The Drainage Piping Dilemma
Julius Ballanco, PE
What happens when the 1.6 gallon per flush water closets start flushing too well? That is what's happening. As everyone has cursed out the 1.6 gpf water closets, the manufacturers have been diligently going about their business improving the flushing performance. So much so that the 1.6 gpf water closets far outperform the old 3.5 models.
This has resulted in some other problems. This newer breed of super flushers are outperforming the drainage system. Unfortunately, plumbing contractors are not keeping up. Additionally, in existing systems, this is a real problem when water closets are replaced.
Before you think I have gone crazy, or am off my rocker, let me explain. One of the big rallying cries for the past 10 years is that people don't crap polypropylene balls. The standard regulating water closet performance includes a test whereby 100 polypropylene balls are flushed. The purists wanted the real thing used so we know the 1.6 gpf water closet will really work. Unfortunately, the real stuff is difficult to use in laboratory testing.
The manufacturers started testing with all sorts of other media. They have finally settled on a bulk media test. Simply stated, the test requires the bowl to be filled with a bunch of junk, paper, sponges, etc. Then, the bowl must remove this bulk media.
While this test is not currently required, every manufacturer is testing to the new protocol. They don't want to be caught short when the test is added to the standard. You may have noticed that, in the past few years, the 1.6 gpf water closets have changed in the way they flush. It's as if the solids are sucked out of the bowl. You don't see any developing siphon action, just an instant sucking of the contents.
I have had the opportunity to measure this sucking action with a manometer. It is not unusual for the suction to reach 5 to 7 inches of a water column. You read that right. I am sure some bowls do better than that, it is just that they are off the scale of my makeshift, field-use, water manometer.
Most of you recognize the pressure differential of plus or minus 1 inch of a water column. This is the pressure the plumbing code stipulates as a trap seal. In a drainage stack you can have pressure differentials in the 5- to 7-inch water column range, but not in the branches. The horizontal branches rarely develop high pressure differentials.
All of our pipe sizing (including vents), fittings and drainage piping arrangements are based on the historical data on pressure differentials. They are also based on the flushing performance of 5 gpf water closets.
Code ConfusionThe first appearance of a problem showed up in back-to-back water closet installations. There were two problems identified: one was that the water closets were interfering with one another during the flush, the other was that the flushing of one was siphoning the other water closet. The main problem was the use of double sanitary tee fittings when connecting back-to-back water closets.
While code writers were skeptical, the codes were changed to address the back-to-back water closet installation problem. As it turns out, the real problem was that one water closet flushing was siphoning the other water closet because of the superior flushing performance of the bowl. The double sanitary tee fitting did not provide the necessary venting arrangement to prevent the siphoning of the back-to-back water closet.
Unfortunately, the plumbing codes are not in agreement with the proper connection for a back-to-back water closet. The International Plumbing Code, International Residential Code, National Standard Plumbing Code, and Uniform Plumbing Code all permit back-to-back water closets to connect to a double combination or a double tee wye fitting. It is with the use of the double sanitary tee fitting that the codes differ.
The International Plumbing Code prohibits the use of the double sanitary tee fitting for connecting back-to-back water closets. The International Residential Code stipulates a distance of 30 inches between the water closet outlet and the fitting. Well, that just precluded the use of the fitting for back-to-back installation since, even with a 14-inch rough-in, you are too close to the fitting.
The National Standard Code says don't use the fitting when there is a problem with the water closets. Finally, the Uniform Plumbing Code says only use the fitting when the run of the fitting is 4 inches and the branches are 3 inches.
Another problem came about when using plastic pipe. The plastic industry has three possible fittings for back-to-back installation -- the double sanitary tee fitting, the double tee wye and the double fixture fittings. Everyone understands the first two fittings, but how do you classify a double fixture fitting?
This fitting has a little more pattern than a sanitary tee, but nowhere near what a combination (tee wye) has. I have always classified a double fixture fitting as a sanitary tee pattern. Based on the plumbing codes, this is the only way you could consider this fitting.
As a result, a double fixture fitting would not be permitted to be used on back-to-back water closet installations. All of the plumbing codes would restrict this fitting, just like the double sanitary tee fitting. This does not go over well with plumbing contractors.
It is well known that a double tee wye doesn't fit in a typical ceiling space. One of the suggestions I would offer is the installation of a double wye fitting to connect back-to-back water closets. Rather than running the drain perpendicular to the wall, offset the piping into a wye fitting. This installation certainly changes the way we pipe a drainage system. The vent can be a circuit vent arrangement downstream of the last double wye. Otherwise, get the builder to provide a soffit to allow the installation of a double tee wye.
More Tests NeededIf you continue to use a double sanitary tee or double fixture fitting, don't complain when the water closets start losing their trap seal. It's not the water closet's fault, it's the drainage piping arrangement's fault.
As for existing installations, this is a big problem. Some water closet manufacturers suggest reducing the performance of the water closet. Isn't that a switch? Rather than finetuning the water closet to flush at peak performance, they recommend you reduce the amount of water in the tank to lower the flushing performance. While that is a quick fix, I really don't like it. I want any water closet to be functioning at peak performance.
It looks like we need some additional testing in the laboratories to see what other options are available for installing the drainage piping for the super flushers. Perhaps a new fitting is in order for back-to-back water closet installations. I hope someone will take up the charge and start a testing program. In the meantime, stop installing back-to-back water closets with double sanitary tees and double fixture fittings.