Just because a manufacturer recommends using PVC does not mean that it is acceptable or safe. He has asked every major manufacturer of boilers and water heaters for data to support the recommendations in their literature for the use of PVC pipe for combustion flue materials but has not received any replies. He said that the company has had numerous complaints about PVC pipes used for venting flue gases, and that they always point out that they do not recommend this usage and that there is no listing for it in their manual. He was quite familiar with this issue and emailed me a link to the company’s technical manual, which discussed all of the physical and temperature limitations of the piping. I called the manufacturer of the PVC pipe in this case and asked the representative a few questions about using PVC pipe as a combustion flue for fuel gases. Although the water heater installation manual we obtained recommended using PVC pipe as a flue material, PVC pipe manufacturers do not recommend this. The water heater had scaled up due to minerals in the water supply this caused the flue gas temperatures to rise, which created the noted problems. This set off the carbon monoxide alarm and prompted a maintenance call. A maintenance man for the university said that some really bad pipes had come apart at the fittings and melted. The flue pipes were obviously deformed from heat, and they were sagging. My inspection revealed a high efficiency water heater with purple/brownish PVC pipes and yellowish PVC flue pipe fittings. There were reported problems of not having enough hot water. The school had hundreds of apartments in numerous buildings with high efficiency water heaters that were installed more than 10 years ago. I recently inspected the mechanical systems in student housing at a Midwest university.
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