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Code Classroom Continued from page 16


teria growth, the flue gas temperature will be about 160 F when the heater is new. As scale builds up and the heater efficiency falls off, the


flue gas temperatures can easily increase to over 350 degrees F. Even if someone had their water heater set at 120 F, with scaling, the flue gas temperatures can rise well above 300 F. Boiler thermostats or burner controls are gen- erally limited to 200 F, commercial water heater ther- mostats or burner controls to 180 F and residential water heater burner controls to 160 F, and all can overshoot by several degrees. As scale builds up on the heating surfaces, the scale insulates the flue gases from the hot water in the system, causing the flue gas temperatures to increase. Some boiler and water heater manufacturers offer stack


or flue gas temperature gauges as a way to see whether the unit is scaling up and losing efficiency, which is helpful for monitoring the flue condition. A temperature sensor or


I have not seen any testing data or an independent test report from a boiler or water heater manufacturer that shows


that PVC piping has been tested and approved for the conditions it will likely see in a water heating or boiler installation.


probe with a high-limit control could be inserted into the flue at the flue connection to the boiler or water heater. This control would shut off the burner if the flue gas temperature exceeds the temperature rating of the flue pipe. A standard will be needed for plastic flue pipes that


should include a temperature gauge and a high limit probe. Then PVC, CPVC and polypropylene flue gas pip- ing can be safely used on high efficiency boilers and water heaters. This would be an answer to the dilemma of cost versus safety Without a standard for proper use of these safety


devices in combination with plastic flue gas piping or without the use of stainless steel flues, plastic flue materi- als can melt as flue gas temperatures rise. Not only is ener- gy lost when this happens but flues can become blocked or disconnected, which can be a carbon monoxide or a fire danger. A family of four died in Aspen, Colorado, in 2008, of


carbon monoxide poisoning from the failure of PVC plas- tic flue pipes on a condensing snow melting boiler system in a rental property. The plastic pipe manufacturer was not at fault, because they had published limitations on the use of their piping, and they had not recommended PVC pip- ing for that application. The boiler manufacturer that rec- ommended using PVC pipe as flue material was a target of the liability claim by surviving family members. I have heard arguments by many contractors that do not


Page 18/Plumbing Engineer


believe PVC flue venting for combustion gases is a prob- lem, but I have seen melted and discolored piping in many of my investigations, so I know it is a problem. I also see the proliferation of recommendations from high efficiency, condensing boiler and water heater equipment manufactur- ers for the use of combustible and unlisted PVC piping products as corrosion resistant combustion flue venting. This approach seems to be a way to lessen the initial


cost of installing a high efficiency boiler or water heater. High efficiency equipment will cost significantly more than less efficient models, so there seems to be a move- ment by manufacturers to promote these unlisted and, therefore, non-code-approved materials over code approved and listed stainless steel flues, which are corro- sion resistant. Some people argue that the mechanical code allows you


to follow the manufacturer’s recommendations. The 2009 International Mechanical Code (IMC) has the following language: 801.20 Plastic Vent Joints. Plastic pipe and fittings


used to vent appliances shall be installed in accordance with the appliance manufacturer’s installation instruc- tions.


Section 802 Vents 802.1 General. All vent systems shall be listed and


labeled. There is no listing for plastic piping for flue gas venting applications. The 2009 IMC also has the following language: Section 304 Installation 304.2 Conflicts. Where conflicts between this code and


the conditions of listing or the manufacturer’s installation instructions occur, the provisions of this code shall apply. Exception: Where a code provision is less restrictive


than the conditions of the listing of the equipment or appliance or the manufacturer’s installation instructions, the conditions of the listing and the manufacturer’s instal- lation instructions shall apply. There is a conflict between sections 801.20 and 802.1.


The water heater manufacturer’s installation instructions conflict with the requirement in section 802.1 that requires all vent systems to be listed and labeled for the applica- tion. PVC pipe is not listed and labeled as a combustion flue pipe material, as noted in the piping manufacturer’s notes above, yet manufacturers of the water heaters and boilers seem to be avoiding the issue, and they continue to recommend the use of PVC flue venting in their installa- tion instructions. Section 304.2 addresses conflicts. The code restriction requiring all flue materials to be listed and labeled for their intended purpose is more stringent language, so the more restrictive code requirement requiring listed and labeled flue pipe materials would apply. There is addi- tional language in the International Fuel Gas Code. I have not seen any testing data or an independent test


report from a boiler or water heater manufacturer that shows that PVC piping has been tested and approved for the conditions it will likely see in a water heating or boil-


Continued on page 20 May 2011


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