The need-to-know on low-flow

August 11, 2010

When Congress passed the Energy Policy Act of 1992 (EPAct) regulating water-flow rates for plumbing fixtures, engineers and building teams were left to determine how to meet the new water-conservation codes while avoiding such problems as waste-line stoppage and "multiple flushing" resulting from the reduction of system pressures.

EPAct specifies that flow rates for newly installed plumbing fixtures be:

  • 1.6 gallons per flush (gpf) for water closets.

  • 1.0 gpf for urinals.

  • 2.5 gallons per minute (gpm) at 80 pounds per square inch (psi) for showers and lavatories.

Better fixtures, better flow

Most of the problems early on dealt with the new ultra-low-flow (ULF) plumbing fixtures, especially gravity-tank water closets. Just a few manufacturers offered water closets of the 1.6-gpf variety and some were so poorly designed that users had to flush two or three times. Several generations of products later, however, low-flow fixtures have improved.

"During the past 10 years, manufacturers have re-engineered their products," says David Viola, technical director for the Plumbing Manufacturers Institute, Schaumburg, Ill. "For instance, water-closet manufacturers have increased the trapway diameters to allow waste to evacuate more efficiently, modified the geometry of the trapway to produce stronger siphoning action and adjusted the amount of water that rests static in the bowl as well as the slope and surface friction of the bowl itself."

Moreover, technologies such as pressure-assisted flushometers and electrohydraulic water closets use water or air pressure for an added boost (see "Water closet technologies," page 50).

Problematic infrastructure

Still, many of today's problems are linked to plumbing infrastructure, namely the size, length and slope of water-supply and waste lines.

"It is paramount that design engineers understand the effect of ULF on the drainage system and water-supply system," says Brian Heideman, manager of Health Care Services for E/A Ross & Baruzzini of St. Louis. "Some designers feel that because we are using half the water consumption, the water-pipe sizing to the fixture can be smaller, which can result in high pressure losses and water hammer in the piping system. ULF flush valves still have relatively the same flow rate as non-ULF flush valves.

"It is equally important to understand the drainage system with ULF fixtures," adds Heideman. "Believing that the waste material will carry as far as with a ULF is not true at all. Basic hydraulic knowledge will tell you that with less water and velocity, the waste material cannot carry as far with 1.6 gallons as compared to 3.5 gallons. Maintaining a velocity of 2 feet per second to allow scouring of the waste piping is very difficult with ULF fixtures."

Adding to the lack of drain-line carry, says Heideman, is the integration of wastewater recycling into a plumbing system. "Water recycling removes grey water-lavatories, sinks and showers-from the sanitary waste system, which reduces the amount of water flow in the waste piping," he adds. "It requires the use of smaller piping and higher slopes to provide greater flow velocities."

Problems also arise when retrofitting older buildings with ULF fixtures. Because these buildings were designed to the old flow rates, the existing drainage lines may not be sloped enough to carry waste contents, causing waste-line stoppage. Furthermore, says Ron George, project engineer with Detroit-based A/E SmithGroup Inc., the size of the existing drain line can cause trouble.

"The old 3.5-gpf water closets work with a 3-in. drain, but often were installed with a 4-in. drain," says George. "So when the 1.6-gpf water closets are installed, the drain line carry is very poor because the wave of water is spread out in the larger pipe."

George says there are several solutions, from replacing the drainage piping with smaller, more sloped piping to installing pressure-assisted water closets.

Fixture installation considerations

"A heavy reliance is also placed on installation and maintenance of ULF fixtures in accordance with manufacturer's instructions and code requirements," says Viola. PMI has identified the following as common problems affecting water-closet performance:

  • Proper tuning of flushing device. Water closets are designed to perform with a specified amount of water; too much or not enough water greatly reduces the ability to remove solid waste. With gravity-tank water closets, tuning consists of setting the water level in the flush tank to the designated level by adjusting the flushing device.

  • Ensure new or existing plumbing system is free from problems. Problems within the drainage and vent system can also have a dramatic effect on flushing performance. Inadequate or obstructed water-closet venting is one of the most common problems. The properly functioning vent promotes the rapid and efficient flow of waste while minimizing adverse pressure fluctuations in the drainage system. A poor or sluggish flush can result if a vent is not properly connected to the drain. Most codes require the vent to be a minimum of 11/2 to 2 inches in diameter, and require the vent to be located within 6 to 10 feet of the water closet.

Other problems are inadequate water-closet drain pitch and obstructed or damaged pipe.

Virtually all plumbing codes require that horizontal drain pipes maintain a uniform minimum slope, free from sags or bellies, to maintain desired flow velocities. Over time, inadequately supported piping can settle, causing sags, misalignment and obstructions.

  • Proper connection to plumbing system. The connection between the drain, or water-closet flange, and the outlet of the water closet can be a cause of poor flush. The joint between the drain and outlet is sealed with a toilet gasket, or wax seal, with or without a plastic horn or other setting compound. Without proper care, these sealing materials can be compressed into the flow passageway, causing an obstruction. Also, if a toilet gasket with a plastic horn is utilized, the gasket must be positioned with the sleeve facing away from the water closet outlet. Otherwise, the horn creates a ledge within the flow passageway and obstructs flow.

Last, an approved drain fitting, or water-closet flange, must be utilized. Many flanges on the market have offsets, ledges and shoulders capable of retarding or obstructing flow.

  • Adequate water required. Water closets require water supplied in sufficient volume and pressure. For instance, most plumbing codes require a minimum of 15- to 20-psi flowing pressure at the water closet's water-supply inlet.

  • Clogged rim holes. Mineral deposits, commonly caused by hard water, and debris collecting in the holes, or jets, under the rim of the water-closet bowl can result in poor flushing performance. The rim holes should be cleaned routinely to avoid mineral deposit build-up.

For more low-flow fixture installation and maintenance information, visit the PMI Web site at www.pmihome.org .

More research needed

Although building teams and manufacturers seem to be getting better at using ULF plumbing, many agree that more research and adjustments to plumbing codes are needed.

SmithGroup's George, for example, has proposed a change to the International Plumbing Code to reduce the length of hot-water piping that does not maintain hot-water temperature via a recirculated line or electric-heat tracing. The change would halve the length from the source to the farthest fixture to 50 feet.

"With the low-flow fixtures now mandated, it takes considerably longer to obtain hot water from non-temperature-maintained hot-water lines than it did in the past, when fixtures had greater flow rates," he says. "For instance, a public lavatory with a 1/2-in. copper water line flowing 0.5 gpm would take 4.1 minutes to get the hot water to the fixture from the water heater 100 feet away. How many people would wait for 4 minutes for hot water?"

         
 

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