As just about everyone in the profession knows, HET stands for “high-efficiency toilets” - a term I still don’t like. Today’s HETs are flushing with a maximum of 1.28 gallons per flush. Why such a strange value? Why not round the number up to 1.3 gallons per flush? Simply because 1.28 gallons equates to a 20 percent savings over a 1.6 gallon per flush water closet. If we continue with the 20 percent savings, the next generation would be the 1.024 gallon per flush water closet. You get the idea.
While the HETs flush with a lower volume of water, they do maintain a requirement to comply with the A112.19.2 standard. This standard still has a requirement for a drainline carry test.
Many years ago, there was an attempt to do away with this test. Opposition from ASPE was the main reason why this test remains in the standard today.
The drainline carry test is good for evaluating the initial flush performance. However, the drainline carry test is only 60 feet long, with performance required for 40 feet. That translates to fixtures only helping to transport the solid waste so far in a drain.
What happens when the horizontal drain is longer than 40 or 60 feet? That is the question that many across the world are starting to ask. There are cries for additional research of the performance of water closets. However, the research of the various styles of HETs is somewhat useless.
More Research
If you go back to some of the original research by Dr. Roy B. Hunter, you will find that water closets provide an initial surge in the drain when they are flushed. Hunter chose to ignore that surge when calculating drainage pipe sizes. The reason being is that the surge helps the flow. If you ignore the occurrence, the actual flow in the drain will be better than the calculated flow.Remember that Hunter was studying water closets that flush between 5 and 7 gallons per flush. Hence, his primary concern was undersizing a drain, not oversizing it.
The more recent studies have shown that the surge from a water closet, including HETs, is only good for 60 to 80 feet. After the waste travels approximately 80 feet, it doesn’t matter what water closet is flushed. What matters is how much water and solids are in the drain, the density of the solids, and the size and pitch of the pipe.
This research has actually been done and placed in a computer modeling program. What the program does not include is the lower flows from an HET. However, that is rather easy to do with the modeling program. It merely takes some time to modify the water closet characteristics.
At the current time, ASPE is addressing this modification to develop new values for solid waste transport. As I write this column, that work has not yet been completed. Hence, I cannot provide you with definitive numbers. However, there are some factors that are known.
Keeping The Flow
As I have previously written, 4-inch drains are rough on HETs. Even with a pitch of 1/4 inch per foot, the solids will drop out of suspension on long runs of pipe if the only fixture operating is the HET. So, if you have a distance of 200 feet horizontally to the sewer, the solids will not make it to the sewer on the first flush with an HET. Again, this is for a 4-inch building sewer.If you add the flow of other fixtures, the solids will flow much better and possibly make it to the public sewer. Pitch the 4-inch pipe more, and the flow of the solids in suspension is much better.
There are still some people who believe that if you pitch the pipe too much, the liquids will run away from the solids. For the umpteenth time, that’s a myth! There is no such thing. The more you pitch, the better the solids will run.
A 3-inch pipe will always be a better selection for an HET. Although there is a question of whether 1.28 gallons is enough water for a long run of 3-inch pipe pitched 1/4 inch per foot, I cannot answer it. This is what the ASPE research will answer.
Is it possible that with 1.28 gallons of water, a 3-inch drain may still have the solids drop out of suspension on a long run? Sure, it is possible. That needs to be determined. There has to be a certain amount of water to move the solids through the pipe. If you get too low a value, the solids stop running. We just don’t know what the number is at the current time.
What does this mean regarding the installation of HETs? As a friend of mine says, keep your horizontal runs short and combine multiple fixtures as soon as you can. This may sound strange, but that is really what we tend to do.
Most designs have vertical drops within a few feet of the fixture. Also, the fixtures are all located together. Most of the time, when a water closet is flushed, the lavatory is used shortly thereafter. The flow from the lavatory helps the flow from the water closet. Add any other fixture, and they help keep the solids in suspension by adding waste to the drain.
When the solids do drop out of suspension, the heavy flow from other fixtures often picks up the solids and continues to transport them down the drain. Just because solids drop out of suspension doesn’t mean there will be a stoppage in the drain. The movement of the solids is very dependent on the flow of all of the fixtures connected to the drainage system.
As we continue to reduce the amount of water flowing down the drain, we need to be cognizant of the design. Our future concern will be oversizing the drainage piping, not undersizing.
