Plumbing Engineer - Columns: February 2013: Sustainable Design
The Authoritative Source for Plumbing, Hydronics, Fire Protection and PVF
Plumbing Engineer - Columns: February 2013: Sustainable Design

Low-flow fixtures and drainline blockages

By Winston Huff, CPD, LEED AP BD+C

Do low-flow plumbing fixtures contribute to drainline blockages? They can, depending on the flush volume. But, the results of a recent study show that other issues, including toilet paper tensile strength and drainline slope, can also cause blockages in drainlines.

In the last few years, many discussions have surfaced about the role that low-flow plumbing fixtures, particularly water closets, play in drainline blockages. In a properly designed and installed plumbing system, solids stay suspended in the fluid when water flows through horizontal waste piping. However, blockages can occur when solids do not stay suspended, and the concern with low-flow fixtures is the reduced amount of water in the pipe to push through solids. Can plumbing engineers design plumbing systems that reduce this risk of drainline blockage?

In November 2012, the long-awaited report, The Drainline Transport of Solid Waste in Buildings, was released by the Plumbing Efficiency Research Coalition (PERC). To download a copy of the report, visit ASPE, along with the Alliance for Water Efficiency, IAPMO, ICC, Plumbing-Heating-Cooling Contractors Association and Plumbing Manufacturers International are members of PERC and support this research.

What problems are associated with low-flow fixtures?

When low-flow water closets entered the marketplace in the early 1990s, the public and the plumbing industry started complaining about blockages in waste systems. It is important to note that many factors can contribute to blockages. Some important factors include the water closet fixture, the waste piping design and installation downstream from the fixture. What causes the blockage: the fixture design, the piping design or both?

Many complaints about the quality of the early 1.6-gallon-per-flush (gpf) water closets were voiced, and it appeared that many low-flow fixtures on the market were built with little research on the best way to manufacture them. Since then, manufacturers have come a long way, conducting intensive research and developing the fixture to the point where blockages at the fixture are less of a concern. This leads designers to ask the next question: What happens after the waste leaves the fixture and flows into the waste piping?

This question led to the PERC research project, which focused on the flow of solids and liquids in the piping (4-inch diameter) connected to low-flow water closets. The PERC tests looked at blockages in the piping that could cause a fixture to overflow, resulting in potential damage in the immediate area around the fixture and on the floors below in multi-level buildings. More importantly, overflows can cause health problems if untreated waste comes into contact with humans or animals. Also, even after repairs to fix water damage from overflows, future growth of harmful mold is a risk, which can cause indoor air quality issues that can result in respiratory problems.

Another problem occurs when waste products remain in drainlines for a long time and become septic. This untreated waste can create harmful gases with which humans should not come into contact. It also creates methane gas, which is a greenhouse gas. The reduced water associated with low-flow fixtures can affect the ability of waste to move through piping and out of the system. While this is an important issue, it was not covered in the PERC testing.

PERC study results

The PERC study brought about some interesting discoveries. For example, many in the industry think that flowing large amounts of water in a drainline will flush out a blockage. However, when 5 gallons of water were used in the testing, the flush failed to clear the drainline in seven out of 39 tests. Thus, using large amounts of water per flush may not be a reliable method to clear a drainline blockage.

One study, conducted by Caroma Dorf, revealed that the wet tensile strength of toilet paper had an impact on drainline carry distances. High-strength toilet paper resulted in shorter transport distances. Thus, the PERC test used two types of toilet paper. One was a single-ply, low tensile strength paper, and the other was a high tensile strength, double-ply paper.

The lower tensile strength paper proved to flow further down the drainline. When flushed, the low tensile strength paper flowed 135 feet down the drainline. But, in comparison, the high-strength paper flowed only 45 feet down the drainline. As a result, if a building is having problems with drainline blockages, the staff could try using a low tensile strength paper to see if this reduces the amount of blockages before investing large sums of money to disassemble the drainline system. This first step could prove to be an inexpensive way to solve the problem.

As stated earlier, the first generation of low-flow water closets resulted in blockages at the fixture, but newer fixture designs have reduced this problem. What about other fixture designs such as flushometer, tank, pressure assist and siphonic? Do they help or hinder drainline transport? The report found that the type of water closet has minimal effect on drainline transport in long building drains.

The PERC study tested 0.8-gpf fixtures, and these ultra-low-flow fixtures demonstrated a significant lack of consistent performance when compared to 1.6-gpf and 1.28-gpf fixtures. Blockages in the line downstream from the 0.8-gpf fixtures were cleared before an overflow occurred. The report states that increasing the drainline slope when these fixtures are used actually increased the number of blockages in the line. However, the data was not consistent, and further research is recommended for 0.8-gpf fixtures.

The 1.28-gpf and 1.6-gpf fixtures demonstrated similar results, so the report states that no problems should be encountered with the use of 1.28-gpf fixtures when all other factors, including pipe slope and design, are identical to those used with 1.6-gpf fixtures. As a result, these fixtures can be used on new and retrofit applications. Any problems will most likely be the result of errors in the piping system installation and the condition of the system.

For example, a sagging drainline may not have been a problem with 3.5-gpf water closets, but when the fixtures are replaced with low-flow versions, the sagging line could be a problem and cause blockages. In the end, the problem was not the fixtures, the problem was the sagging line. The study showed that long drainlines and toilet paper have a greater effect on blockages than the fixture discharge characteristics.

The results of the PERC testing are very important because government agencies, municipalities, companies and sustainable organizations are recommending, and sometimes requiring, the use of low-flow water closets. This was a concern in the industry because of the lack of available test data about how the fixtures perform. The results of this study should encourage other groups to recommend 1.28-gpf fixtures. It also should encourage designers, contractors and inspectors to look for other problems that cause drainline blockages, such as sagging piping or incorrectly vented piping, early in the installation process. This way changes can be made with minimal effort.

Winston Huff, CPD, LEED AP BD+C, is a project manager, plumbing fire protection designer and sustainable coordinator with Smith Seckman Reed Consulting Engineers in Nashville, Tenn. He serves as an ASPE representative on the ICC Green Construction, Energy and Water Code Development Committee and is on the U.S. Green Building Council’s Water Efficiency Technical Advisory Group. He was the founding editor of Life Support and Biosphere Science and has served as its editor-in-chief. He also is editor of Me Green You Green (, a LEED credit databank.

Plumbing Engineer Twitter
Digital Editions
Timothy Allinson
Designer's Guide
Sam Dannaway
FPE Corner
Ron George
Code Update
Winston Huff
Sustainable Design
Bristol Stickney
Solar Solutions
Joseph Messina
Engineer's Notebook
Max Rohr
Alternative Energy
2014 Sponsors