The commercial restroom has been at the center of an ongoing debate for the better part of the last century.

Through increased innovation and product improvements, architects, engineers and installers are becoming increasingly educated on the merits of plumbing technology, enabling the plumbing community to draw an added number of comparisons between piston valve and diaphragm valve technology to determine which system is superior.

Yet, in reality, one technology is not inherently “better” than the other. The determining factor for picking the correct flushometer technology ultimately comes down to operating conditions and specific applications.

The diaphragm flushometer was invented in 1906 by Sloan, which then created piston technology 20 years later. Diaphragms are the dominant commercial restroom technology in contrast to pistons. They are estimated to account for 75% or more of all flushometers sold in the U.S. However, piston technology is the preferred technology in a number of markets and case-by-case applications. The more information that can be gathered will ultimately ensure successful valve selection and operation.


Flushometer functionality

Before exploring how to select the right type of flushometer technology, it also is beneficial to understand how each system operates. The theory of operation in each is fundamentally the same. Each technology has an upper control chamber and a lower supply chamber connected by a bypass.

The bypass hydraulically connects the upper control chamber and lower supply chamber in both the diaphragm and piston valves. This bypass is a small hole or orifice that is no larger than a pin hole, typically measuring approximately 0.02 in. in diameter. A flexible rubber disk separates the upper from the lower chambers in a diaphragm flushometer, while a molded cup separates the upper from the lower chambers in a piston flushometer.


Valve operation options

When selecting and installing any flushometer, it’s imperative that the valve model matches the pressure and water volume requirements of the plumbing fixture. The key to successful valve operation is matching the correct flushometer to your installation and operating environment.

When debating whether a diaphragm or piston is best for your next commercial restroom project, there are three main factors to consider: restroom traffic, water quality and operating conditions.


Restroom traffic tips

In high traffic environments such as airports and sports stadiums, diaphragm technology (all static seals) is the preferred solution, as its static sealing mechanism proves more durable when compared to the sliding dynamic seal in pistons, which can deteriorate under conditions with heavy traffic.

Take O’Hare International Airport in Chicago for instance, where nearly 75 million passengers passed through in 2016. By utilizing diaphragm technology for wall-hung water closets and for standard high-efficiency urinals, the airport’s restrooms are able to accommodate the hundreds of thousands of people passing through each day.

Meanwhile, in low traffic situations, both diaphragm and piston technologies are practical solutions, offering similar water-saving solutions with nearly identical lifespans. Ultimately, if the determining factor was simply heavy or light traffic, diaphragms would be a universal solution. However, additional environmental factors such as water quality and operating conditions play a role as well.


Water quality counts

Poor water quality often compromises performance and reliability, whether it be solid particulate matter, hard water (dissolved salts) or aggressive treatment from your water source.

These factors bring wear and tear to the physical material of the flushometer and both diaphragm and piston technology will be affected over time. This tends to be where diaphragm technology is often the preferred solution over pistons. Not only are diaphragms superior to pistons in water conditions with a high level of particulates (due to the sliding dynamic lip seal in pistons) that can deteriorate in a piston, but diaphragms hold up better in water conditions with a high level of chloramines as well thanks to thick diaphragm sections.

UA Local 130 Training Center in Chicago installed diaphragm flushometers because of the water conditions. The union plumbing school uses reclaimed water and treated rainwater in the facility, making diaphragm flushometers — especially ones designed specifically for the harsher conditions of reclaimed water — the ideal choice.

Piston deterioration is influenced by the sealing material composition and the operating water supply conditions. As the main piston lip seal deteriorates from constant abrasion, bowl-flushing performance slowly decreases as the lip seal becomes compromised and develops small leaks while also causing a tendency for the piston to shut off quicker. In addition to delivering a short flush — which can impact bowl performance — this fast shutting can cause a banging or hammer when the valve closes. However, the flexing action of the captive diaphragm does not exhibit this same quick shut off as the diaphragm wears. When determining if a flushometer requires maintenance, frequent indicators of deterioration include leaking or a flush cycle that is either too long or too short.

Most people don’t really know what the water quality or water supply conditions are in their building. They assume the system has been designed properly, but don’t truly know. Due to the fact that diaphragm valves are more accommodating than piston valves and can better handle changes and fluctuations in a water supply system, they are often the better choice when a universal product is needed.


Answering operating condition questions

Harsh operating environments can cause a flushometer that is not engineered to perform at those conditions to malfunction or operate at low efficiency and, in turn, waste water. That’s why it’s of the utmost importance to select the proper flushometer technology for your commercial restroom.

In low-pressure and weak-system situations where flow tends to be at a fixture-required minimum or periodically below the minimum fixture requirements, piston technology is the preferred solution. In high-rise buildings where the water tank is often situated on the roof — a common practice in international markets — piston technology is a practical solution on floors with low pressure. Conversely, diaphragms are the better choice in high-pressure situations where a system could have a very high static pressure condition, but not recover quickly after a flushometer initiates a cycle with the system struggling to recover to the initial high gpm (weak system capacity).

While pistons often have a slight performance advantage in low- pressure situations, today’s ultra-high-efficiency water closets typically require higher minimum operating pressures than their higher flushing predecessors. These minimum bowl pressure requirements are typically well within the parameters where the flow from a diaphragm valve is as sufficient as the flow from a piston valve.

Newport News Public Schools in Newport News, Va., installed piston flushometers throughout the school system because of lower water pressure conditions. During the massive renovation, designed for improved water efficiency, more than 1,400 new piston flushometers were installed.

Despite the preconceived notions of each technology — as well as when and where each should be utilized — one factor that remains consistent is that each system is highly engineered to meet water conservation goals. With high-efficiency toilets producing an average flush volume of 1.28 gpf, piston and diaphragm technologies are putting the commercial restroom on the front lines of water conservation. But in order to effectively fight the water conservation — and in some instances water scarcity — battle, it’s important to understand which flushometer technology to specify when planning your next commercial restroom project.

Diaphragm and piston flushometers can each bring plenty of value in their own right, but it’s up to the engineers and contractors to put these innovative technologies in the right environments to succeed.