Posted on March 10 2022
Piston or diaphragm? If you are having constant issues with your flushometers, or they are wearing much faster than they should, then you should read this article and figure out if the flush valve type you have installed is the best type for your unique situation. This is also a great read for anyone deciding which flush valve they should use for an initial installation. Read on to learn the history of diaphragm and piston valves, how they work, their differences and similarities, what a flush curve is and why it matters, which valve performs best in certain conditions and why, and some Q&A’s.
The History of Diaphragm and Piston Flush Valves
The first ever diaphragm flushometer was invented by William Elvis Sloan in 1906, and was called the Royal flushometer. Instead of a gravity tank, it utilized incoming water pressure to flush, which used less water and energy. Next came the piston valve, invented in 1928, also by William Sloan. It worked better than the diaphragm flushometer in low pressure and hard water environments. The most popular diaphragm valve models used today are Royal, Regal, and Sloan. The most popular piston valve models are Gem and Crown.
How a Diaphragm Valve Works
Incoming pressure to the upper chamber seals the diaphragm down over the lower chamber
Activating the handle causes the diaphragm to flex, releasing water into the fixture
The diaphragm re-seals as the upper chamber re-pressurizes
How a Piston Valve Works
Incoming pressure to the upper chamber seals the piston down over the main seat
Activating the handle causes the complete piston to slide up, releasing water into the fixture
The piston re-seats as the upper chamber re-pressurizes
Valve Differences and Similarities
Both diaphragm and piston valves do the same thing: they control the flow rate and volume of water, and reset quickly so they are ready for the next flush. What is different is their method of internal sealing: a diaphragm utilizes a static, flexing seal, and a piston utilizes a dynamic, sliding lip seal.
Why does using different seal methods matter? Because they change the flush curve, which is created by the flow rate over time. The different curves created by the different valves are shown above.
- For a diaphragm valve, The flexing action provides a smooth open and close cycle. Having a gradually decreasing flow at higher pressures prevents a “hard close.” A slower close of the flush cycle helps prevent noise.
- For a piston valve, The slide action delivers most of the water to the fixture at the beginning of the flush cycle. A quick in-rush of water into a old style washdown fixture at less than 25 PSI could help to clear the bowl.
Variables that Impact Valve Selection
- Water Supply: Flow, pressure, properly sized (strong), under sized (weak)
- Water Quality: Chlorine/Chloramines, pH, sediment
- Fixture Type: High efficiency (HE), Closet, Urinal
- Usage Patterns: Frequency, Volume
Design Elements and Operating Performance Comparison Charts
Use the following charts to help identify which valve type makes the most sense for your application. In the ‘Design Elements’ chart, the green check mark indicates which valve has the better design (if they are not equal). In the Operating Performance chart, the green check mark signifies which valve is better for certain conditions.
- The static sealing mechanism of the diaphragm valve can require less maintenance than a piston valve would (assuming the components used are the same).
- Piston valves will work better in systems when water pressure can drop lower than 25 psi.
- Modern fixtures requiring 25 PSI or more tend to benefit from diaphragm technology
- In low pressure (<25 PSI), undersized (weak), and older fashioned systems pistons have an important role to play
In summary, Sloan designed both a diaphragm and piston valve because they have strengths in different areas. These differences in how they operate make one valve a better choice than the other depending on what conditions they will encounter when installed in various applications.
Buy all of the Sloan valves and valve repair parts needed for your facility from Sloanrepair.com. We carry all of the units and parts Sloan makes, with thousands of Sloan items in stock and ready to ship. Place your order online, or if you don’t see the model you are looking for listed in our webstore, give us a call to place your order.
Phone Hours: Mon-Fri 8:00AM - 4:00PM EST
Sloan Piston versus Diaphragm Q&A
The Q&A’s listed below are directly from Rolando Zambrano, Associate Product Line Manager at Sloan and Don Yurkovich, Regional Sales Manager at Sloan.
Q: What is the cost differential between diaphragm and piston flushometers?
A: This comes down to features and benefits. Royal (diaphragm) and Crown (piston) have the most features and benefits and have higher price points. Sloan (diaphragm) is our middle tier. Regal (diaphragm) and Gem (piston) are our competitive flushometers.
Q: Can you adjust the water volume per flush by replacing the diaphragm or piston?
A: Yes, the diaphragm or piston is the primary component controlling the gallons/liters per flush.
Q: What is the best solution for hard water where the owners refuse to treat their water?
A: Diaphragm flushometers with filtered bypasses and synthetic rubber components are recommended for harsher conditions. Sloan would advise using our Royal or Sloan series flushometers for this application.
Q: How does adjusting the flow rate with the stop affect the diaphragm flush curve?
A: The flush curve is controlled by the diaphragm kit or the piston kit (in the case of piston flushometers). Adjusting the stop in either does not affect the flush curve. Adjusting the stop “dials in” the flushometer with the particular fixture to make sure that the water being supplied properly clears the fixture and does not create any issues with splashing, “rooster tailing,” or overflow.
Q: Do many facilities have low water pressure in the USA?
A: Some do for various reasons. If the facility has newer low- flow fixtures (as mandated after 1997), they would need to have at least 25 PSI of flowing pressure for the fixture to evacuate properly.
Q: Is there a reason many other manufacturers are supplying piston flushometers vs. diaphragm?
A: Due to local water conditions (low pressure) in many international markets, piston flushometers are sometimes preferred there and are imported by other brands from low cost suppliers. They are also easier to manufacture. Remember, almost all of these are based upon Sloan’s original 1928 piston design. Sloan offers both diaphragm and piston flushometers that are manufactured in the USA. In most applications, the diaphragm is the best choice.
Q: Is there any noise difference between the two types of flushometers? Which is quieter?
A: The noise from the flush is created by the fixture, not the flushometer. In our laboratories, when we activate either type of flushometer without a fixture attached they sound the same.
Q: What is the maximum water pressure recommended for the flushometers to operate correctly?
A: The limit is set by the fixtures to which the flushometers are applied, and not by the flushometers themselves. Local plumbing code dictates the maximum, which is usually around 80 PSI maximum water pressure.
Q: Do any of these flushometers work with salt water, and which type would you recommend?
A: Sloan manufactures other flushometers called Dolphin and Naval that we recommend for salt water applications (call to order).
Q: Can I adjust a flushometer using the control stop?
A: The water volume from the incoming water supply can be adjusted with the control stop. The gallons per flush (gpf) can only be modified by changing the piston or diaphragm.