Does Replacing Toilet Fixtures Save Water?

January 03, 2012
January/February 2012
A version of this article appears in the January/February 2012 issue of Home Energy Magazine.
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Thousands of units of low-income housing were retrofitted for energy and water savings through the American Recovery and Reinvestment Act. Still, multifamily housing remains the poor stepsister of single-family housing when it comes to making homes more water efficient. My company, Koeller and Company, studied the water savings achieved with toilet fixture replacements in an urban apartment complex. Because we feel that a sustainable building should be both water efficient and relatively easy and inexpensive to maintain, we also wanted to find out whether these replacements would reduce maintenance service calls.

The Mendelsohn House apartment project in downtown San Francisco conducted a toilet replacement program in 2010. We used the data that Mendelsohn House collected for the analysis that follows. Mendelsohn House is a Todco Development project located just three blocks south of Market Street. It consists of 189 apartment units, most of them occupied by senior citizens. At the time of construction, in 1988, all units were fitted with gravity-fed toilets using 3.5 gallons per flush (gpf).

Koeller_Fotolia_28129785_MOld inefficient toilets can be replaced with new ones without sacrificing performance and user satisfaction. (©Peter Kurgan,

High-Efficiency Toilets (HETs)

In 1999, the first high-efficiency toilets (HETs) were introduced to the North American marketplace. These were all gravity-fed dual-flush units. HETs are defined as toilets with an effective flush volume at least 20% below the federal maximum of 1.6 gallons; HETs therefore flush at 1.28 gallons or less, on average. In 2001, the first pressure- assist single-flush HETs were field-tested in California by manufacturers and the water utilities. Upon completion of those trials, and subsequent product refinement, the first pressure-assist HETs were introduced into the marketplace in 2003. All of those first units utilized the Sloan Flushmate IV pressure vessel and flushing system.

Since the introduction of the Flushmate-equipped HETs, many toilet manufacturers have refined their bowl designs and improved the hydraulic matching of the Flushmate unit with their fixture bowls. As a result, flush noise levels have been much reduced and pressure-assist fixtures may now be found in residences, hotel guest rooms, and other areas where noise is an important selection factor.

Toilet Replacement Project

In April 2010, Mendelsohn began a toilet replacement project in which the 22-year-old non-efficient gravity-fed fixtures in the building, 192 in all, were replaced with 1 gpf Kohler two-piece pressure-assist fixtures equipped with the Sloan Flushmate IV system, in one of two models: Kohler's K-3519 Highline Pressure Lite, with a K-4304 ADA bowl and a K-4484 tank; and Kohler's K-3531 Wellworth Pressure Lite, with a K-4303 bowl and a K-4484 tank. Both models are EPA WaterSense-certified and -labeled HETs, ensuring top performance and user satisfaction (see "The Real High-Efficiency Toilets Have Arrived," HE Water/Energy: Linking Efficiency Efforts (Special Edition), '07, p. 10). A licensed plumber performed installation of the replacement fixtures. Replacements were begun in April 2010 and completed by May 4, 2010.

Koeller_Figure1Figure 1. Mendelsohn water use declined dramatically after toilet replacement.
Figure 2. Quarterly water use dropped by 1 million gallons.

Study Results

Mendelsohn's site administrator (manager) tracks water use and maintenance service calls on a monthly basis. Using data collected both before and after the toilets were replaced, it was possible to calculate the reductions in both water use and maintenance calls. Mendelsohn has little irrigated landscape, so water use is confined almost entirely to individual apartments, and to heating and cooling systems. Using meter data from utility bills and in-house maintenance records, we identified a downward trend in water use after the toilets were replaced. Maintenance calls related to toilets declined as well. No other significant water efficiency measures were implemented during or immediately after replacement.


Water Use

Figure 1 shows water use by Mendelsohn over the study period. Figure 2 shows quarterly and annual average water use before and after replacement. The reduction amounted to approximately 4 million gallons per year, equal to approximately 56 gallons per day per replaced toilet.

The 1999 AWWA Research Foundation Residential End Use Study determined that the average person flushes a toilet approximately 5.1 times per day. Subjects in this study were residents of single-family homes, where some people work or go to school for part of the day. However, retired people, who spend most or all of their day at home, populate the Mendelsohn complex almost entirely. For this analysis, we therefore assumed an average flush count of 7.1 per day per person. Average occupancy is two persons per dwelling unit.

When these values are multiplied by a savings of 2.5 gallons per flush, (assuming a reduction from 3.5 gallons to 1 gallon), the calculated (or theoretical) savings for a two-person household with one toilet is about 36 gallons per toilet per day.

While many toilet replacement studies have found greater-than-expected savings, the magnitude of the difference between the calculated savings (36 gallons) and the measured savings (56 gallons) strongly suggests that the additional 20 gallons of savings were achieved as a result of other factors. Here are three possible explanations.

First, the replaced gravity-fed fixtures had been in use for approximately 22 years. Over that period of time, flush volume adjustments are typically made by maintenance personnel to correct for reported performance problems. These adjustments sometimes involve replacing fill valves, flappers, or entire flush valves. For this reason, it is likely that the flush volume of the gravity-fed fixtures may have been greater than 3.5 gallons at the time of replacement. The actual flush volumes and the physical condition of the replaced fixtures were not measured in this study. If the flush volume is greater than 3.5 gallons, savings will likewise be greater; if the flush volume is less than 3.5 gallons, savings will likewise be less. In this latter case, flush performance may be sacrificed, and double flushing will waste more water, as explained below.

Second, as other studies involving the replacement of gravity-fed toilets have shown, there are often leaks within the tank. These leaks may occur through the flapper or the overflow tube. In the case of old toilets fitted with ballcock-type fill valves (as opposed to pressure-resistant pilot-type fill valves), changes in water pressure can cause the fill valve to open, releasing water into the tank. This raises the water level above the overflow point, and the water drains through the overflow tube. (This usually occurs in the very early morning, when municipal system pressures are at their highest.) Replacing an aging gravity-fed toilet with a new pressure-assist toilet eliminates these leaks, reducing water use.

Third, frequent double-flushing to clear waste results in excessive water use. Poor flush performance in both aging and new toilets can lead to double-flushing. However, the older fixtures are more prone to poor flush performance, particularly if incorrect replacement parts have been installed in the tank, or the flush volume has been modified, as explained above.

Service Calls for Maintenance and Repairs

Service calls for maintenance also decreased when the gravity-fed toilets were replaced. Mendelsohn residents expect that their toilet will always perform satisfactorily, usually without regard to water consumption. When they are dissatisfied, they will frequently put in a service call, leading to a visit by maintenance staff. The Mendelsohn site administrator also closely tracks service calls for toilets. In the 14 months prior to replacement, there were 80 service calls for the old gravity-fed toilets. In the first 6 months after replacement, there were 5 service calls for the new toilets. In the following 11 months, there were no service calls for the new toilets.

Flush with Success

Any project that replaces aging inefficient toilets with HETs will save water, and the Mendelsohn project was no exception. However, the savings that we found in this study were higher than the savings that would be expected solely from a flush volume reduction of 2.5 gallons. The other factors noted above probably go far to explain the extra 20 gallons per day of achieved savings.

The secondary major benefit of the replacement program was the 93% reduction in service calls. We attribute this reduction to improved flush performance and fixture reliability, and the elimination of frequent repairs.

learn more

Find out more about the Mendelsohn Apartments, located at 737 Folsom Street in San Francisco, by going to

To find out more about the replacement toilets described in this article, as well as other similar high-performance fixtures from other manufacturers, go to At this site, you will find performance data on over 2,500 different toilet models from over 100 brands and manufacturers, including a full listing of all WaterSense-certified models.


Old inefficient toilets can be replaced with new ones without sacrificing performance and user satisfaction. This is particularly true when pre-1992 toilets are replaced with new EPA WaterSense models; in the case of the Mendelsohn project significant savings showed up immediately in the water bills. Regardless of where they are installed, we recommend that all 3.5 gallon and 5.0 gallon toilets be replaced with new high-performance WaterSense models, which are independently tested for flush performance and durability. Rebates or other financial subsidies are often available from local governments and water providers to offset replacement costs.

John Koeller is a registered professional engineer with extensive experience in water-efficient technologies and products. Widely recognized as a water efficiency specialist, Koeller is a consultant to numerous water providers, green-building organizations, and private-sector firms. He is codeveloper of Maximum Performance (MaP) testing for toilet fixtures.

A very special thank you goes to Willie Abasta, site administrator for the Mendelsohn project. Abasta was of immense assistance in furnishing information on the replacement program and its effects, including historical records on water use and maintenance service calls.

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