Do you flush P traps before maintenance?

[Timothy LaDuca]

I recently made a plumbing discovery. My father is/was a big DIYer. Whenever he needed to do maintainence under a sink and remove the P-Trap, he'd get a bucket to catch all the nasty water that was in the P-trap and above it to the height of the drain line. I did some experimenting and I realized I could "flush" my kitchen sink by simply filling it and waiting for any turbulence to die down and removing the stopper (carefully as to not introduce excess turbulence) the water would drain and "flush". I knew it was flushing because there was such a sucking sound when the water finished emptying the sink (which sound I don't normally hear). Curious, I removed the P-trap, suspecting the water in the P-trap should be no higher than the top of the bend. Indeed that was the case. No mess, just a little water in the bottom of the P-Trap. Is this plumbing 101 or just a waste of time? I demonstrated this to my father (before investigating the P-trap, nor even thinking of it) and he was impressed by the sucking force (as was I) which won't happen if there is turbulence because you get angular momentum and a whirlpool (funnel) which prevents the water from evacuating at full force and/or introduces air into the flow preventing a complete siphoning.

 

[Leejosepho]

...he'd get a bucket to catch all the nasty water that was in the P-trap and above it to the height of the drain line.

I did some experimenting and I realized I could "flush" my kitchen sink...when the water finished emptying the sink...the water in the P-trap...no higher than the top of the bend...just a little water in the bottom of the P-Trap. Is this plumbing 101 or just a waste of time?

If I were not opening the drain to clear a clog that would prevent what you have mentioned, I could possibly cause the same siphon effect at our kitchen sink where the vent is a few feet away since the sink is under a window. But I suspect you might not get the same effect at a bathroom sink where the vent line might be immediately behind the trap. So, what you have here is Hydraulics 101 as a way to check how well a given drain line is vented!

 

[Sponsor]

 

[Timothy LaDuca]

If I were not opening the drain to clear a clog that would prevent what you have mentioned, I could possibly cause the same siphon effect at our kitchen sink where the vent is a few feet away since the sink is under a window. But I suspect you might not get the same effect at a bathroom sink where the vent line might be immediately behind the trap. So, what you have here is Hydraulics 101 as a way to check how well a given drain line is vented!

Ah. Thanks. Indeed my kitchen sink is under a window as well. Interesting.

 

[hj]

If the level in the trap drops below the outlet level, it either was subjected to siphonage or inertia, both of which are prohibited by the plumbing codes, and good installation practices. The "sucking sound" IS air being aspirated by the flow of the water. The sucking sound is air being aspirated by the water flow, and you cannot prevent the Coriolis Effect which creates an eddy current. Any turbulence caused by filling the sink is completely irrelevant at the drain opening where the flow is occurring.

 

[Timothy LaDuca]

If the level in the trap drops below the outlet level, it either was subjected to siphonage or inertia, both of which are prohibited by the plumbing codes, and good installation practices. The "sucking sound" IS air being aspirated by the flow of the water. The sucking sound is air being aspirated by the water flow, and you cannot prevent the Coriolis Effect which creates an eddy current. Any turbulence caused by filling the sink is completely irrelevant at the drain opening where the flow is occurring.

The level of water is not below the bend, so the trap is still effective. I don't see why that would be a problem.
The Coriolis effect is not applicable at such small scales as the effect is negligible.

http://en.wikipedia.org/wiki/Coriolis_effect#Draining_in_bathtubs_and_toilets

If you let the water settle you will not get a whirlpool, it is always due to turbulence. No whirlpool leads to faster draining which causes the P-trap to "flush". [source: my own experiments with kitchen sink drains]