Sink S trap into open drain

[Lancelot1]

We have an S trap on our utility sink that exits the house into an open drain outside. So its not a closed system as air will join the exiting water before it enters the sewer. So is that why we dont have a P trap with vertical air vent valve under sink?

 

[Madrab]

Nope, either trap will do but an S trap usually has a deeper water seal and will be less liable to emptying if there is self syphoning when you have an open ended waste pipe.

With a properly ventilated waste and soil system a (vertical) vent under the sink wouldn't normally be required. An anti vac trap is sometimes used with longer or multiple runs.

 

[Lancelot1]

Thankyou Madrab. As the drain is open with air gap then there is no suck on liquid to get a syphon going? Plus do we need a trap at all under sink as the outside open drain in the ground will have water barrier to stop smells coming into open air around house and the sink drain pipe hovers over the drain like a giant tundish. So a water barrier is not needed under sink? Maybe just a small bulb trap to catch wedding rings? The S trap that is there takes up so much space plus it is lowered by a few inches of pipe which has an unused spigot! All our sinks in new kitchen extension go out into open drains instead of to a closed soil pipe.

 

[conny]

The trap, (which ever one you use), will help prevent spiders/flies etc gaining entry to the waste pipe and pop out through the sink plug hole.
Something I think your wife would appreciate.

 

[Lancelot1]

Haha good point Conny.

 

[Madrab]

Swap it for an anti vac bottle trap.

 

[Lancelot1]

Thank You Madrab. That is what i will do. Is the anti-vac bit necessary in my case or for the sake of an extra £1.50 i may as well have that feature?

 

[Madrab]

When you have an open ended pipe, as when the waste runs into a gulley, the high flow of the slug of water emptying out of the sink can create a vacuum in the pipe, that vacuum can then 'pull' on the trap drawing out the water seal. An Anti vac valve just stops that from happening, also stop the trap from gurgling.