A question about washing machine traps

[Macman]

In the thread below entitled Washing machine trap, Terry states that the trap cannot go in the basement because it has to be above the floor level of the floor on which the washing machine is located. I was wondering if Terry, or someone, could explain to me why this is. I'm just a lurker who very occasionally posts, and it's just a matter of curiousity.

 

[Terry]

If the standpipe is too tall, it siphons the trap.

Plumbing code, knowing this, has limitations on how long a standpipe can be.

 

[Sponsor]

 

[Macman]

Terry, forgive my ignorance, but it's not yet making sense to me. I don't understand how a standpipe can siphon. Are we talking about a too-tall standpipe clearing the water out of the trap? How does this work? How tall is too-tall? I'm not trying to be a pain, but this has me interested.

 

[hj]

trap

It will not siphon the trap, but the inertia of the falling water WILL carry it through the trap, and the end result is the same as if it had been siphoned.

 

[Terry]

It will not siphon the trap, but the inertia of the falling water WILL carry it through the trap, and the end result is the same as if it had been siphoned.

And then there won't be any water in the trap.
I think hj worded it better.

 

[Macman]

Ah! Okay. Now this makes sense. It would seem to me that it would take a pretty good volume of water to make this happen, but I can understand that it could be a problem. Thanks for the answers.

 

[Jadnashua]

Washing machines, especially newer ones, pump the water at quite high velocity.

 

[Redwood]

Momentum!

 

[NHmaster]

Mr. Newton I believe. A body in motion tends to stay in motion.

 

[Jimbo]

I tried to get my head around this problem, and I think it would take an engineer to explain it.

I suspect that it would all revolve around the vent.....

 

[hj]

vent

Of the five ways that a trap can lose its seal, a vent, or lack of one, is only one of them.