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Do Sensor-Activated Taps Save Water?

Article by Plumbing Connection Autumn 2012

A new study reveals that sensor activated taps may not save as much water as is believed.

Over the years, there has been much debate among North American water efficiency professionals, manufacturers, green building advocates, and others regarding the water savings associated with using sensor-activated valves (also known as ‘hands-free’ or ‘touch-free’ valves) in restrooms. This includes the three main types of valves found in commercial restrooms, i.e., flush valves for urinals, flush valves for toilets, and flow control valves for tapware (faucets).

While it is commonly accepted that these sensor valves are more hygienic than manually operated valves, there remains some question as to whether or not they are more water efficient.

Because of this uncertainty, our team selected a multi-tenant office tower building in Hillsborough Country, Florida as an ideal candidate to study the ‘real world’ situation. The purpose of the project was to evaluate the effectiveness of sensor-operated valves to save water. The project included a comprehensive pre- and post-auditing program involving physical inspections, sub-metering, data logging, and maintenance staff surveys. It evaluated changes in water demands when manually operated toilet, urinal, and tapware valves were replaced with sensor-operated fixtures. The study was conducted over a 23-month period beginning February 2007 and concluding January 2009.

To help quantify any change to washroom water demands related to the introduction of sensor-activated plumbing fixtures, the project was divided into four phases:

  • Phase 1: pre-monitor water demands of washrooms with existing manually operated fixtures,
  • Phase 2: monitor water demands after manual tapware replaced by sensor activated tapware
  • Phase 3: monitor water demands after manual urinal flush valves replaced by sensor-activated valves, and
  • Phase 4: monitor water demands after manual toilet flush valves replaced by sensor-activated valves.

While the results achieved in this relatively small-scale project many not necessarily be indicative of results that might occur in other projects, they clearly indicate a significant increase in water demands when manually operated plumbing fixtures where converted to sensor-activated ‘touch-free’ models. The total average daily demand of the men’s and ladies’ washrooms almost double from 2,475L to 4,700L per day when all tapware, urinals and toilets were concerted to sensor-activated units.

That there was no decrease in water demands when the flush valve were converted to sensor-operated did not come as a surprise, as one would not expect there to be fewer flushes (most of us have experienced ‘phantom flashes’ with sensor valves, something that is not possible with manual vales). However, the measured increase of 54% in water demands when sensor-activated toilet flush valves were installed was much higher than expected. As for the tapware, water demands increase by 31% when the manual fixtures were replaced by ‘touch-free’ fixtures.

Prior to Hillsborough, two other studies had been performed that also indicated sensor-activated taps in public setting were not water savers.

The first of those studies was conducted by Thames Water Research & Technology in 2000 as the Millennuim Dome in London1. Over the period of one year, tap (faucet) use was measure for 240 taps, including both manual and infrared (sensor-operated) fixtures. Results showed the following:

Average water use per user visit (litres):

Manual – 0.9

Infrared – 1.8

Increase – 100%

The study summarises tap use with this statement: “Surprisingly over the year the conventional swivel top (manual) taps used significantly less water than the purported more efficient types, with each user of the swivel top (manual) taps using, an average, just less than 1 litres of water.”

The second study was conducted in 1997-1999 and was directed at photovoltaic water heating2. As part of the analysis, tap use was again measured for one year each for manual and sensor-activated taps. The following results are for taps rated at 8.3L per minute (Lpm):

Average Hot Water Consumption Per Day Per Washroom (litres):

Manual – 372

Infrared – 587

Increase – 58%

Study report for all three projects many be downloaded for our MaP testing website:

Unfortunately, tapware manufacturers in North America continue to claim “touch less” taps save water. The results of the studies referenced above clearly contradict those claims. There is little doubt that automatic sensor-activated taps are not the optimum choice when it comes to water efficiency. However, the question remains: why is the intuitive choice for automatic faucets not necessarily the water-efficient choice? One reason is that ALL automatic censor-activate taps are set to open at their full flow rate (average of 4.6Lpm in the case of the Hillsborough study) while manually-operated taps are typically used at flow rates much less than 3.0Lpm (users rarely fully open manual taps, possible to avoid splashing).

It should be noted that the US national standard for tapware installed in commercial installations (such as in the Hillsborough building) sets a minimum flow rate of 1.9Lpm. Other studies have shown that is both manual and sensor-activated taps are fitted with 1.9Lpm flow regulators, water use will be virtually the same. A disadvantage of the maximum 1.9Lpm flow rate for commercial installations, however, is the likelihood that hot water will never arrive at the tap in more circumstances.

In conclusion, we recommend that manufacturers of sensor-activated valves and taps promote these fixtures based upon their true attributes, hygiene and touch-free convenience, rather than fictitious water use efficiency claims.

Article written by John Koeller, P.Eng. and William Gauley, P.Eng.

1Thames Water Research & Technology, 2002. The Millennium Dome “Watercycle” experiment: to evaluate water efficiency and customer perception at a recycling scheme for 6 million visitors.

2ASHRAE, 2002. Field Test of a Photovoltaic Water Heater, Report No. HI-02-8-3

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