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I had a plumber come out and install a drain line and hook up the supply water for a washing machine. This is the state he left the project: enter image description here

How am I supposed to hook up the waste line? Do I need to drill out that center hole?

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Interesting box; I haven't seen that before. Is the intent to catch any potential leak from the hot and cold water connections, or is this just intended to permit pretty installation behind wallboard? – keshlam Jun 9 '14 at 17:53
Pretty installation behind wallboard. – Adam Davis Jun 9 '14 at 18:19
As a side issue, did the plumber install any shut-off valves on the hot and cold supplies, upstream of the connection points shown? – DJohnM Jun 9 '14 at 18:31
@User58220 Per the product info sheet there should be shut-offs in there, it looks like they're just not visible due to the angle of this photo. – Andrew Medico Jun 9 '14 at 19:20
@keshlam The info sheet I linked above claims "raised drip guard contains and directs potential leaks down the drain". No idea how well it actually works, though. – Andrew Medico Jun 9 '14 at 19:21
up vote 10 down vote accepted

It should be a knockout, meaning that a hard sharp blow will pop it out. The plumber should have done this before he glued the drain line in place, since knocking it out now could cause the plug to fall down the drain. This could lead to clogs in the future, since the trap is glued and cannot be disassembled.

Put the tip of a flathead screw driver on the knockout, and tap the back of the screwdriver with your hand or a hammer. Try to get it so that only part of the knockout breaks free. Then grab the loosened disk with a pair of pliers, and twist it free. Be very careful not to drop it down the drain.

Once the hole is open, simply hang the washing machines waste line in the hole. The washer should come with a piece of plastic used to make a hook in the waste line. Install that, and hang the waste line in the hole.

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The spec sheet for the box claims "Oversized KOs will not fall down drain". I'm not sure know how that actually works here though. – Andrew Medico Jun 9 '14 at 19:30
You can also cover the knockout with tape, with a wire or string under the tape. This would allow you to retrieve the knockout if it falls into the drain. – David Wilkins Jun 9 '14 at 19:33
@AndrewMedico Yeah that certainly doesn't look oversized in this case. However that may have something to do with GuestPost's answer - the box appears to be upside-down, and the port on the bottom (now top) seems to be larger and could potentially have a larger KO. Just my best guess though. – thanby Jun 10 '14 at 12:11

I would say that the box might be upside down, as the drip guard appears to be on the top. However, they are often designed to be used either way.

Nevertheless, yes, there should be a knockout. It is possible the plumber did not want the possibility of sewer gas coming back up while you wait to attach the washer, so s/he left the knockout in. It would have been better to remove the knockout and insert a plug that you could remove.

And I concur about the hope that there are shutoff valves above somewhere. Clearly the plumber used that space for air-hammer arrestors.

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You can just barely see the tip of the shutoff valve handle to the left of each open threaded connection. They're integral to the fitting. – Timbo Jun 9 '14 at 22:19
I would say it's two sided and not upside down. Just pop that plug and hang the hose there. Nice touch having the water hammer reducers installed. I have them on mine too. – sborsher Jun 11 '14 at 19:16

The water hammer devices are designed to hold air to stop water hammer. Your valves are upside down and will not stop water hammer. If water hammer is not an issue you can either knock out the plastic or drill it. You should have removed the knockout prior to installing it since when you knock it out the part might fall into the drain blocking it. You installed an "S" trap that cannot be easily removed to clear the drain. PVC fittings are available to make it removable. It might be advisable to cut the existing pipe, instal a sweep "Y" with threaded cap for easy clean out then a removable "S" trap. Just make shure you leave enough of the PVC thatin the box to have enough room to make the splice. Remove the drain knockout before doing the drain pipe.

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The plug is there to simplify pressure testing the drain line. That is done when a new plumbing system is installed or amended (as for a remodel adding some plumbing) by inserting a pneumatic plug (or at Home Depot), inflating, and filling the system with water.

enter image description here

Once the plumbing inspector has verified the pipes maintain several minutes of steady water level (indicating no plumbing leaks), the system can be drained and then the knockout can be (ahem) knocked out.

I would use a sharp flatblade screwdriver and a hammer to punch the screwdriver piercing through the knockout and then pry it up.

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