whats this

So here is a Whats This. can anyone help me out here. under the laundry room sink there is a SS braided supply line running from a shut off valve to the sink drain…

Nice cross connection :roll::roll:

what were they trying to do?

Trap primer.


Agree on the cross connection concern. Never understood why it is allowed for trap primers.

Put a rubber plug in the drain hole for crying out loud.

Thanks Jeff, never have I seen one before.

Usually see them for dishwashers, but the ‘stub’ is angled ‘up’, not ‘down’.


OK… So I am not familiar with this… so I looked up the definition.

Wikipedia says
“A trap primer (or trap seal primer) is a plumbing device or valve that adds water to traps. The water seals in traps are needed to prevent sewer gases from entering buildings, but because this water is exposed to the air, it is subject to evaporation over time in infrequently used floor drains, leading to the release of sewer gas into the environment. The trap primer mitigates this problem by injecting water, either directly or indirectly, into the trap to maintain the water seal indefinitely.”

Wouldn’t you need a backflow preventer on this in case the pipes clogged and the water level raised?

It seems that the water valve would need to be turned on/off manually each time or else the water would just keep running. Wouldn’t it be easier to just turn on the main faucet?

Seems like these are used mainly for floor drains

Let us know when you make sense out of it Dennis. :wink:


My understanding is these were originally intended for use in remote sinks, such as a basement utility sink that gets little use, and/or for vacation homes. Not saying it is right, just what I have been informed of.

Interesting, I will say that the home was equipped with a mop drain system in the floor across from this, I wonder if there is a correlation

LOL sorry about the pool picture, it has nothing to do with it (hopefully)

Wait a minute – I know — I think that the pool fountains must be connected to the trap primer line ---- This keeps the pool water evaporation in check.

Yes it is a trap primer. Usually for floor drains to prevent sewer gas

If you held the plug in the drain and turned the valve on it would be trap cleaner :slight_smile:

It’s not a cross connection.

It’s a trap primer that diverts some of the water draining out of the sink into the trap of the floor drain to keep water in the floor drain so the house doesn’t smell like sewer gas.

Makes sense now. Thanks.

Not sure why a valve is needed.