I have a read a few times on this forum over the years that a fixture will drain better without a vent? I think it was mostly stated regarding a toilet's vent, but I thought I read another time regarding another fixture?

My understanding of vents was the example of sucking water into a drinking straw then placing your finger on top of the straw, then releasing your finger, watching the water now fall out of the straw. Meaning the equalized air pressure between the atmosphere and the inside of the drainage pipe was a good thing for drainage.

I realize vents have other uses like allowing the methane gases to escape to the atmosphere and also preventing p traps from siphoning, but I thought it improved drainage dramatically too.

Could you explain in more detail when a drainage pipe flows better without a vent?

With a vent, water drains by the force of gravity, which is not very strong with minimum pitch/slope. Without a vent, drainage occurs by gravity AND air pressure differential combining. If you connect a hose to a tank's drain valve, and lay it horizontal on the ground, it will drain by gravity, the same as if it had a vent. BUT, if you were to hang the hose over the edge of a slope, then the air pressure differential between the hose connection and the lower end of the hose will increase the rate of flow considerably. I once went to a house where the owner was doing his own plumbing and he wanted me to run a 3/4" drain from the tub on the second floor to a connection in the basement. I told him that once flow started that a pipe would suck everything in the room, including his baby in the tub. A bit of hyperbole but he got the idea.

all I know is when I pour a bottle of beer into a mug it runs slower without a vent. But if it's pitcher it fills faster with the large opening with air. But the bottle creates more foam in the mug.

There is air above the outlet of the fixture so there is no vacuum as there is with a bottle.

A bottle without a "vnt" is a closed system. A drain without a vent is a system that is open at both ends, basically an inverted "U" siphon.

the purpose of the trap is to trap sewer gas IN the pipe and NOT let the gas into the room

the purpose of the vent is to prevent the water in the trap from 'siphoning away'

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Water always flows better thru a drain when it is properly vented, You can put this to the test yourself. Hook up a hose to the water heater drain, run it to a floor drain, turn off the water going to the water heater, open the drain on the heater and let it run for 1 min and watch the flow dissipate to a dribble, now open a hot water faucet closest to the heater and then notice the difference in the flow, You have just created a vent in a closed system, the same applies to the drain system in the house.

PlumbCat TM 2003


Plumbermandan

noooooo ........ you have merely broken the vacuum


a vent installed on the heater drain will NOT make it drain any faster

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

I like sum's answer best.

>all I know is when I pour a bottle of beer into a mug it runs slower without a vent.
>But if it's pitcher it fills faster with the large opening with air.
>But the bottle creates more foam in the mug.

Put the "vent" at the point where the hose is connected and see if there is any difference.

Hi Folk, First, you mentioned about the atmosphere. The pressure at the top of your water pipe is equal to atmospheric pressure, so no fluid can flow through it at that state. When you open your tap and create the pressure difference between the atmosphere and the pipe in your sink, the pressure difference forces the water to flow through the pipe. When you put your finger on the top of the pipe, you obstruct the fluid flow. Hence, water can not flow. When the fluid flow is obstructed, the pressure difference is lost and the atmosphere at the top of the pipe is equal to atmospheric pressure. When the pressure is equal, the pipe is said to be 'water seal'. This is how we test for leaks. If you push your finger tightly on the pipe and water stopped flowing, then there is a leak at the site where you are blocking the pipe. The air in the pipe is at the atmospheric pressure and the water in the pipe is at a higher pressure. The water in the pipe will want to flow out until its pressure is equal in the pipe. Hence, you can test for leaks by just blocking the pipe with your finger. When the pressure is equal in the pipe and atmosphere, there will be no flow of water.