Bypassing P-traps?

I have a love-hate relationship with indoor plumbing.  Pressurized water on demand is awful nice and all, but getting the stuff out gives me headaches.

A couple bits of background info.  One, we eat a rather high-fat diet: plenty of butter, lard, duck and goose fat, olive oil, and whole milk.  Oh, and I kind of like ice cream.  This is great for us health-wise (excepting, perhaps, the ice cream), but it seems to cause our pipes a fair share of consternation.  Two, our only outgoing plumbing is gray water from the kitchen and bathroom sinks, tub/shower, and washing machine.  There is no toilet sewage.  The gray water is conveyed through a large PVC pipe out the side of the hill east of our house.  Three, our kitchen sink is chronically slow draining.  Usually it's workable, with the need for occasional plunging, treatment with baking soda and vinegar, washing with boiling water, and, about a year and a half ago, a desperation call to a local plumber who plunged quite vigorously and finally dislodged the offending obstruction with a rather beefy drain snake.

Anyway, a couple days ago we seem to have hit a wall of sorts.  The sink clogged up and just would not drain.  So after two days of my patient wife washing dishes in the bathtub, I finally prioritized dealing with the sink.  I took apart the P-trap, drained it, then ran a drain snake into the pipe.  I got it in about 12 feet or so until I could go no longer--whether I hit a clog or a bend I don't know.

So I put the bits back together and started running water again.  After only half a gallon, give or take, the sink was backing up on both sides.  Now, I don't know much at all about plumbing, but this just didn't seem right.  There is clearly no way the water hit the clog and then started backing up, since a scant half gallon had gone through.

So my question is twofold:

One, what is going on here?  How is the sink clogging so quickly?  Is it an airflow/venting problem?

Two, would bypassing the P-trap do us any good?  The purpose of the trap is to prevent sewage gasses from coming back up through the drains, which seems no concern given our particular situation.

As a last resort, we figure we may as well disassemble the bits under the sink and just collect the runoff into a 5-gallon bucket to be dumped as needed, but bypassing the P-trap, if it would work, would be far preferable.  It doesn't make sense to me how that would make a difference, but I may as well ask, right?

If your drain pipe is 1.25 inches in diameter, and 12 feet long, then it would hold 3/4 gallon of water.

Joseph Lofthouse wrote:
If your drain pipe is 1.25 inches in diameter, and 12 feet long, then it would hold 3/4 gallon of water.

The water didn't get near that far down, based on how much was backed up into both sides of the sink.  I don't believe the half gallon even got through the P-trap.

Wes Hunter wrote:The water didn't get near that far down, based on how much was backed up into both sides of the sink.  I don't believe the half gallon even got through the P-trap.

Then seems like the pipe is completely clogged so that not even air can get through it, and the p-trap is acting like a piston... Removing the p-trap wouldn't make a difference until the blockage is removed.

With ample plunging I'm able to force the water down the drain.  Am I possibly, then, just piling water on top of an existing clog?

I haven't looked around the crawlspace lately, so I don't know the exact layout, but I'd guess there is upwards of 30 feet of 1" pipe before it meets the 3" pipe that runs out down the hill.  If it were completely blocked, wouldn't the remainder of the pipe fill up really quickly?

Hey Wes I might be able to help. I've done my fair share of plumbing while remodeling houses, but I will say that I am not a plumber. So 30 feet of 1 inch is really an excessive length to run 1 inch, and that would not pass code here in Tennessee. I bet if you cut that 30 feet of 1 inch to remove it, you'll find it full of grease. 1 inch is only used right under the sink in the cabinet, where it usually meets 2 inch stubbed out at the wall/floor. I recommend removing that 1 inch and replacing it with 2 inch, you'll thank yourself years down the road. This is easy to do, and I know you can do it in less than a  day with a helper. Glueing together PVC is easy, and I know you can do this successfully. If this is something you want to do, I will gladly provide instructions on what to buy and walk you through putting it together. While P-traps are often the first location of a clog, they're there to prevent foul odors from wafting up in your sink and stinking up the house, and bypassing them is really not a good idea.

I've been thinking about this while making breakfast, Wes are you sure that's 1 inch PVC and not 1-1/4 or 1-1/2? Reason I ask is standard basin drain fittings aren't manufactured to fit 1 inch.

If it is actually 1" pipe, that is part of your problem.  Kitchen sink drains are normally 1.5" to help avoid clogging from grease and fat.

If it is in fact 1" pipe, I'd start by replacing it all with 1.5", which will also remove the clog.

Then to avoid getting clogs in the future, STOP rinsing grease and fat down the sink.  Not only will that clog up your pipes again, but it will also clog up the soil where ever the grey water ends up.  If you have a garbage disposal, stop using it.
Pour the oil and fat off into a can/bucket, scrape the food/grease/etc. off the plates before you wash them.  Use a good biodegradable soap to help emulsify any remaining fat/grease so it doesn't stick to the pipes.

As for your current clog, it sounds like the drain is clogged before it gets to the vent pipe. so the P-trap is creating an airlock.  
If the drain is really 1.5" instead of 1", you can temporarily remove the P-trap so that you can try getting hot water to the clog to help dissolve it, but if it's 12 feet away the water will likely cool before it gets to the clog.
You could try using a drain snake with a spring type screw on the end and see if you can hook into the clog and pull some parts of it back out.  Keep doing that until the snake get's through the clog and then use a wider bit to bore out the clog.
If your drain pipe runs through your crawl space, you might try inspecting it from below.  Try measuring out 12 feet and see if there is an elbow or Tee, that might be where you clog is.

Do NOT permanently remove the P-trap.  It's purpose is to keep sewer smells and insects, etc. from entering the house through the drains.  Even grey water smells.

One final thought, if you are having chronic problems with clogs, sometimes the best solution is to simply replace all of the drain pipe...and then stop flushing oil&grease down the drain.

Yes, my mistake James.  I'm not looking at it at the moment, but it's not 1".  From memory, I'd guess the piece that goes through the floor is 2" OD, roughly.

We don't wash oil/grease down the drain in large quantities.  What goes down is the residual left on plates/bowls/silverware after their contents were scraped into the pig bucket.  My guess is that, though it goes down in small quantities, the fact that we cook and eat every meal for 5 people at home, plus our high-fat diet, results in a relatively large amount of fat and grease going down the drain over time.

In that case, what are some practical solutions?  Wiping every dish dry with a paper towel seems excessive, though I suppose those would make decent fire starters.  Or should I replumb the kitchen line with larger-diameter pipe for extra insurance?  Or perhaps a more regular schedule of rinsing the pipes with vinegar or boiling water or whatnot?

Peter VanDerWal wrote:As for your current clog, it sounds like the drain is clogged before it gets to the vent pipe. so the P-trap is creating an airlock.

For what it's worth, the P-trap leads into the drain pipe that goes through the floor.  From that same junction, the air vent goes upward so that the actual vent itself is nestled as high in the underside of the sink as it can be, between the two basins.  The level of the air vent is higher than the level of the sink drains.  The understanding I got from the guy who did the work is that this isn't ideal, but short of a complete overhaul it was the best option, and it did improve the sink's draining for a little while.

Another point of not removing the P-trap, even if you only have clean graywater going out to daylight, is that in the winter you would get cold air blowing up the sink drain. I experience this with my laundry room floor drain. It has a "trap" configuration built in, but since there is usually no water in it, there is no obstacle to cold air.

Grease buildup in pipes is almost inevitable, happening one micro layer at a time. Since it sounds like you have a more than minor clog, since you tried to snake it and hit a road block, one solution is to go buy a couple PVC couplings, and cut away that drain pipe, mechanically remove the blockage, then using those couplings glue it all back together.

Wes Hunter wrote:
For what it's worth, the P-trap leads into the drain pipe that goes through the floor.  From that same junction, the air vent goes upward so that the actual vent itself is nestled as high in the underside of the sink as it can be, between the two basins.  The level of the air vent is higher than the level of the sink drains.  The understanding I got from the guy who did the work is that this isn't ideal, but short of a complete overhaul it was the best option, and it did improve the sink's draining for a little while.

Is this vent pipe open at the top? There ought to be a studor valve on top. An open top won't affect it's function as a vent, the studor valves keep foul odors for coming in, but if bad smells have not been an issue I wouldn't worry about it.

Tracking down the problem. I see a couple of potential issues. It will require removing the ptrap once again.
Remove the ptrap and funnel water into the vertical pipe. If it Flows freely the problem is either a clog between the ptrap and sink or in the jury rigged vent pipe. If the main drain ever had a clog in the past the water would have backed up into the ptrap but also up the vent pipe as well. Grease is lighter than water so it would also get deposited in the vent as much as the ptrap. Try snaking the vent pipe as well as poking a hanger or something in the short pipe between the ptrap and the sink.

Also check that your main drain slopes at least an 1/8" per foot but ideally 1/4" per foot. Also check that the main drain has no sags along its length. It does not need to sag much to have standing water with eventual grease building up there also. My bet is a sag or level main drain or clogged vent and more than likely the vent is clogged.

Since this is gray water there is really no need for a ptrap because you will never get sewer gases coming in......only critters!

Wes Hunter wrote:

Peter VanDerWal wrote:As for your current clog, it sounds like the drain is clogged before it gets to the vent pipe. so the P-trap is creating an airlock.

For what it's worth, the P-trap leads into the drain pipe that goes through the floor.  From that same junction, the air vent goes upward so that the actual vent itself is nestled as high in the underside of the sink as it can be, between the two basins.  The level of the air vent is higher than the level of the sink drains.  The understanding I got from the guy who did the work is that this isn't ideal, but short of a complete overhaul it was the best option, and it did improve the sink's draining for a little while.

Ok I'm familiar with that, forget what it's called, but it's basically a one way air valve, it lets air in, but not out.  It's designed to let air in when the sink drains so the moving water doesn't create suction and pull the last bit of water out of the p-trap.
Anyway, same thing, because it doesn't let air out (only in) it forms an airlock with the p-trap.

As long as the drain pipe doesn't have any sharp bends in it, you might be able to scrape most of the gunk off with a power auger and the right end, something like the end shown in the bottom right side of this picture:


If there are any tool rental places near you, they might rent augers.

It definitely sounds like you’re dealing with more than just a simple clog here, especially given that barely half a gallon of water is causing a full backup in both sink basins. That suggests the obstruction might actually be in the trap itself or immediately downstream of it, rather than 12 feet down the line. A partial clog combined with heavy buildup from fats and oils (which tend to congeal in cooler pipes) can make even small amounts of water behave unpredictably.

To your question about bypassing the P-trap, technically yes, it would allow water to flow more freely if the trap is the restriction point. But it’s not a long-term fix, and even though you’re only dealing with gray water, omitting the trap could still invite odors, insects, and bacteria back into your kitchen over time. In setups like yours, especially in rural or off-grid homes, some folks do opt to bypass or modify traps, but they usually add some kind of waterless trap alternative or venting system to avoid the smell and hygiene concerns.

That said, your issue might not be with the trap or the clog alone. It could be an airflow problem, as you suspected. Without proper venting, water can’t move efficiently through the system and will push back up, even if the pipe isn’t fully blocked. A blocked or nonexistent vent can cause symptoms just like a clog.

If you’re not seeing results with a snake or boiling water, and you’ve already cleared the P-trap manually, it might be time to have a professional evaluate the slope of the pipe and check for hidden buildup or venting issues. If you’re in Singapore or nearby, you could click here to get in touch with licensed professionals who specialize in these kinds of drainage problems. Sometimes a camera inspection is the best next step, it saves a lot of guesswork.

Hope you find a clean solution soon. No one wants to be washing dishes in the bathtub for long!

Another reason to keep a trap installed: if something small like a ring gets dropped in the sink, getting it back from the trap is better than taking apart greasy pipes in the crawl space.

Can I ask, why the hi fat diet?
Conventional wisdom is that such a diet can cause other problems for you.