CHOOSE THE RIGHT MATERIAL FOR THE APPLICATION
Value engineering is a process meant to optimize the resources available without compromising the quality of the building. Unfortunately, it has become a cost-cutting practice that can compromise the building and sometimes result in higher costs for the owner. This is especially true when it comes to substituting plastics for cast iron.
A study of plumbing engineers found that 61% of value engineering requests they received were not in the client’s best interest. To aid in the decision-making process, here are a few resources that can help you decide whether to use cast iron or plastic pipe and fittings. These tools can also be used to help protect the engineer’s specifications.
Factors to consider
Choose the right material for the right application.
MAXIMUM WORKING TEMPERATURE
Plumbing codes restrict discharges into sanitary lines to 140 F, but the reality is that commercial kitchens can sometimes exceed these guidelines. Consider that possibility on projects where PVC is being substituted.
Compensation for PVC thermal expansion and contraction should be designed into systems and is generally accomplished through offsets or expansion joints. The movement of cast iron is close to that of structural steel and concrete and usually requires no compensation.
The denser a material, the better it is at blocking the transmission of sound. Sound attenuation is especially important in main drainage stacks and storm systems in multifamily housing, office buildings, hotels, and hospitals.
Proper maintenance is required to keep DWV systems performing optimally. Periodically, drain-cleaning devices using auger bits are required to remove blockages. Repeated use of these devices could damage PVC pipe walls and fittings.
Defined by failure mode, piping systems are classified as either rigid, such as cast iron, or flexible, such as PVC. Calculating maximum burial depths for flexible piping systems is very complex, and PVC must always be installed per the requirements of ASTM D2321.
DWV piping systems require consistent fall without flat or low points that could impede flow. The additional cost of hangers, anchors, and other materials, plus the labor to install them, is also a significant consideration when it comes to specifying materials.
COMBUSTIBILITY & USE IN PLENUM SPACES
Plenum spaces require careful attention in terms of safety, design, and building codes. PVC, like all plastics, is a combustible material, while cast iron is noncombustible and can be installed in plenum areas without further consideration.
The cost and complexity of penetrating fire-rated assemblies add significant costs to the installation of DWV systems in commercial construction. Consider the additional care and costs involved in using combustible materials.