Monday, February 27, 2006

Kitchen Island Drains 101

I recently discovered that the drains on kitchen island sinks present a unique problem because the island is in the middle of the room. Normally a sink is against a wall and it is easy to run drains and vents. Vents are an important part of a smooth running drain. Think of a two liter bottle of water. If you turn it completely upside down the water drains out of the bottle unevenly and there are frequent “burps” as air forces it’s way in to the bottle to replace the water that is draining out. On the other hand, if you tip the bottle only half way air is allowed to flow in the top part of the bottle opening while the water flows out the bottom half of the opening. The water flows smoothly because air and water are displaced simultaneously and at an even rate.
Download in the Play Store

Because the island is in the middle of the room you can’t run the vent straight up in the air and out the ceiling of the kitchen. Instead you use the drain pipe as a vent in a “loop vent” This was all explained to me on Sunday when I went over to my neighbor Gary’s house to talk with him about plumbing the kitchen island. Gary’s garage is stocked better than any hardware store or plumbing store in the area. If I don’t go to Gary’s for free plumbing parts I am admonished by him for going to a hardware store and buying plumbing supplies. I feel kind of funny about imposing on him all the time so I go over and ask a simple question first. “Hey Gary, should I run inch and a half or two inch for the island drain”. Inevitably, within a matter of minutes, we are out in Gary’s garage laying the entire drain and water system out on his floor. That is exactly what happened Sunday and I left a half hour later with enough parts and equipment to do the whole thing twice. The policy is always the same. “Take 2 of everything and return what you don’t need”. Here’s what I got on Sunday. What’s not in the picture is the 15-feet of ABS and assorted segments of half-inch copper.

All of this is leftovers from past jobs so Gary has no problem giving it away for free to friends and neighbors. It is incredibly helpful to have a highly skilled plumber who is a heck of a nice guy with a well stock garage living right across the street. You just have absolutely no idea.

Anyway, back to the drain. Here is what it looks like under the cabinet.

You can see that there are 2 drain pipes that go in a loop. The pipe on the left is 2-inch ABS and acts as the real drain. You can see the spout I will hook the sink up to towards the bottom. The pipe on the right is inch and a half ABS and acts as the vent. Notice how at the top it is pointed and not just squared off. This is important to meet code. You use 2 45-degree elbows and 1 90-degree elbow to achieve this.

Here is a drawing to show how the drain works under the floor.

Just like the bottle of water that is only tipped half way, the horizontal drain is only filled half way with water. The top part of the drain pipe allows air to flow back to the drain preventing gurgling sounds and a slow draining sink.

So now that everyone is an expert on kitchen island sink drains it’s time to learn some plumbing parts slang. If you’re gunna walk-the-walk, you gotta talk-the-talk. So, the little do-hicky in the picture below is used to support half-inch copper pipe under the house. The slang term for this thing among plumbers “Flying Butt-Hole”. So now as homework for the Kitchen Island Drains 101 class I want each and every one of you to go down to the hardware, go back to the plumbing supply area, and with a clear and confident voice announce to the sales clerk, “Give me a half a dozen flying butt-holes!”


Trissa said...

Interesting- thanks for the tutorial! We used a studor vent, which is a one way valve and allows air to enter when needed, but gravity keeps it closed and prevents sewer gases from leaking out. We use it for the sink in the peninsula and we haven't had any problems with it at all. I'm sure we'll use another one when we complete the half bath. I'll try to paste a hyperlink to their site, but if it doesn't work, just search for Studor Vent and their site will come up... I can't wait to ask for a half dozen flying buttholes at the hardware store!

Greg said...


I used a studor vent in the upstairs bath in my house. Maybe it is only a California thing, but according to code I'm only allowed to use one "in-line" vent in the house. I'm not sure why but that is the code. The loop vent is another way to do it and still meet code.

Becky said...

he he

Flying buttholes :)

I'd better go out of town to ask for those or I'll get a "reputation"

Mike said...

Don't forget to ask about their nipples while you're there.

tom, said...

I'm pretty sure you need another tee on the vent side of loop under floor tying into existing or new vent stack

Greg said...

I'm pretty sure you need another tee on the vent side of loop under floor tying into existing or new vent stack

I have seen this done in some applications. It is my understanding that this gas vent is not required in residential kitchen islands. I think the logic is that the island drain is connected to the other drains past the other vents. Therefore, gas is vented out of the drain well before it gets to the island. The loop vent is primarily for a smooth running drain and also to prevent the trap from being siphoned empty.

Libby Ornani said...

the loop vent to be legal in ca must access true vent on the vented side if the loop and have a cleanout on the vent side of the rise in the island wall behind sink base should look like two drains looking straight on at it........ outside vent to vent riser to santee to 45 to 90 down to 45 to santee to drain CA is very anal about the vent to outside to start

Greg said...

Well, that's not true. I've already addressed this, so I won't go in to it again.

The only thing I will point out about your statement is that there is no such thing as a "CA code". There is no state level residential building code. Not for drains or anything else. Building codes are city or county level. Not state.

Anonymous said...

There is in fact a State Building code in most states, it's just that the local codes are derivative of it, and so you don't look directly to the state code very often. I'm an architect in California, and the CBC is a base document to all local building codes here. here's a link to the California Building Standards Commission, for your edification:
Now, you'll almost never encounter a state inspector, but the state code does impact the local codes and agencies, as they will enforce the state code where the local has no specification beyond it.

Unknown said...

"I'm pretty sure you need another tee on the vent side of loop under floor tying into existing or new vent stack" I agree to Greg.

Emergency Plumber Melbourne