Plumbing for low-flow toilets and drains

A common trend today in North America is the usage of “low flow” (also known as “high efficiency”) toilets. Low flow models are chosen by many ecologically aware consumers and frequently required by many state, provincial, and national plumbing laws because to their evident advantage of using less water during operation ΑΠΟΦΡΑΞΗ ΣΤΗΝ ΗΛΙΟΥΠΟΛΗ.

The amount of water used per flush in toilets has changed throughout the years from 3.4 US gal. (13 L) to 1.6 US gal. (6.1 L) to the current, more popular revised norm of 1.3 US gal (4.9 L). The task of creating toilets with better designs to provide proper flushing while consuming less water has been met, and the resulting decreases in usage of the priceless resource of fresh water have been significant.

Low flow toilets, however, may have an adverse downstream effect on the plumbing system, notably on drain piping, designers and contractors involved in commercial construction are now understanding. This is why.

Cast iron, copper, and PVC are the three most often utilised piping materials for sanitary drains in commercial construction. Numerous factors influence the choice of piping, but material and installation costs as well as code acceptance play a major role. Regardless, the surface smoothness qualities and interior dimensions of these 3 piping materials vary, leading to noticeably varied flow capacities. Although it has historically been demonstrated that all three piping materials can handle the flow capacity of 13L toilets, this may no longer be the case with the low flow variants.

The Manning Flow Coefficient, a measure of the surface roughness of piping materials for gravity flow, is used in drain systems (N). The smoother the pipe surface, the smaller the N factor. For toilet drains, the standard pipe size in North America is 3″. 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).

The flow capacity for 3″ pipe (flowing full) in each material, calculated using the Manning Formula for a typical slope for drain plumbing (1/4″ per foot), are as follows: PVC: 86 US gpm, cast iron: 62 US gpm, copper: 69 US gpm, and their respective metric equivalents are 3.93 L/s, 4.38 L/s, and 5.41 L/s. Therefore, it is a fact that 3″ diameter copper and PVC will have much larger flow capacity than the same nominal dimension of cast iron pipe (25 percent for copper and 38 percent for PVC), regardless of the flow source. Cast iron’s flow capacity will actually degrade over time due to its susceptibility to internal corrosion from the acids frequently present in sanitary sewage, whereas copper and PVC will retain flow capacity due to their high levels of resistance to internal corrosion.

It seems sense that, in the case of toilets, decreased flow down the drain lines will significantly increase the likelihood of sediments building up in the sanitary flow or, worse yet, obstruction. Contractors today have frequently decided to reduce the slope of the drain pipe to be nearly flat in order to keep sediments suspended longer and enhance the likelihood that adequate drain flow will occur in order to fix this operational problem. Building owners should be concerned about this since, in addition to being against the law in most places, it could result in other odour or sanitary problems if sewage is allowed to sit in drain lines for an extended period of time. Only repeated flushes of the low flow toilet in the future might ultimately move the sewage into a vertical stack and then into the sewer beneath the ground.

What steps may be taken today to prevent blockages or flow issues? Consideration should be given to replacing cast iron laterals with either copper or PVC for significantly improved flow capacity in existing buildings with cast iron laterals when upgrading to low flow toilets is possible. For the same reasons, while specifying the more modern, high-efficiency toilets for new construction, designers should be aware of this problem and pay closer attention to the drain piping’s flow capacity as a key deciding element in the choice of piping materials.