Venting Systems

What you need to know about standards and testing for systems of fuel-fired appliances.

Energy codes are driving the installation of boilers and domestic hot water heaters that are high-efficiency condensing Category IV appliances. The venting for these appliances tends to be factory-built, corrosion-resistant steel or PVC, CPVC, or PP. The following article discusses two critical issues regarding venting and possible solutions. First, there is no nationally recognized standard for plastic venting in the U.S. Second, there are no nationally recognized test criteria or standards for field testing installations of factory-built or plastic venting. 

The problem with plastic venting

First, a brief summary of basic definitions is in order. From the International Code Council (ICC), a chimney is “a primarily vertical structure containing one or more flues, for the purpose of carrying gaseous products of combustion and air from a fuel-burning appliance to the outdoor atmosphere.” A chimney can be factory-built (a listed and labeled chimney composed of factory-made components, assembled in the field in accordance with the manufacturer’s instructions and the conditions of the listing), a masonry chimney, or a metal chimney (similar to an unlisted NFPA 211-compliant smoke stack).

A vent is “a pipe or other conduit composed of factory-made components, containing a passageway for conveying combustion products and air to the atmosphere, listed and labeled for use with a specific type or class of appliance.” The 2015 International Mechanical Code (IMC) Commentary states, “In Code terminology, vents are distinguished from chimneys and are usually constructed of factory-made listed and labeled components intended to function as a system … Some appliances are designed for use with corrosion-resistant vents, such as those made of plastic pipe and special alloys of stainless steel.”

So what is the plastic pipe that conveys combustion by-products from the appliance to the exterior? It certainly is not a chimney. The IMC Commentary indicates that it is a vent; however, it also says a vent is listed, is labeled, and is a system. 

Forgiving our friends at the ICC for the deviation from UL’s definition of labeling, to what standards exactly are we listing and labeling? In 2012, UL provided a white paper entitled “Venting Gas-Fired Appliances” to help provide some clarity. Among others, it mentions the following standards:

UL 103: Standard for Factory-Built Chimneys for Residential Type and Building Heating Appliances

UL 959: Standard for Medium Heat Appliance Factory-Built Chimneys

UL 1738: Standard for Venting Systems for Gas-Burning Appliances, Categories II, III, and IV

These are some of the standards to which vents (and chimneys) are tested. UL indicates that “U.S. codes permit Category IV gas-fired appliances to be vented using unlisted plastic vents where such vents are tested and covered under the specific appliance listing.” By definition, an unlisted plastic vent is not a vent, at least applied to non-gas-fired appliances defined in the IMC. 

We also should consider section 501.14 of the International Fuel Gas Code (IFGC), which states, “The design, sizing, and installation of vents for Category II, III, and IV appliances shall be in accordance with the appliance manufacturer’s instructions.” 

Additionally, 502.1 of the IFGC states, “Plastic vents for Category IV appliances shall not be required to be listed and labeled where such vents are as specified by the appliance manufacturer and are installed in accordance with the appliance manufacturer’s instructions.”

Does this language absolve plastic pipe manufacturers of liability in cases where unlisted plastic pipe venting fails? This is unclear, and no doubt the U.S. courts will provide a response. What is clear is that appliance manufacturers are required by code to provide information to design, size, and install venting for their appliances. 

However, in general, it does not appear that appliance manufacturers are providing instructions for venting systems. 

Let’s explore one well-known and respected Category IV appliance manufacturer’s installation instructions. Its table of materials includes the following standards for vent pipe and fittings:

  • PVC Schedule 40, 80: ASTM D1785
  • CPVC Schedule 40/80: ASTM F441
  • Polypropylene: ULC-S636     
  • Stainless steel AL29-4C: UL 1738

ASTM D1785 “covers poly(vinyl chloride) (PVC) plastic pipe, Schedules 40, 80, and 120, for use with the distribution of pressurized liquids only,” according to the abstract. Additionally, Section 1.2, Note 2 of the Scope states, “This standard specifies dimensional, performance, and test requirements for plumbing and fluid-handling applications only. It does not include provisions for the use of these products for venting of combustion gases. UL 1738 is a standard that does include specific testing and marking requirements for flue gas venting products, including PVC.” ASTM F441 also explicitly states that the standard is for “pressurized liquids only.” 

ULC-S636 is a Canadian standard that provides some hope for a template of a U.S. or international standard, but it is noted that the temperature ratings listed in ULC-S636 exceed the working temperature ratings of many plastic pipe manufacturers.

Factory-built steel venting generally conforms to UL 1738. On a side note, Section 502.1 of the IFGC requires Category II and Category III appliance venting to be tested to UL 1738.

This appliance manufacturer does not provide a conformance standard for plastic fittings, nor is a support spacing listed. Plastic pipe manufacturers publishing hanger spacing generally base their requirements on the pipe being filled with liquid. Table 305.4 of the IMC lists the horizontal hanger spacing at 4 feet on center. Something as simple as vent support is not being provided by appliance manufacturers, which is a major problem.

Included in the appliance installation instructions is the following: “Installation of a PVC/CPVC vent system should adhere to the PVC/CPVC vent manufacturer’s installation instructions supplied with the vent system.” However, the U.S. plastic pipe manufacturers I spoke with discourage the use of their products for appliance venting, so it is unclear exactly what vent manufacturer instructions need to be complied with regarding plastic venting. I also spoke with a Canadian plastic pipe manufacturer that instructs conformance to ULC-S636. 

In summary, the industry has applied drainage pipe to venting fuel-fired appliances, but these vent systems have not been fully tested and are certainly not listed and labeled in most instances. Appliance manufacturers are now being held responsible for the vent system design, sizing, and installation, but their installation instructions are generally incomplete regarding fittings and support spacing. Also, most of the standards cited are explicitly limited to liquid applications. 

What would improve the safety of plastic venting? The vent and appliance manufacturers need to develop and agree on testing protocols. Appliance manufacturers need to explicitly test and list the venting to their appliance as well as provide complete installation instructions. Appliance manufacturers also need to place temperature-limiting devices on the appliances that can only be changed by factory-authorized personnel. This will prevent operators from overriding supply water temperatures and hence the maximum temperatures to which plastic venting is exposed. Finally, contractors need to be trained to install plastic and steel venting by the vent manufacturers.

Appliance vent testing

So, how do we test plumbing sanitary piping? If we decide to conform to the International Plumbing Code (IPC), we might fill the pipe with at least 10 feet of water, 5 psig of air, or smoke and see if it leaks. However, most plastic pipe manufacturers prohibit testing their pipe with air, and in the 2012 IPC, air was prohibited as a test fluid for plastic piping. 

Of the pipe manufacturers I spoke with, all of them espouse the need for safety and workplace protections, which would prohibit the use of air for testing. It appears that litigation and the fear of lawsuits have driven pipe manufacturers from permitting plastic piping being tested with air. This leaves those in cold climates to test with water and antifreeze, waiting for temporary heat or warm weather, or to choose materials other than plastic. Oddly enough, the Technical Standards & Safety Authority (TSSA) of Ontario, Canada issued a 2007 Advisory permitting the use of air to test plastic venting. 

How do we test ductwork? The engineer might specify the duct seal class, leakage class, and pressure class to provide the parameters to construct it and then require compliance with a standard such as the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) HVAC Air Duct Leakage Test Manual. In short, you blow air into the duct at a specified pressure and see how much it leaks — not all that much different from a smoke test of sanitary piping.

Appliance venting carries the products of combustion that can, and in unfortunate conditions do, kill humans. What do the IPC, IMC, IFGC, NFPA 211, most vent manufacturers (including plastic pipe), and appliance manufacturers say about testing installed appliance venting? Well, they say virtually nothing. However, NFPA 211 requires smoke testing of masonry chimneys, and the New York City Mechanical Code requires smoke testing of chimneys. The NYC MC smoke test requires “a pressure equivalent to ½-inch (13-mm) column of water,” but this violates the plastic pipe manufacturers’ recommendations and the code prohibition against testing with air.

Until 2016, I have heard of no manufacturer of factory-built metal venting agreeing to pressure testing of their installations. For many decades we have tested drain, waste, and vent piping and ductwork to improve energy efficiency/leakage, but we do no field testing of the system that can cause deaths. This appears to be a problem.

However, this certainly is not a technical problem, and we have the skill set to test appliance vents. There may be many root causes driving the trend against testing — from manufacturers of piping (venting) and appliances not wanting to be placed at a competitive disadvantage, to raw liability for individual manufacturers. One solution is for existing organizations to step up to the plate and drive consensus on a test standard. Some organizations that could facilitate resolution are the Plastics Pipe Institute (PPI), ASPE, ASHRAE, NFPA, UL, ICC, and a host of others. 

In an effort to address this life-safety issue, I identify core activities to help. First, manufacturers need to train the installers of factory-built chimneys and other systems. Training those who actually install the materials can only help reduce failures. We have pre-installation conferences for roofing and fire-smoke dampers — why not for venting? This would appear to solve the immediate short-term need. In the long run, manufacturers may need to certify their installers. 

Second, we need to test the installed vent. Visual observation coupled with the human sniff test does not equate to testing. Almost all AL-294C and other steel vent systems I observed post-construction have leaked. You may want to inspect yours when you are done reading this. 

Finally, the manufacturers must take responsibility for their products and certify in writing that the installation conforms to their installation instructions. If we can do it with kitchen floor assemblies, we can certainly do it with venting.

Working with Pathfinder Engineers ( and Precision Vent (a vent distributor,, we arrived at consensus language to achieve the above. This language is:

FABRICATION: Prior to the installation of AL 29-4C double-wall flues, there shall be an onsite pre-installation training conference. The flue manufacturer shall send a factory representative to the site to review installation procedures with the contractor and the owner’s field representative prior to commencement of installation.   

The representative shall be from the manufacturer’s factory, not the local sales representative. 

Topics to be covered at the pre-installation meeting shall include: 

  • Joining of Materials
  • Pitching Pipe
  • Hanger Systems
  • Draining Condensate From Low Points
  • Repair and Replacement of Unsatisfactory and Leaking Joints
  • Dry Fitting the Flue Components without O-Rings Installed Prior to Final Assembly

The manufacturer shall provide a written certification of the chimney installation once it is complete.

Testing of flues for CO leakage: The contractor shall provide an independent testing agency to “sniff test” the joints of each flue assembly for CO leakage while operating the boilers and water heaters. Defective joints and assembly shall be repair, replaced, and re-tested.”

We have included this language on multiple projects. It has been limited to AL 29-4C venting, but certainly could be expanded for other materials. The concentration limit of carbon monoxide is not yet defined, although OSHA 1910.1000 requires a maximum exposure of 50 parts per million (ppm) for general industry. Bluntly, if the vent is leaking, it should be fixed.

Additionally, a sniff test cannot be readily used for the vent assembly prior to the appliances being on site. In a tight construction schedule, it is common for the factory-built chimney (or plastic) to be installed and enclosed with brick prior to appliances being energized. As an alternate for testing the vertical vent, the NYC smoke test could be used with the manufacturer’s approval. We have done so with a pressure of not more than 2 inches of water column.

The final message here is to test appliance vents. You also may want to “retro-test” your vents in the immediate future.

This article was originally published in ASPE Journal 2015, American Society of Plumbing Engineers. All rights reserved. 

Doug Page PE, CPD, MPA, LEED AP BD+C, CEM, has been involved in the plumbing and mechanical industries for more than 33 years and is currently an engineer with the EME Group. Page received his BS from Clarkson University and has an MPA from the University at Albany/Rockefeller College. He is a Code Enforcement Official and a past member of the NYS Plumbing, Mechanical, and Fuel Gas Code Technical Subcommittee. He holds several industry certifications including CPD and is Past President of the Capital Region New York ASPE Chapter. Page can be reached on LinkedIn or via email at

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