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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
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    Pretty installation behind wallboard. – Adam Davis Jun 9 '14 at 18:19
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    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
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    @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. – nobody Jun 9 '14 at 19:20
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    Hope he didn't melt the insulation off the wires in the conduit behind the water pipes. The copper and conduit should not be touching as they will corrode each other over time. – user23025 Jun 10 '14 at 20:20
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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. – nobody Jun 9 '14 at 19:30
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    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
  • Saved my bacon with this post. To keep it from falling in, I drove a wood screw into it and tied some twine to the top, and the other end to my thumb. Whacked edge with screwdriver, pried it out, done. – tobybot Mar 3 '18 at 14:24
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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. – Tim Sparkles Jun 9 '14 at 22:19
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    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
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Let's try to simplify/clarify for someone reading this now.

A. The box is NOT upside down. There are identical KNOCKOUTS on top and bottom to accomodate water supply from above or below. The drain could have been on just the bottom side, but that's just the way they made it.

B. Several ways to remove knockout without it going down the drain. Knock in with screwdriver (as suggested), then pry up and grab with pliers. One could drill a coulple of holes and secure with zip tie or string, etc etc.

C. Clearly there are shutoff valves. I believe I see the red handle on left and blue on right. These are 90 deg ball valves.

D. The water hammer arrestors are NOT upside down and will work just fine. They can be installed "at any angle" per the mfgr website:

http://www.ipscorp.com/plumbing/watertite/protectivesystem/hammerarresters

They are mechanical - involvong an air filled bladder and spring, I believe.

E. Note that the trap is glued up. This should be fine as washer drains are inherently self-cleaning. (Yes the trap is there to block sewer gasses. It only works once it is filled with water.

** A footnote to those who have floor drains in basements/utility rooms. You need to pour a few cups of water into them once in a while. This will flush out the dust/dirt, but most importantly keep the water level up. Especially important in dry climates where the water can evaporate quite quickly.

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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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    That's not an "S" trap, it's a "P" trap. – Tester101 Jul 25 '16 at 11:29
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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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If you were immediately hooking up a washing machine then the plumber might as well have knocked it out. If not then it is good he left it in to keep sewer gases from backing up into the house.

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    The trap at the bottom keeps sewer gases from backing up in to the house. – mmathis Aug 15 '17 at 13:58
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    @mmathis if there is no water in the trap it won't keep gas out. – mlapaglia May 12 '19 at 16:36

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