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What is a trapped TPR (Temperature and Pressure Relief) valve at water heater?
Friday, July 6, 2018
A common defect we find when a water heater has been replaced by someone other than a professional plumber is that the TPR valve discharge pipe has been trapped, which is not allowed by the building code. Recently we had a homeowner ask us why we were calling out a trapped TPR discharge pipe when they couldn’t find any clogs, crimping, or obstructions in the line—because they misunderstood the meaning of a “trap.”
A plumbing trap is a section of pipe that allows a volume of water to stay in place after the pipe has drained by gravity. A P-trap under a sink is one kind of trap that is both useful and required, because the pool of water keeps sewer gas from rising up into a home from the sink drain.
But a trap in the discharge piping from a TPR valve—which allows standing water to sit in a the pipe after the valve has closed, likely grow bacteria in it over time, and then drain backwards into the water heater if the TPR valve is opened when there is no pressure in the tank—is a safety hazard. An example is shown at the top of the page, with an arrow indicating the height of a column of water that will remain in the pipe, and another one below.
The International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC 2803.6.3) both have a list of eleven requirements for TPR valve discharge piping, but the two that relate to this defect state that the piping must “be installed to flow by gravity” and “not be trapped.”
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