The drains we install could be affecting water closet performance.

Mention the acronym “HET” in a room full of older plumbing contractors and stand back to see the reaction. Many will grumble about another water closet fiasco just like the mandate of 1.6 gallon per flush water closets in the early 1990s.

HET, meaning high-efficiency toilets, always evokes an emotional response. But, before you get too excited, you need to realize that the manufacturers are not going to repeat the mistakes made when the federal government mandated 1.6 gallon per flush water closets. They are smarter and better at designing water closets.

There is no denying that up until the mandate of 1.6-gpf water closets, very little had changed in the flushing of the fixture. It seemed that the manufacturers just kept reducing the amount of water used to flush until they reached a point where the water closets didn’t flush anymore.

Engineering took over and newer designs started to appear. As they greatly improved the flush of the water closets, they realized they didn’t need 1.6 gallons of water. They could use much less water. Thus, the beginning of a 1.28-gallon-per-flush water closet or HET.

One of the important factors that the manufacturers kept in place were the performance tests for the water closets. There hasn’t been a lowering of the flushing performance. An HET must perform at the same level of efficiency as a 1.6 gallon per flush fixture.

Another fact to consider is that the manufacturers are voluntarily converting to HETs. They do not want to see an overnight mandate. They prefer to have the industry switch because the fixtures are more water-conserving, as well as efficient in their flushing performance.

So what is the downside? Where will we see headaches?

It's In The Drain

I recall a discussion I had with one manufacturer’s engineer who said it is their responsibility to get it out of the bowl and our responsibility to get it out the pipe to the public sewer. The “our” meaning the remainder of the plumbing profession.

Of course, this is not the general opinion of the manufacturers. They have an interest in the piping design so that they are not blamed for a lousy water closet.

What needs to occur is a major change in our thought process. We, as an industry, have always had a love affair with 4-inch pipe. If you think about it, 4-inch drains have been the mainstay of the industry. What is often forgotten is that, on long runs of 4-inch pipe, no matter what water closet is installed, when there is a single flush, not all of the solids make it out on the first flush.

Everyone tries to install the best flushing water closet. However, after 80 feet of run in a 4-inch drain, every water closet is the same. The flow converts to gravity following the precepts of the Manning equation. With 1.6 or 1.28 gallons of water, the velocity and depth of flow is not adequate in a 4-inch drain.

What this means is that a 3-inch drain should become the modern 4-inch drain. In other words, 3-inch drains should become the mainstay. The same single flush in a 3-inch drain will keep the solids in suspension until it reaches the public sewer.

Part of the problem is the plumbing codes. Some codes still limit the number of water closets that are permitted to connect to a 3-inch drain. Of course, this is silly. There really is no justification for placing an arbitrary limitation on the number of fixtures that can discharge. The ICC International Plumbing Code has no limitation. You can connect as many as 13 water closets to a 3-inch drain. So, why does it work under one code and not the other?

Down To A Tee

Another piping problem that has occurred with HETs is the use of 3-inch double sanitary tees to connect back-to-back water closets. Just about every plumbing code prohibits this installation. I recall watching one installation where such a fitting was installed. When you flushed the water closet in the men’s room, the water in the bowl of the water closet in the ladies’ room was sucked dry. This is a result of the super-flushing performance of the fixture. (By the way, the fixtures were properly vented. It was simply that the bowl was creating a negative pressure of 7 inches of a water column.)

Unfortunately, there is no way to predict when the use of a 3-inch double sanitary tee or double fixture fitting will create a problem. But, it can occur.

The other change in mindset is the pressure of the water supply. For years, tank-type water closets were listed as requiring a water pressure of 8 psi. With HETs, the pressure demand jumps to 20 psi (or possibly greater, depending on the style of water closet). In most cases, 20 psi is available. If there is a shower in the bathroom, 20 psi is already required for the shower valve.

While you may think that less water means lower pressure, the opposite is true. Less water means more pressure demand by the fixture.

It is not possible to change the piping behind the walls and floors in existing installations. But, for new installations, we have the opportunity to get the piping installation right. Remember: smaller drains and larger water lines.

When it comes to single-family dwellings, there is no reason for not switching to 3 inches as the largest size drain for the entire system, including the underground. I don’t care how large a McMansion is built, a 3-inch building drain and building sewer will do the job. Plus, it will do it better than a 4-inch drain.

So if you have a problem with HETs, don’t curse out the manufacturers, curse out yourself for being unwilling to install the optimum size drain. Think 3 inches!