What you need to know before going super-low-flow in the shower

Low-flow showerheads are a highly tempting proposition for remodelers looking to boost the efficiency and sustainability of their properties. Know how to choose and spec them appropriately.

July 31, 2009


Super-low-flow fixtures’ design should place the showerhead as close to the user as possible.

Low-flow showerheads are a highly tempting proposition for remodelers looking to boost the efficiency and sustainability of their properties. Install a showerhead with a flow rate of 2.0 or 1.5 gallons per minute, rather than the conventional low-flow 2.5 gpm, and — presto! — you have cut water consumption by 20-40 percent at that outlet.

But making the move to enhanced low-flow showerheads is not as cut-and-dried as simply replacing a 2.5-gpm showerhead for a more efficient model. Without careful consideration of proper design, the installation of super low-flow fixtures can lead to performance and safety issues, including scalding and thermal shock.

Life, health and safety considerations

The overwhelming majority of building codes currently require automatic temperature compensating (ATC) shower valves, which keep water temperatures at a safe and comfortable level, regardless of changes in system pressures and temperatures. This generic term encompasses three types: pressure balance, thermostatic and a combination of the two. The predominant standard is ASSE 1016, which establishes the testing protocol that these valves must meet for certification.

To be fully certified, an ATC shower valve must not permit the water temperature to fluctuate up or down by more than 3.6 degrees at the nominal pressure of 45 pounds per square inch (psi). For example, if someone flushes a toilet, thus diverting cold water from a 105 degree shower, the water temperature may spike uncomfortably high, but only briefly. Within the first five seconds, the outlet temperature must settle between a safe and comfortable 101.4 degrees and 108.6 degrees.

What does all this have to do with the new low-flow showerheads? The problem is ASSE 1016 was written for showers with a flow rate of 2.5 gpm, and it does not require testing at other flow rates. A shower valve that works just fine with a 2.5-gpm showerhead may not work well with one rated at 1.5 gpm. This, in turn, can lead to thermal shock — a sudden significant change in temperature that may cause the bather to slip and fall — or even cause scald burns.

Still, many current shower valves are able to accommodate showerhead flow rates lower than 2.5 gpm. Which makes and models? No one knows for sure. If you look at a shower valve, you won’t likely find reassuring language that “this valve provides temperature compensation protection down to a flow rate of 1.5 gpm.” Absent such confirmation, you don’t know how low the gpm can go for a given showerhead and still keep the outlet temperature within the ± 3.6 F threshold.

The simplest route to safety is to buy the shower valve and the showerhead in a package from the same manufacturer. That way you know the combination complies with ASSE 1016. Some of the main-line shower valve manufacturers are producing low-flow showerheads, so your chances of getting a pair that work together are improving.

If you decide to mix valve and showerhead products from different sources, and the valve manufacturer’s instructions do not explicitly say that the product will work with certain low-flow rates, call the factory for a clear-cut decision. If that manufacturer cannot accommodate, look for one who can.

When in doubt, change the spec upward and settle for more modest water savings. Your chances that a given valve and showerhead will work effectively together are much greater at 2.0 gpm than at 1.5 gpm.

Low-flow in an existing bathroom

Going low-flow in an existing bath presents a far more daunting challenge than with new construction. Most of the time, installers have no idea what type of valve is in the wall behind the shower and they have no way of finding out. If the valve is older, it won’t likely function properly with a modern low-flow showerhead.

Some retrofit applications don’t even have ATC valves because they use two-handle faucets, which cannot protect against changes in temperature and pressure. Now, you may think: Why not go low-flow anyway? In either case, you can’t protect against dramatic fluctuations in temperature, but by substituting a low-flow head you’ll at least be using less water.

Bad idea. Low-flow means less water is coming out of the shower, so the system has less thermal mass associated with the water for a given period. Consequently, the system’s reaction to a given temperature and pressure change will be more extreme — that is, greater and faster. Your likelihood of getting scalded rises, and the thermal shock potential is more serious, too.

The only appropriate way to retrofit a shower with a two-handle faucet is to eliminate the outdated faucet and install a new valve and showerhead.

Design considerations

Low-flow showerheads inevitably impact the size and layout of the showering environment, as well as the positioning of the fixtures and fittings within it. By definition, a low-flow showerhead delivers less heat, in the form of hot water, to the body if the shower water temperature remains the same. Droplet size and aeration also impact heat retention within the shower enclosure. As a result, it is not unusual for users of low-flow showerheads to complain of being cold.

A major factor is the distance the water must travel from the head to the body: the longer the distance, the bigger the drop in water temperature. Someone who is four feet tall will experience a decidedly different temperature than someone who is six feet. To overcome this heat-loss problem:

  • Place the showerhead as close to the person as possible, or
  • Specify a height-adjustable shower fixture so that the bather can pull the shower closer.

If the bather cannot get closer to the showerhead, the only comfort option is to turn up the hot water temperature, thus using more energy. That’s not a tradeoff any green-minded designer, contractor or owner wants to make.

Careful attention should also be paid to the design of the showering compartment. As flow rates drop, air currents within the compartment and from the surrounding environment exert a proportionately larger effect on user comfort. A low-flow showerhead will provide less warmth in a large or open shower area than in a small or fully enclosed compartment.

Low-flow showering technology promises to have a major impact on water consumption in the U.S. during the next decade — certainly a welcome development. But safety and comfort are as equally important as water conservation when it comes to installing super-low-flow fixtures. Proper design and installation will ensure that all three needs will be met.

This article originally appeared in our sister publication Building Design + Construction.

Author Information
Shawn Martin is the Technical Director for the Plumbing Manufacturers Institute, the international trade association representing the manufacturers of most plumbing fixtures and fittings used in North America. An engineer by training, Martin is an expert in water efficiency, clean drinking water and green building practices. Contact Martin at smartin@pmihome.org.


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