Many homes have water heaters in the basement—or some other part of the home.
Sometimes the TPRV (Temperature Pressure Relief Valve) drain on the water heater runs to the exterior—sometimes it merely runs to the floor. As has been discussed before, the drain can not run up hill because water will be trapped against the valve leading to possible failure of the valve.
TPR Valves that only drain to the floor can cause damage to interior finishes if there is no acceptable place for water from the TPRV to drain to—like a floor drain. Current regulations require pans with drains under the heater if damage could be caused by leaks from the heater.
In some jurisdictions, when the TPRV drain can not run to the exterior due to the exterior grade being higher than the valve on the tank, the plumber will sometimes install a “PRV” (Pressure Relief Valve) on the plumbing system. This valve is only sensitive to elevated water pressure—-a condition that would relate directly to increased water temperature. The PRV is set at a lower pressure so that in the event that the house water system pressure was to exceed safe levels, the valve would open to lower the pressure. Being set at a lower pressure it will “blow off” before the higher setting of the TPRV, which might damage the finished interior of the home in the event that the TPRV were to open.
This next picture shows a typical installation.
Note the normal TPRV on the side of the tank with the drain going down to the floor area. In the picture one can see the top of the foundation and how it would be difficult to run the drain from the TPRV to the exterior without running up hill. Up near the ceiling there is the PRV and its drain to the exterior just above the top of the foundation.
Charles Buell, Seattle Home Inspector
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