Strange (Not Strange?) Sink Drainage Problem

[Opifex]

I've been remodeling my kitchen and just this week I finally got the plumbing installed under the sink. I think it is a fairly standard configuration: a double sink with a garbage disposal feeding into a single P trap, and from there down into the sewer line. Our sewer pipes are large cast iron I believe. But after a day of use, the water has started to back up into sink and garbage disposal. We never had this problem with our old faucet. So what has changed? First, the new faucet is about 25% higher flow/volume. Also, it has been about 6 months since we ran any water down that sewer line. I haven't been up on the roof to inspect the vent, but I'm sure it's fine. I snaked the line as far as I could go, maybe 15 feet or so, but the problem persists.

My theories are: (1) something hardened in the sewer line while the sink was not in use for the past 6 months. I could see some iron debris build-up in the sewer drain line under the sink. (2) The sewer line was always partially clogged, but we did not notice it before because our faucet was not as high flow.

Any other ideas?

 

[Opifex]

Here is a picture for context.

 

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[Reach4]

Your theories each make sense. The cure in each case is to get somebody to clean the drain pipes with a bigger, more dangerous machine. I see that you have a cleanout plug, so that should be useful.

Cleaning from the roof could be the choice if there is a vent right above the kitchen sink. Is there a window above the sink?

How much water do you put in before the backup starts? How quickly does the water recede? I presume that the backup level pretty much equalizes in the two bowls if the water rises high enough to have water in the right-hand bowl.

 

[Opifex]

Yes, the plumbing is vented to the roof, not directly above the sink, but a few feet to the right. I saw the vent when I had the drywall open (as I recall, there are two 90 degree turns because there is a window above the sink).

As to your other questions, I thought it strange that when I first disconnected the disposal from the P trap (to isolate the issue), I was able to run the water full blast for a good 30-60 seconds without any water backing up. But after some additional troubleshooting and partial reassembly, it started backing up after only about 10 seconds. The water appears to equalize and recedes fairly quickly.

Here is another view with the drywall removed and directly below in the crawl space:

 

[Reach4]

Interesting. No obvious clog points that I see. You could probably rod that yourself if you can get that cleanout cap open. A vent problem will not cause this to not drain.

I presume you have not seen backup into your tub or shower.

One more thing... if what you see is not really a backup from one bowl to the other, but instead this is only slow to take water into the disposal, it could be that the rubber disposal splash guard is not letting air out. Try pushing in a flap, and if that does the trick, you can trim one or two flaps a bit to let air pass.

 

[Opifex]

Yeah, those sewer lines are pretty big. I did disassemble everything and snaked it through the main sewer entrance (i.e., I mean the drain hole under the sink). The cleanout is snug. I doubt I will be able to get that off without cracking the pipe (or myself). Is there some reason to use the cleanout and not the other hole?

We do not have water backup into the tub or anywhere else.

I don't understand what you mean by "slow to take water into the disposal." I don't see how the disposal could cause the problem because I completely disconnected it from the T and just ran water down the other sink. After about 10 seconds on full flow, the water starts to come out of the T.

 

[Reach4]

I don't understand what you mean by "slow to take water into the disposal."

I meant that the water was not going down the disposal, but water from the non-disposal side was not having a problem. That does not represent your situation.

Yeah, those sewer lines are pretty big. I did disassemble everything and snaked it through the main sewer entrance (i.e., I mean the drain hole under the sink). The cleanout is snug. I doubt I will be able to get that off without cracking the pipe (or myself). Is there some reason to use the cleanout and not the other hole?

Reasons to use the plugged hole:
A little bigger hole. You will will more sure that the snake is going down, rather than up the vent. You can supply water while you are cleaning.

To remove the plug, you would want a big wrench rather than a slip joint pliers. However I cannot guarantee you will not break a pipe.

If you fill one of the bowels half way, how long does it take for the water to recede 2 inches?

 

[bigb56]

How old are those iron pipes? Mine were from 1952 and when I finally changed them about 10 years ago the 2" from the kitchen/laundry was about 75% blocked with only a small opening in the center, the rest was hard packed deposits all the way around. We never could use the laundry stand pipe, had to use the tub and let the washer fill the tub then the tub would drain slowly. Once I changed it all to new ABS everything now drains like a cyclone.

 

[Opifex]

How old are those iron pipes? Mine were from 1952 and when I finally changed them about 10 years ago the 2" from the kitchen/laundry was about 75% blocked with only a small opening in the center, the rest was hard packed deposits all the way around. We never could use the laundry stand pipe, had to use the tub and let the washer fill the tub then the tub would drain slowly. Once I changed it all to new ABS everything now drains like a cyclone.

I'm sure we have something like this going on in our sewer lines. I believe the house was built in the early 70s. When we first bought the house about 6 years ago, we had a sewer scope performed and as I recall there was no problem at the time on the "main" sewer line. But the existing dishwasher did back up shortly after we moved in due to a lot of food debris and whatnot in the drain lines. We installed a new dishwasher and did not have any further draining problem until now. Come to think of it, our new dishwasher would occasionally fill with a small amount of standing water when not in use (which is most of the time), but washing dishes was never a problem. I will have to see if I can put a flexible camera down there and see what I can see.

 

[Opifex]

If you fill one of the bowels half way, how long does it take for the water to recede 2 inches?

I have everything taken apart at the moment. I will let you know once I've had a chance to troubleshoot further. Thanks!

 

[Opifex]

Eureka! As I was inspecting the drain line I noticed some caked on iron debris just where the water enters. I chipped it off with a flathead screwdriver and snaked the sewer drain once more for good measure. Evidently the debris buildup right where the water enters was enough to restrict the flow after a period of time (or maybe it was the snaking?). Anyway the problem appears to be fixed! I hooked everything up and ran the water full blast on both sinks, and the water is not backing up anymore!

 

[Opifex]

Ugh! I spoke too soon. It was draining perfectly all day with constant use, and then started to back up! What could be going on?!

UPDATE: Okay, I got some more intel from the wife, and here's what happened. She washed some coffee grounds into the disposal, noticed the water starting to slowly back up, ran the disposal, and then the other sink backed up. With our old 1/2 HP disposal and plumbing configuration using two P traps, we never had any issue whatsoever with food waste, coffee grounds, etc. getting stuck in the plumbing or causing the other sink to back up. The water level has since cleared and everything is draining fine again.

So, I am now wondering if the nearly level plumbing connection from the disposal to the Tee is the issue, or perhaps the new 1/2 HP disposal (a Moen) is too underpowered for our needs (or possibly overpowered, pumping too much water into the adjacent shared pipe). Thoughts?

 

[Breplum]

Folks, never leave old galv. water or drain pipes in place. ALWAYS demo out and replace to code, which for a kitchen is 2".

The pipe is totally rust-built-up and accumulates scum very fast.​

 

[bigb56]

From your pics it looks like you can get under the house. Start cutting sections of the iron pipe out and swapping in ABS or PVC Do as much at a time as you can. I'll bet you'll be surprised when you see a cross section of that old pipe.

 

[Opifex]

The pics I posted were older. I have replaced 90% of the galvanized water supply lines. The sewer lines have not been upgraded yet. That is a much bigger job than I am prepared to undertake myself.

 

[bigb56]

It's really pretty simple, you can just buy ABS and cut sections of the iron out with a good cutting disc or a Sawzall with a Extreme blade then piece it back in with no hubs. There are chain snaps but I find grinders and Sawzalls to work fine. You can do bits at a time, even if it takes years, and as you move along replace the no hubs with solvent couplings. The worst restrictions are usually right at the fittings.

Once you replace a small section you'll see how easy it is and it will give you confidence! Just copy the iron using ABS, if there are any non compliant fittings or methods, or if you see a better way, change it. For instance, changing a standard vertical 90 to a sweep if space allows.

 

[Opifex]

Simple for you to say, bigb56! Haha, that pipe looks heavy! Is ABS generally preferred to PVC for underground sewer lines?

 

[Reach4]

Simple for you to say, bigb56! Haha, that pipe looks heavy! Is ABS generally preferred to PVC for underground sewer lines?

Your picture is not underground sewer lines. For your drain stuff, you would replace galvanized with plastic. That plastic would be ABS or PVC depending on what is common in your region.

I suspect the cast iron is not the problem.