It's a trap that's designed for water to hold back the sewer smell. When it's actively used, the water accomplishes this. Every type of drain that goes to wastewater must have a p-trap.

Thank you and would you please tell us about the difference between S-Trap and P-Trap?

Hi Netmailer...

A simplified explanation is that S-traps are illegal and are not allowed to be used in most states nowadays as they were usually used to go straight into the floor without a VENT and the traps had a tendency to siphon (empty) and allow sewer gasses into the home. Even when vented, these traps still have/had a tendency to siphon as a result of the second curve in the trap... see image.

P-traps are legal everywhere and are supposed to be used in conjunction with a vent and do not usually siphon as a result of the vent and the required minimum of 2 pipe diameters from the trap to the vent connection... see image.

More questions? Let me know, OK?

Mark

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You made it very clear. Just one more question. You saying S-Trap is illegal.. I live in Melbourne/Australia. As far as health concerned I must believe the rules are same here too. But why do I see S-Trap toilet suits(Toilet pans) sold on Ebay if it's not legal. Does it really not matter what type of trap they come with if it's the pipes that meant curves to form the traps?

Hello Mark,

You explained it very well as usual and sorry I Can't rate you it will not let me here is the message ERROR: You must spread some Reputation around before giving it to massplumber2008, Thanks.

Best Regards,
John

To expand on what Mark said. Perhaps he should have said that all plumbing fixtures except toilets have P traps. The integral trap that is built into the toilet is an S trap. Note what happens when you flush and you will see why S traps are illegal for all other fixtures.

When you flush, water from the tank flows into the bowl causing the water in the bowl to rise above the level of the top of the S bend. Note that it does this rather quickly. This initial surge of water fills the discharge pipe. This water flowing down the pipe then sucks or siphons the remaining water out of the bowl, emptying it.

The tank then refills the bowl, but this is in a measured amount and it does it slowly so as to not start the siphoning action again. The water in the bowl seals out sewer gases.

The S shape of the toilet trap, the bowl being part of the S shape, is what causes the bowl to be emptied completely.

If your sink has a S trap the same thing would happen. The water flowing down the pipe would pull all of the water out of the trap.

So toilets have a trap built in and are not separately trapped. The shape of the toilet trap is what causes the bowl to be emptied.

(Assumption: Toilet pan is what we call bowl and toilet suit is bowl and tank)

In Australia there are the two most widely installed tiolets are either S-trap which is through the floor, P-trap that is through the wall. The other two types are Skew trap or Universal Trap and both of these can be installed as either an S or P trap with the right fittings.

Great explanation by Harold! Couldn't have said it better and wish I had.
In the US we also have rear discharge toilets. However, these are used mainly in public bathrooms. Most of our toilets discharge through the floor which is a "S"trap, (see image}

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