Today across North America, there is a popular trend towards the use of 'low flow' (also known as 'high efficiency') toilets. With the obvious benefit of using less water for operation, the low flow models are preferred by many environmentally conscious consumers and often mandated by many state, provincial and national plumbing codes.
Throughout the early 1990's until today, the volume of water used per flush in toilets has evolved from 3.4 US gal. (13 L) to 1.6 US gal. (6.1 L) to today's increasingly common new standard of 1.3 US gal. (4.9 L). Toilet manufacturers have met the challenge of improved designs to get adequate flushing using less water and the resultant reductions in consumption of the precious resource of fresh water have been noticedable.
However, designers and contractors involved in commercial construction are now realizing that low flow toilets may have a negative downstream effect on the plumbing system, specifically on drain piping. Here's why.
There are essentially three commonly used piping materials for sanitary drains in commercial construction – cast iron, copper and PVC. The selection of piping is due to a myriad of factors but driven mostly by material and installed cost as well as Code acceptance. Regardless, these 3 piping materials have varying surface smoothness properties and varying internal dimensions and as a result, markedly different flow capacities. Although the flow capacity of all three piping materials has historically been shown to be adequate with 13Lilets, this may no longer be the case with the low flow models.
Piping materials for gravity flow as in a drain system are characterized by a surface roughness coefficient known as the Manning Flow Coefficient (N). The lower the N factor, the smoother will be the pipe surface. The pipe size 3 "is uniformly used across North America for drains fromilets. Critical data for each of the 3 piping materials are: cast iron (ID = 3.10", N = .013); copper dwv (ID = 3.03 ", N = .011) and PVC Sch. 40 (ID = 3.04", N = .009).
Using the Manning Formula for a commonly used slope for drainage piping (1/4 "per foot), the flow capacities for 3" pipe (flowing full) in each material are as follows: cast iron – 62 US gpm; copper – 69 US gpm; and PVC – 86 US gpm (metric equivalents are 3.93 L / s; 4.38 L / s and 5.41 L / s, respectively). Thus, regardless of flow source, it is a fact that 3 "diameter copper and PVC will have significantly more flow capacity (25% for copper, and 38% for PVC) than the same nominal size of cast iron pipe. being acceptable to internal corrosion from the acids commonly found in sanitary sewage, its flow capacity will actually worsen over time where both copper and PVC will maintain flow capacity due to high levels of resistance to internal corrosion.
With respect to toilets, it stands to reason that less flow through the drain lines will greatly increase the chance of accumulation of solids in the sanitary flow or worse still, blockage. Contractors today have many times resolved to correct this operational problem by lessening the slope of the drain pipe to be near flat in order to keep solids suspended longer and increase the chances of proper drain flow occurring. This should be of concern to building owners as it not only would be a Code violation in most areas, it may introduce other odor or sanitary issues by sewer remaining for prolonged periods of time in drain lines. Only multiple future flushes of the low flow toilet may contribute to the ultimate transport of the sewage into a vertical stack and subsequent delivery into the underground sewer.
So, what can be done today to avoid flow problems or blockage? For existing buildings with cast iron laterals where upgrading may be done to low flow toilets, consideration should be given to replacing the cast iron with either copper or PVC for much improved flow capacity. For the same reasons, on new buildings, designers should be aware of this issue when specifying the new high efficiencyilets and give increased attention to the flow capacity of drain piping as a major factor in the selection of piping materials.