Everybody is washing their hands lately; it is the thing to do. Washing hands is a simple and effective way to prevent the spread of infection. As plumbing engineers, we know and understand the importance of public safety. Our task is to understand and specify the devices used by the public — in this case, handwashing faucets.

Since their commercial inception in the 1980s, hands-free faucets have become commonplace in public locations. Based on my investigative discovery (I went out in public and washed my hands), I found that a good proportion of hands-free, sensor-type faucets do not work so well. The purpose of this article is to understand how sensor-type faucets work and what we can do to ensure they serve the public and help prevent infection.    

In sensor-type faucets, sometimes known as hands-free or touchless faucets, the water flow is activated by placing your hands in the path of a proximity sensor, which then activates a solenoid valve. The solenoid valve opens and closes based on a prescribed algorithm so the person can use the sink faucet for its intended purpose. 

In an age of germ-free conscientiousness, I have been surprised by conversations with nonengineers who actually prefer manual faucets. The main reason is that manual faucets are dependable; you know they will work when you turn the handle and you can wash your hands for as long as you like.

Specifications for faucets are typically defined in Division 22. The base specifications I am familiar with designate many of the standards we would expect to ensure a quality project. These standards come from the National Sanitation Foundation (NSF), the American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME). 

One specification even lists a National Fire Protection Association standard, NFPA 70 — likely to ensure that the faucet does not cause a structure fire or, half-joking aside, that nobody gets “zapped” when they work on the product. 

Standards such as ANSI and ASME make certain the faucet will meet the dimensional and material qualities expected of a commercial product. Meeting the NSF standards will help ensure that the faucet will not cause adverse human health effects due to the components in the product. What about the standards making sure the faucet works consistently and dependably? Can any of us just purchase an infrared sensor, battery pack, solenoid and plug it in to get the water to flow?

Anatomy of Sensor-Type Faucets

Sensor-type faucets are comprised of four main parts: the spout, sensor, solenoid and power source. The first component, the spout, would appear to be straightforward. Keep in mind, though; we are engineers. “Simple” is a carrot on a stick that is often just out of grasp. Spouts can include filters and aerators. They also may act as the carriage for mounting sensors and electronics. Some manufacturers even mount the power source, photovoltaic cells, onto the spout body. 

We need to balance all our client’s needs when we configure the spout body. Aerators may be specified to limit flow or limit the risk of infection. The desire to place a laminar-flow aerator in the base of a spout body may not work with a manufacturer who designed its solenoid to be placed in the spout body for ease of maintenance. Designing a lavatory system to operate with ultra-low-flow as a driver may not be desirable for faucets regenerating their batteries with a turbine-type hydrogenerator. 

The spout is the key legacy component to any faucet; it is the first thing anyone looks at when they wash their hands.

The device that switches a hands-free faucet on is typically an infrared sensor. It includes an emitter and receiver, usually housed behind that little red window we wave at frantically when the faucet does not work. Sometimes the sensor device is cleverly tucked away at the end of a spout. 

According to the manufacturers I spoke with, sensor technology is not the leading cause of malfunction. While they said that the sensor is one of the first things people assume is the problem, they claim sensor technology is rather straightforward and dependable. 

Once the infrared beam from the sensor is reflected to the detector, a solenoid valve gets actuated. This is what allows water to flow through the spout. The solenoid is an electromechanical device subject to the most movement in the hands-free faucet assembly. For this reason, solenoid valves should be on a periodic maintenance schedule for facilities. 

Solenoid valve designs are different between manufacturers. Common to the dependable operation of solenoid valves is typically a pre-filter, sometimes shaped like a small wire-mesh cone. Solenoid valve assemblies can be sensitive to small grit and other water impurities. 

One of the more common reasons for sensor faucet failures is the improper installation of the solenoid filter and improper flushing of the domestic water system before startup. Some manufacturers include a “wiper spring” to clear out the solenoid after each use. Construction materials also play a factor in how long a solenoid lasts; silicon seals are commonly used for longevity. The solenoid is also the component using the most power.

The power source for sensor-type faucets is one of the most unique and emphasized components of the faucet. For health-care designs, many of my colleagues specify sensor-type faucets using AC power on an emergency circuit. This would seem to create the highest level of dependability. 

When it comes to designs where DC power is the preferred choice, each manufacturer has its strategy. Some manufacturers claim that standard AA batteries are the best choice since they are readily available and provide a better load distribution profile than custom-type batteries. The designs using lithium-type batteries will claim operational lives of eight, 10, 12 or even 15 years. 

One of the keys to battery life is how the circuitry of the faucet is designed. A hands-free sensor faucet's electronic features, including the sensor and the solenoid valve, can only operate through the power source. There will be a low, consistent power drain for the sensor and a larger power drain for the solenoid valve. 

Capacitors are typically employed in the design to help store an electric charge and dampen the spike in power demand when the solenoid is actuated. Photovoltaic cells or hydrogenerators may be used to help top off the power stored in the capacitor.

The last electronic portion of hands-free faucets is the programmable component; this is sometimes referred to as the controller. Similar to many electronic devices, the controller seems to have gotten smaller over time. 

My question to many of the manufacturers I spoke to was: “Why can’t the faucet just turn on when your hands are under it and turn off when you pull them away?”. I didn’t quite get a straight answer. Probably the most satisfactory answer was that the faucets come preset from the factory and you shouldn’t need to worry about it. 

After digging into how sensor faucets work, I wonder if it doesn’t have more to do with the switching technology of the sensors. Maybe they only operate as a one-way switch to trigger a time sequence. 

Specifying Quality and Dependability

What makes me skeptical is the branding of all the different smart features. I’d like to see the faucet work dependably before I dig into custom programming.    

Advice from one manufacturer requires a reset by unplugging the power source and data cable. It reminds me of the advice the cable company gives you after you sat on hold for a while and are just about to throw the cable box out the window. When it comes to a service we pay for, such as cable TV, we are willing to do what it takes to make sure the service works. Unfortunately, if a sensor faucet does not work, the public may just not wash their hands.

As we specify sensor-type faucets, we should be familiar with the standards ensuring quality and dependability, not just the extra features that look good on a sales sheet. As engineers, we should be careful not to specify products that cause “feature fatigue,” discussed in a Harvard Business Review article as to why manufacturers add so many programming options (https://bit.ly/3mWCSV5):

“One reason is to serve their own efficiency goals. To begin with, adding features costs next to nothing. As faster and faster chips offer ever-increasing memory capacity — at a lower cost — engineers can’t resist the temptation to equip existing electronic components with more functions. Of course, they are not looking at the whole equation, which includes the intangible costs of reduced usability.” 

Sensor-type, hands-free faucets offer many benefits to society. They have the potential to conserve water and allow people with physical conditions such as arthritis to wash their hands much easier. As you imagine users in a busy public restroom, sensor faucets probably allow many more people to wash their hands more efficiently than if everyone had to manually turn on and off the handles or wrist blades.     

Specifications for the inclusion of sensor-type faucets may not include standards that define the quality and dependability of the electronics controlling the faucets. Arguably, this led to robust innovation by many of the leading faucet manufacturers. No third-party testing or results study is available that I am aware of to define the reliability of operation. 

Additional quality standards placed in Division 22 specifications by plumbing engineers could stifle innovation while at the same time increasing quality and dependability. At a minimum, plumbing engineers should understand the technology they are incorporating in their designs.

The evolution of hands-free faucets is reminiscent of computers. Some of us may not recall a time when our computers at work would crash at a moment’s notice. When the IT professional would tell you to reboot, it was anyone’s guess whether your work was saved or not. Computers have come a long way in their dependability because we all use them and, arguably, depend on them. 

The public also depends on faucets to wash their hands and prevent the spread of infection. As plumbing engineers, we should do our part to understand how they work, specify the right combination of features for the clients we work for, and help ensure that sensor faucets are dependable.