Water Closet Testing

A ToolBase TechNote

Background

In the interest of fostering water conservation initiatives in their respective water service areas, Seattle Public Utilities, Seattle, Wash., and the East Bay Municipal Utility District, Oakland, Calif., engaged the NAHB Research Center, Inc., to undertake a series of performance tests and evaluations on a variety of residential and commercial toilet fixtures.

One of the objectives of the study was to develop information on flush performance, water savings reliability, and physical characteristics that will assist the consumer in evaluating products and making purchase choices. This TechNote presents a synopsis of the flush performance results.

A variety of commonly available toilets and newly developed fixtures were tested to determine flushing performance, flush volume, trap diameter, water spot area, and other characteristics. The majority of tested fixtures were designed to flush at the standard 1.6 gallons per flush (gpf).

The models tested in this study were, to the extent possible, obtained from retail outlets or commercial plumbing distributors. To evaluate any differences from unit to unit of like models, two units of each model were obtained and, when possible, from different sources. The testing included nominal 1.6 gpf gravity, pressure-assist and vacuum-assist models, as well as a few special models such as dual-flush, flapperless, and air-assist units.

Results

The graph below shows the distribution of how the toilets performed in the test. A low Flush Performance Index value indicates better flushing performance.

Several complaint toilets, removed from residential dwellings because of performance problems, were tested to gain a benchmark as to what index value might be expected from a toilet that performed poorly in actual use. The complaint toilets test yielded flush performance indexes that ranged from 32 to 43.

The majority of the models tested performed much better than the complaint units. The following models had an average flush performance of index 10 or lower (better).

Determining a threshold for satisfactory performance was beyond the scope of the study. However, an examination of the data suggested 10 as reasonable criteria for selecting the better performing toilets. Any threshold of acceptable performance is likely to vary from consumer to consumer.

Additional Information

The flush performance testing utilized a series of floating or sinking sponges as well as paper wads to simulate the waste loading in the toilet. Five different loading levels were used ranging from a substantial challenge to a minimal load. Each toilet was flushed a number of times at each loading level. After each flush the number of test media remaining in the bowl was counted. An average was determined for each level and summed to calculate the Flush Performance Index. The reader is also encouraged to consider the obvious differences between this testing with sponges compared to human waste. The test media differ in size, uniformity, consistency, surface characteristics, and amount and type of paper compared to actual toilet usage. All of these variables affect the flushing performance in actual use.

By the very nature of their use, there is a significant degree of randomness and chaos associated with toilet loading and flushing in actual use. There is additional variability associated with the test method in preparing, loading, and flushing. A number of factors could influence the results of the testing including placement/arrangement of the media, level of media saturation, and the rate at which and length of time the fixture's flush handle is depressed. These variations are also common in actual fixture usage.

Replicate testing on 8 units of the listed models showed average differences in the flush performance index of about 3. A complete error and uncertainty analysis was beyond the scope of this study. Some models showed significant differences between the two units of the same model. Since only two units of each model were tested, these results should not be considered as fully representative of the typical or average production of the models tested.

Considering the above comments, the flush performance index may be used as a general indication of the expected performance, but cannot be used to precisely predict performance of one model compared to another in a particular individual's home.

Because flapper valves typically require replacement several times during the useful life of a toilet fixture and the likelihood that the consumer will install a generic replacement flapper, the flush volume of the tested units was also measured after a generic replacement flapper was installed (not all models would accept the generic replacement). A number of models showed significant increases in gallons per flush with the generic flapper. Consumers will need to be conscious of obtaining appropriate replacement parts in order to maintain the water conserving benefits of the low flow toilet.

Disclaimers

Neither the author, reviewers, nor Seattle Public Utilities, the East Bay Municipal Utility District, and the NAHB Research Center nor their employees make any warranty, guarantee, or representation, expressed or implied, with respect to the accuracy, truth, effectiveness, or usefulness of any information, method, or material in this document, nor assume any liability for the use of any information, methods, or material disclosed herein, or for any damages arising from such use. Readers use this report at their own risk.

Neither Seattle Public Utilities, the East Bay Municipal Utility District, nor the NAHB Research Center or any of its accrediting agencies endorse products or manufacturers. Trade or manufacturers' names appear herein not as an endorsement but solely because they are considered important to the object of this project.