Potable water is a real commodity these days. Although more than 70% of the earth’s surface is covered with water, only about 2.5% actually is suitable for human consumption. A renewable resource, fresh water restores itself solely through evaporation and precipitation. And as drought spreads across the globe — from the southwestern U.S. to India and the Middle East — the focus turns toward preservation.

And yet water conservation is nothing new. The first federal legislation on the topic dates back to the 1992 U.S. Energy Policy Act — still the national standard for plumbing fixtures today — that cut the traditional 3.5 gpf down to 1.6 gpf. States such as California recently have set higher efficiency standards, mandating that all faucets installed not exceed 1.2 gpm flow and toilets not exceed 1.28 gpf.

At the same time, third-party certification systems have created an organized, voluntary standard for water conservation and made it fashionable for commercial buildings to earn accreditation. From the most notable, the USGBC’s LEED Rating System, to the WELL Building Standard, Green Globes, Estidama and Net Zero Energy Buildings (NZEB), today’s rating systems have become a new benchmark more than even a prized endorsement.

Unfortunately, in their noble quest to conserve water, these standards have inadvertently led to additional building complications as they relate to overall plumbing system design and performance. Dictating water consumption at the fixture level only, today’s efficient rating standards force engineers to rethink back-of-house plumbing infrastructure design that encourages both true water savings and continued, sustainable operations. With the right engineering know-how, plumbing infrastructure and accurately specified fixtures, sustainable standards can still succeed in today’s buildings.

Fixture operation

For low-flow fixtures to deliver the same performance as an older fixture, a higher pressure supply is required at the fixture. For example, a toilet flush valve operating at 3.5 gpf used to work on 15 to 20 psi. Now, 1.6 gpf and lower flush valves need a supply pressure between 25 and 30 psi to achieve the pressure necessary to move waste out of the system. While this may not be a big deal for residences or commercial buildings three stories or less, getting an extra 10 pounds of pressure at the top of a downtown high rise could add significant capital costs to a project. For this reason, special attention must be paid to cold water supply systems serving urinals and toilets when it comes to low-flow fixtures to achieve the desired results.

• Hot water supply and return systems: Simply put, low-flow fixtures require more time to get hot water to the consumer from the source. Flushing out uncirculated hot water supply lines takes time and the lower the flow rate, the longer the flushing time takes. For example, a 3/4-in. hot water supply to a sink, featuring a traditional 2-gpm aerator on the faucet can be as far as 44 ft. away from its hot water source and still procure hot water for the occupants in just 30 seconds. When this flow rate is cut in half to 1-gpm, the source must be 22 ft. closer to the faucet to maintain the same 30 seconds of wait time for hot water. The latter requires an extra 22 ft. of piping and a balancing valve to achieve the same “time to hot water.” This necessitates additional hot water return branches, balancing valves and cost to systems that in the past have not been as complicated. 

  Third-party certifications are well intended, but perhaps unintentionally have opened a window to performance issues in the name of water savings.

For this reason, low-flow aerators should be specified in locations where they won’t irritate occupants. For example, when specified in task locations — at showers, kitchen sinks, etc. — low-flow faucets will take up to double the amount of time to achieve the same results, such as rinsing hair or filling a coffee pot. Although all that is required to achieve the water conservation designation from a third-party certification is to specify and install the low-flow faucets, in task applications they likely won’t actually achieve the ultimate goal of saving water.

• Urinals: Using low-flow urinals — or a lower amount of water to evacuate the waste from a urinal — leaves a higher concentration of urine in the wastewater stream. Where this is the case, the wastewater becomes more acidic and can be corrosive to a building’s metal piping. In some cases, this could even result in deterioration of the vent system serving the waste piping. Vent piping assumes a lot of evaporation from the waste stream, which can corrode the piping from the inside out, especially when made of metal. This problem is further heightened with waterless urinals where there’s no method of cleaning the urinal after each usage. In this case, foul odors may overtake a bathroom.

• Toilets: While the current federal baseline, established by the 1992 U.S. Energy Policy Act, remains 1.6 gpf, there is a real trend to move this standard to 1.28 gpf, a push from toilet manufacturers, and building owners and operators alike who are looking to champion water conservation.

The biggest challenge here is the capability of a 1.28 gpf toilet to carry solid waste down a horizontally-pitched waste pipe. ASME A112.19.2, which sets minimum performance requirements for toilets regardless of flush volume, requires waste pipe to be carried 40 feet based on an average amount of flushes during the performance test. While the higher flush rate toilets easily can carry waste more than 40 ft., today’s low-flow fixtures are highly engineered to meet this standard. This has resulted in great consideration by plumbing engineers to understand where water closets are located in relation to building stacks and main lines carrying larger waste streams.

Another important function of flushing is to cleanse the bowl. While the 1.6 gpf likely will evacuate the bowl, it won’t clean it the same way the 3.5-gpf fixture once did. And the 1.28-gpf toilet will make it even harder to get the bowl clean. Where this is the case, the occupant typically will flush a second time. Now, the low-flow flush used 3.2 gallons or 2.56 gallons, respectively.

User satisfaction shouldn’t be discounted

A building owner or operator considering specifying low-flow fixtures should ask themselves the following questions:

• How long are occupants willing to wait for hot water?

• Will the low-flow toilets need additional plumbing infrastructure support to operate effectively?

• Will the facilities personnel need to rot out the plumbing lines more often with low-flow fixtures?

• What are other potential repercussions of specifying low-flow fixtures?

While the answer to some of these questions will be universal to all buildings, many will be specific to market or industry. Customer-service-focused facilities such as retail and hospitality, or medical facilities such as hospitals and doctor’s offices, likely will have very different water needs and demands than a commercial office building or residential property. 

Third-party certifications are well intended, but perhaps unintentionally have opened a window to performance issues in the name of water savings. To solve this conundrum, fixture manufacturers continuously are striving to improve the design of low-flow fixtures so they will gain better traction. Until then, there is a chance that a building’s plumbing system performance and user satisfaction may suffer without the proper insight of low-consumption fixtures by the plumbing engineer.

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