Here is the installation diagram from the manual for the Liberty 404 drain pump. It clearly states "Note: a trap shall be used between the fixture and pump, a flange type is recommended."

On page 2, the diagram shows an s-trap inlet to the pump. Won't that be siphoned out, leaving the sink and washer stand pipe vulnerable to nasty pump basin smells? I can't figure out if the outside air vent on the side of the pump basin would somehow break the siphon. Wouldn't the trap vent need to be above the trap weir? Seems more logical to just inlet from the side with a standard p-trap arm.

Why would Liberty suggest such an s-trap?

Appreciate any expertise.


[www.sumppumpsdirect.com]

The unit WILL work as designed IF you follow the installation EXACTLY.


The 's' trap discharges ABOVE the water line in the basin and is 'functionally' vented by the basin vent.

==============================================

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

An"S" trap, by its definition is NEVER vented by ANYTHING. It will ALWAYS siphon if conditions are right.

So you mean the head of water in the basin being fed by the outside air vent will break the s-trap siphon? How is that any different than an s strap under a kitchen sink that eventually meets a vent downstream? Will Libertys s trap work because the vent is right at the end of the s trap arm? I thought an s trap will siphon unless the vent is above the trap weir.

The 'inlet' side of the 's' trap is open to atmosphere in this application.

the laundry sink drain strainer


It will work as designed.


@ hj: picture an old style floor mounted 'slop sink' with an integral 's' trap

==============================================

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

Picture ANY toilet "with an S trap" An "S" trap s an "S" trap, PERIOD. NOTHING, other than extremely slow flow will prevent its siphoning.

siphon JET

the jet hole MAKES the WC siphon


but


we agree on the reasons that 's' traps and plumbing do not play well together

==============================================

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

Washdown toilets did NOT have a jet hole but they still flushed

not very well, that is WHY they were replaced by siphon jet type


.................. over and out

==============================================

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

Appreciate the toilet examples guys. But a toilet s-trap doesn't meet a vent until further downstream. If you go back to Liberty's diagram the drain pump places the vent just below the s-tram arm, on the side of the pump. I guess you'd call this a vented s-trap? I don't plan on installing it that way, Im going to just come in from the side with a p-trap. But it's food for thought?

I think it comes down to the placement of the vent in relation to the trap weir and arm. As the water exits the s-trap its immediately pulling outside air from the vent and the head of air above the water line in the basin. I believe this would break a siphon effect.

But I also still think if you fill the laundry sink with a large enough volume of water you could empty the s-trap.

This guy shows how an unvented long arm s-trap will absolutely siphon:
[structuretech.com]

I cant get the website pic to post here but there are plenty of examples online where people throw an AAV onto an s-trap, usually near the end of the trap arm to break siphon. Or how about a crown vent on the top of an s-trap?

you should have NO or a SHORT arm 's' trap in your application

just install as per manufacturer's instructions and be done with it

==============================================

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



Edited 1 times.