Kitchen Sink Plumbing
Posted by Wayne
Most people who look under their kitchen sink just see a mess of plastic tubing, and I was one of them until I worked as a new construction plumber.
The new plumbers all had to run water pipes, and the more advanced guys installed drainage and kitchen sinks. I think the reason is that there is just more code surrounding the drainage, and the water lines basically just have to not leak so any idiot can do it. I was assigned to kitchen sinks pretty early on, and I probably did about 30-40 in my short lived plumbing career.
So here’s what you’re looking at when you open up the cabinet under your sink:
The purpose of this image is to show how the drainage is connected, however the image is inaccurate in that the two sink drains would obviously be parallel to the back of the cabinet in real life (unless you have a really funky sink).
The drainage in my house (and likely any older house) is like the image on the left. I understand that plumbing code used to say that every drainage pipe had to vent out the roof of the house. In new plumbing (at least in Colorado), you can have certain drainage pipes with only a charcoal filter attached to the top, and that would be visible inside the cabinet if it were the case.
In Colorado, there are normally two pipes coming off the main drainage pipe for the kitchen sink. The lower one would be for the dishwasher drain (a tube comes out of the dishwasher and is fed down into the open top of the pipe), and the upper one would be for the sink drain(s) and garbage disposal.
I worked with a plumber from New York and he told me back there they were allowed by code to hook the dishwasher drain right into the garbage disposal, eliminating the need for the dedicated dishwasher drain.
All drains contain a P-Trap. I was told that this is because it’s shaped like a P, not because it traps pee. I personally think they should call it a ‘U’ trap to avoid confusion. The purpose of the P-Trap is to keep sewer stink from coming back up your sink drain. Water just sits in there and blocks the stink. The water can evaporate over time though.
Theoretically (and I have smelt this in person), if you leave a shower or sink unused for years, the water will evaporate and you will believe a dead animal is lurking. If you turn on the water in said shower or sink for a few seconds this will go away because it will refill the p-trap.
The new sink we bought only had one drain and the old sink had two drains. This required some slight modifications to the existing plumbing, and now all the sink drainage runs through the garbage disposal (see image below). Those with a keen eye will notice that the p-trap is on backwards. This is a result of the new sink being so much deeper than the old sink. It was either flipping the p-trap backwards or ripping into the wall to modify the drain heights (if I want to stick with the code). One time when I was installing a kitchen sink I pointed out to a Master Plumber that whoever had been there before me had installed the drain too high, and he told me to just flip the p-trap backwards and it would work, so I’m keeping my fingers crossed that he knew what he was doing.
The only bad thing I can think of that would happen here is that because the ‘u’ part of the trap is now deeper than normal, food from the garbage disposal might start gathering in there and stinking. I haven’t noticed anything so far. I think if I did start getting smells I’d just seal off the upper drain permanently and attach the garbage disposal to the lower drain and attach the dishwasher hose to the garbage disposal. I’m all for sticking with the code, but if it’s good enough for New Yorkers to have one drain for the sink and dishwasher then it’s good enough for me too.
I was able to salvage the old garbage disposal, but the rubber baffle (the rubber you see when you look down the garbage disposal) was worn out. I bought a replacement baffle and as far as anyone using the sink can tell, it’s a brand new garbage diposal.
Installing the new faucet and fixtures was the most difficult part of the job, and not because it’s complicated to understand but because it is physically complicated to lay up under the sink and reach up there to work. Damn these broad shoulders!
There are also times when it is necessary for another person to hold the fixture above while you’re working below to make sure everything stays in place while you’re tightening it down, otherwise things will not stay in a straight line.
After
One Response to “Kitchen Sink Plumbing”
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February 12th, 2008 at 10:35 pm
Michigan is the same as New York also - one drain to rule them all. Your post made me go look under my sink in detail for the first time. I was surprised by what I found in terms of complexity. Good grief. Thankfully, I don’t see a scenario where I will need to remodel this kitchen.
I do love the charcoal filter venting thing-a-ma-bobs and just learned about them with the new house in Colorado.