Article
Stainless Steel and Plastic Special Gas Venting Systems: Which Is the Best Option, and Why?
Finding the safest and most efficient gas venting exhaust systems for your high-efficiency boilers and hot water heaters can be challenging. For example, plastic is less expensive and may appear, at first glance, to be just as safe. But when you look at the total cost, along with regulatory requirements, installation guidelines and more, the answer is not as obvious. In fact, when you look closely at the differences, stainless steel products have a significant advantage over plastic ones. Let’s dive in deeper and see why.
The Importance of Safety and UL Listings
Safety is the most important aspect of a special gas venting system, but it is often overlooked. Gas-fired boilers and hot water heaters are used in high-occupancy buildings, including large residential complexes, schools and hospitals. While these venting systems have low emissions and relatively low exhaust gas temperatures, they carry the poisonous by-products of combustion, which are virtually undetectable without a carbon monoxide detector. Special gas venting systems must be clearly engineered, specified, installed and inspected to ensure building safety. The materials these systems are made with are critical.
Special gas venting systems constructed from stainless steel materials must be UL 1738 listed to comply with National Fuel Gas Code/NFPA 54, which requires stainless steel constructed products to be able to handle corrosive condensate, withstand high operational temperatures of 480° F or higher, and be pressure tested/rated. These stainless steel venting systems are often replaced by less expensive PVC, CPVC and PP products.
This choice presents risks, however, as a special gas vent constructed of plastic cannot meet the UL requirements—a UL 1738 listing requires a product to pass at temperatures unobtainable for plastic materials. Simply put, plastic is much more sensitive to high heat than metal. Some manufacturers claim their plastic products comply with UL 1738 standards, but this claim is, at best, misleading. Plastic products are, in general, ETL listed. This is a similar but easier threshold than the required UL standard.
Adhering to UL standard temperature requirements is extremely important when it comes to plastic products due to plastic’s sensitivity to high heat. One popular workaround has been adding an automatic system shutdown feature—in which the system turns off when it reaches a dangerously hot temperature in order to avert a system meltdown. This is a simple setting on the boiler that can be done by most maintenance staff or a boiler tech. Unfortunately, this can force appliances to shut down during peak periods, when building heat is needed most. Because it can withstand much higher temperatures, a metal gas venting system will not require a similar automatic shutdown feature.
Other Limitations with Plastic for Special Gas Venting Systems
Plastic materials make plastic pipes and tubes extremely sensitive to temperature changes, which can cause plastic to expand and contract. This significant thermal expansion and contraction requires a system layout that can accommodate it. This means you should restrict the use of:
Long Runs
When used for long-length runs, horizontal and/or stacks can experience significant expansion. This can cause the system to sag, bow or move.
Changes in Direction
Each change in direction places significant stress on the joints and elbows. This greatly increases the risk of system damage from expansion and contraction.
Insufficient Supports
The recommendation for most plastic materials is to use a support every five feet of a horizontal run and every other floor on stacks. This is significantly greater than the support needed for metal systems.
Always Choose Metal for Improved Safety and Design
Plastic materials for venting can appear ideal from a cost and functionality standpoint but neglect safety, design responsibility, product lifespan and installation concerns. A simple straight-up system configuration with various safety mechanisms in place can mitigate some of those concerns but is often unrealistic to install and can greatly increase costs. The end result might be a system that is more costly and less safe.
Stainless steel UL 1738 listed products, when produced by a reputable manufacturer, are safer, perform more consistently, and may even come with a lifetime warranty (something a plastic system is unlikely to offer).
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