe a little more mock up that might explain what’s going on.

Plumbing Trap

First of all, every single plumbing fixture has a trap- a water seal. So when it leaves the sink you might see a trap like this and what you don’t see is there’s always water right in the trap here.

We know it’s as a P-trap. Correct. And there’s also a trap built into a toilet. So this is a cut-away right here, you can see the water seal right here and that prevents sewer gas from coming up inside the building. And this is what’s being pulled away in his case when it raining, how does that happen?

That’s right. Think about if this washing machine discharges right here, a big slug of water comes right down here, through the trap, comes to this point right here and now it goes down the drain. Now, the important point on any plumbing system is also the vent- this right here.

This is a pipe that goes up through the roof for air. That’s right. So we can break the vacuum of this slug of water coming down right here. Let me show why the vent is important. Now, this would simulate the big slug of water with an unvented fixture and if I do that, see a gurgle, gurgle, gurgle- it’s looking to pull air down but can’t find it. That glugging is sucking for air. Something like that. So it’s looking at that negative pressure that is creating that vacuum.

And let me create as if there is a vent on this. So now we’re actually allowing air to be introduced to the system as the water leaves, right? Yeah. See how it perfectly drains? Oh, beautiful. No gurgle. So we got the air that it needed, the relief that it needed, okay? Let me take you back here now. So now, in the case of Frank, here is that big slug of water, it comes through right here.

I don’t know if they actually have a vent. It may be that the original staller… I’ve seen before, they just say “Ah, we don’t need a vent” and what happens is the old washing machine didn’t put that much water down. The new one has a big slug of water that’s coming this way.

Now, it’s coming down and it’s looking for air, trying to create a back pressure. So that big slug of water comes here but it’s trying to pull and can’t find air anywhere else because there’s no way to relieve that over there. So his washing machine is connected to all of these pipes that are behind the wall? Absolutely. So here comes this pulling, pulling, pulling; where is it going to look for air? It could look for air right here and try to pull that water seal backwards down this way. So the vents are really important. So in this mockup, here is the main vent right here.

That vent, if it existed should come up and connect back into here and then go out through the building. A solution for Frank might be to be sure we have a vent. That vent could be clogged- if he has a vent. That could be an animal that got inside and died. If they doesn’t have one, these are allowed in some states.

Air In The Plumbing Trap

This is interesting where it could go here. It allows air in to break the vacuum but doesn’t let sewer gas come out- a mechanical vent. You don’t have to run this all the way up to the roof, you going to just do that. That’s right? But there’s a bunch of ways that you can lose your trap seals- all of the trap seals.

There are other ways in this? There is about seven of them. So, one of them is oscillation. You’ve got this plumbing system right here and you got this pipe going through the roof. Yes. Now you got crazy wind blowing over the top of it that could push down this way. It’s like blowing on s straw?

Right. So now, what it could do, it makes the water seal, in any of the traps, bounce. So now if it bounce- I have a really tight house that’s near the water. The wind blows like crazy; on those days, we can see the water seal going like this. Now if it bounces enough, the water bounces up here and just all of a sudden, over time, it would make this be just low enough to release some sewer gas. Got you. It may happen also inside these traps but you wouldn’t see it. Right. Because it’s underneath the sink and it’s certainly not clear. That’s right.

So oscillation is the second one? Right. Now, there’s also a siphonage. These are no longer allowed. This is called a full house trap. Well, hang on a second, we love siphons, don’t we? Because that’s how a toilet works. That’s how they work but, think about this: if you have a siphonage in the presence of a short leg and a long leg.

So now, if I have water coming down here and I have a complete full house, which is illegal in almost every part of this country, the water would come here and there would be a short leg, right here, of water and then a long leg right here. When you have that long leg, what does it do? It would actually pull that water right out of the trap. Too much velocity going down here that sucks it out of there. Correct. That’s right.

Another one is called capillary traction. Here is the water seal inside here and there may be times that there’s some foreign matter that gets down inside the trap and look what happens. Over time, the water could wick up through that string and just slowly empty that trap of the water and rip it down the drain. Capillary action. An easy one is evaporation. Yeah. Everybody goes away for the winter and now what happens?

This water- the really dry house- would evaporate over time, you have sewer gas coming here and over here. Okay. So last one or is there one more? Well, there’s also momentum. You don’t see that much anymore. In the old days, we might have put a trap down in the basement and a lavatory in the second floor, and the water would come down and we have so much velocity it would go right through the trap.

We don’t see that too much anymore. And I guess on some level, if we’re getting this sewer gas smell, it might be as easy as just refilling these traps but certainly, now we know sort of how to find the problem. We just look at these vents. That’s right. Remember, every fixture has a trap, so you’ve got to have water in every one and you can’t see it.

So if you think you’re smelling it from some place, go fill those traps again. So will you write all that up and send it to Frank or? I will send him in an email and I’ll fax it to him. Good. Great information. Thank you.

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