Design engineers must know the proper application of cast iron and PVC during the value engineering process to ensure a cost-efficient solution.
by Eric Knauth, PE, LEED AP, and Troy Johnson
In few construction projects in today’s market is the initial cost not a factor. Many projects are over budget even early on during the design phase, and unless the project has extreme budget issues, cutting scope is usually not an option. Instead, value engineering (VE) of the building’s systems hidden in the walls and above the ceilings takes precedence over that of the visible finishes.
Is there indeed value in value engineering? Does it result in reduced construction costs for the owner? On the other hand, does the construction cost increase while resulting in an inferior or potentially less desirable product? These are complex questions that differ based on each line item in the VE process. In some cases, it may even be referred to as “de-value engineering.”
Value Engineering Piping Options
Several items typically are considered during VE discussions of the HVAC, plumbing, and electrical systems, and one of which is utilizing PVC in lieu of cast iron piping. Many engineers specify cast iron because they know it’s an appropriate material for most commercial buildings—but why is that?
Every product/material has its place. PVC is not the right product for all applications, and neither is cast iron. The design engineer must be informed of the pluses and deltas (pros and cons) of each material to ensure it is used in the correct application to result in a successful project. Unfortunately, due to VE, PVC is being incorrectly used in places where the design engineer never intended it to be utilized.
In looking at piping as a potential VE item, a few factors in the design and product selection will impact construction cost. Additional factors that may not have an initial cost implication, including longevity, temperature rating, expansion, and codes/standards, should be discussed as part of the final VE decision. Those factors are discussed below, and at the end of this article we will analyze a hypothetical 10-story office building and compare the cost of cast iron vs. PVC.
Value Engineering Items Having a Direct Impact on Construction Cost
One of the biggest differences between materials is noise. Even with noise wraps and insulation, PVC is at least 10 decibels (dB) higher than cast iron (see Figure 1). (Note: for every 10-dB increase, an individual perceives a sound as twice as loud.) In our cost analysis at the end of this article, we took this into consideration by providing an insulated cost for aboveground PVC sanitary and storm piping.
Firestopping can be a major cost for building owners. Cast iron is relatively inexpensive to firestop, which can be as simple as filling the annular space with fire caulk or mineral wool. With PVC, an intumescent material, in addition to fire caulk, is required, and the price point of the intumescent material and associated firestopping increases dramatically when the pipe size increases.
Additionally, you must consider the life expectancy of the various firestopping materials, as well as the shelf life of these prior to their installation. Many municipalities are requiring third-party certification of these materials and the contractor’s installation prior to building turnover.
Further, because life safety is a major issue when it comes to building development, it cannot be overstated that PVC is deadly when burned. Long before reaching its melting point, PVC begins to smoke and produces hydrogen chloride (HCI), an incapacitating irritant reported to be intolerable at concentrations above 100 parts per million (ppm). Once ignited, PVC then produces hydrogen cyanide (HCN) by reacting with nitrogen in the atmosphere. HCN can be distributed in the body within seconds, and death can occur within seconds or minutes after significant exposure (such as during a fire).
According to most authorities and plumbing codes, it is sufficient to support horizontal cast iron pipe at each joint. Supports should be adequate to maintain alignment and prevent sagging and should be placed within 18 inches of the joint. Vertical components shall be secured at each stack base and at sufficiently close intervals to keep the system in alignment and to adequately support the pipe and its contents. Riser clamps, sometimes called floor or friction clamps, are required for vertical piping in multistory structures for each floor not to exceed 15 feet, according to the Cast Iron Soil Pipe and Fittings Handbook.
Depending on the model plumbing code being followed, mid-story support for PVC will either always be required (2018 UPC) or be required for tighter floor-to-floor construction (2018 IPC) than that of cast iron. Plumbing codes follow hangar spacing requirements for PVC pipe at a maximum horizontal distance of every 4 feet for all sizes. Additionally, every change in direction should be supported. This results in about double the quantity of hangers required for PVC over that of cast iron.
Other VE Items to Consider
The following items often do not have a direct correlation to construction cost, but they must be included in VE discussions as they impact the design, performance, and/or installation of a system.
PVC has a maximum operating temperature of 140°F, at which point approximately 21 percent of its initial strength remains. At temperatures above 140°F, PVC piping can become pliable and fail. On the other hand, cast iron has a maximum temperature of 212°F, which is limited by the neoprene gasket inside the coupling or joint.
Expansion and Contraction
All buildings must account for expansion and contraction. PVC will expand 3.6 inches per 100 feet of pipe with a temperature change, or nine times the rate of cast iron. Cast iron expands and contracts at a very similar rate to the steel and concrete in a building, meaning the cast iron isn’t outpacing the building’s structure (see Figure 2). Because PVC is solvent-welded and a rigid system, failure to account for expansion can result in catastrophic failure of both the plumbing system and the firestopping system referenced above. Expansion joints or expansion loops are costly and should be added to the cost.
Sustainability, green building, decarbonization, etc., are hot topics today and will continue to increase in popularity. U.S.-made cast iron is between 96 and 98 percent recycled material. Foreign-produced cast iron uses mostly virgin material, as does PVC pipe resin. Domestic cast iron production reduces the impact on our landfills, and in fact, even if PVC is recycled, it cannot be reused in the creation of new PVC pipes as current standards only allow for virgin material due to the variables of differing PVC compositions.
Codes and Standards
Various national, state, and local codes and standards dictate the usage of one material vs. another. For example, the 2019 California Plumbing Code says that per the state’s Department of Health Care Access and Information (formerly the OSHPD) and the Department of Housing & Community Development, PVC installations are not allowed or are limited to not more than two stories of accommodation. NFPA 90A: Standard for the Installation of Air-Conditioning and Ventilating Systems also impacts the material, requiring noncombustible piping when sharing a shaft with HVAC ductwork.
Some wastes, such as waste from beverage stations in which sodas/syrups could sit in the trap, start compromising cast iron, while PVC will not be affected. Acidic soils may require standard cast iron to be wrapped, while no special provisions are required for PVC. Other materials on the market will also withstand these conditions, including stainless steel and high-performance, domestically produced, epoxy-coated cast iron.
The above information only represents part of a VE discussion for cast iron vs. PVC. Below we compare the cost of a 10-story generic office building with core toilet rooms utilizing conventional waste and venting with two 4-inch sanitary and two 4-inch vent stacks. Off the cast iron stacks, a combination of cast iron and DWV copper is utilized, whereas for the PVC estimate, the entire system is PVC. Quiet Shell was used on the PVC for the aboveground sanitary for noise reduction. The cost analysis (see Figure 3) only includes rough plumbing for the waste system; no finish plumbing or costs for fixture carriers are included. All estimates were quoted from a commercial contractor in Texas, acknowledging that material costs and labor will change by region across the country.
The cost consideration includes noise, firestopping, hangers, and underground installation requirement (per ASTM D2321) to make it a true, apples-to-apples comparison.
Figure 3 shows that the installed costs of cast iron compared to PVC are nearly identical. In this case, cast iron is actually cheaper than PVC. So where is the cost savings? Unfortunately, the savings often come from cutting corners on the PVC installation to include improper firestopping, improper underground installation, and provisions for noise considerations not being included. These items change the value of the building for the end user and put the owner at potential risk for future litigation.
How Can the Design Engineer Protect the Owner?
You must know the right questions to ask when cost issues arise. First, verify that all costs are included and not just material costs. Such costs may be included by one subcontractor, but another subcontractor may have additional costs that are not included. Second, how are changes to firestopping and noise going to affect the building and its end use? Once all of this is factored in, is there any value added to the project to change materials for a minor cost difference? The owner and their team should know all of the facts before making a calculated and educated decision.
Both cast iron and PVC have a place in the market. However, when value engineering from one to the other, ensure that value is indeed created, proper costs are returned to the owner, and the VE does not incur additional costs in the future. It is incumbent on the engineer to ensure that acceptance of a VE item does not result in de-value engineering of the project.
About the Authors
Eric Knauth, PE, LEED AP, has been in the plumbing industry for more than 20 years and is the Plumbing Department Manager for EXP. He is responsible for quality assurance, quality control, manpower staffing and planning, plumbing engineering and design, peer reviews, and plumbing commissioning. He is a Past President and current Board member of ASPE’s Central Florida Chapter and is a graduate of the Milwaukee School of Engineering.
Troy Johnson is a Regional Manager for McWane Plumbing Group. His primary focus is on educating engineers, architects, and building owners/developers on plumbing industry issues and solutions. He has worked for McWane Plumbing Group for approximately 10 years. Troy graduated from Texas State University and is a 10-year member of the DFW ASPE Chapter.
The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.