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What are the code requirements for the discharge piping from a Temperature-Pressure Relief (TPR) valve at a water heater?

Thursday, March 7, 2019

Temperature-Pressure Relief (TPR) valves rarely go off; but, if one does, it’s a big deal. Super-hot and high-pressure water gets released to keep the tank from exploding like a bomb. So, both the International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC P2804.6.1) have the following requirements to make sure that there are no obstructions or the possiblity of injury from the scalding water if the valve opens. Here they are outlined below. And, unfortunately, we have an abundance of examples of how not to do it.

1) The discharge pipe cannot be directly connected to the home’s plumbing drainage system, but an air gap located in the same room as the water heater can used to connect to the drainage system. An air gap eliminates the possibility of backflow from the drain into the discharge piping. The photo below shows TPR and catch pan drains that dump into a laundry sink without necessary minimum 1-inch air gap above rim of sink. To read more about it, see our blog post What is an indirect waste receptor?

2) The pipe should not be smaller than the outlet of the valve and must run full-size (no reductions) to the termination. 

3) Each TPR valve must have its own discharge piping, not shared with any other devices or equipment. In the example below, an upper pressure relief valve drain (not visible behind vent connector) has been tied into the drain for the lower one.

4) Termination of the discharge piping can be to any of the following:

5) The piping must discharge in a way that does not cause injury or damage. When there is no TPR discharge piping, or it terminates horizontally, anyone unlucky enough to standing in front of the water heater when the valve opens will be severely burned.

6) Must terminate in a location that is readily observable by the occupants of the home. If the water heater thermostat fails to turn off at its temperature setting and continues to heat the water indefinitely, the TPR valve will open and let the overheated water out; but, if the termination of the piping is not readily visible—such as if it is located under the house—the water will continue to drain until the homeowner gets an amazing electric or gas bill and goes searching for the reason. Also, sometimes the valve fails by opening partially and leaking a continuous small stream of hot water, like the one shown below, under an older mobile home..

7) The piping must drain by gravity and not be trapped.  A plumbing “trap" is a part of a pipe where water remains in place after gravity has drained the rest of the water. Because this stagnant water could sit for months and possibly drain backwards into the tank if the TPR valve is opened manually and there is no water pressure at the time, introducing bacteria into the tank, the pipe must completely drain by gravity. A trapped TPR dischage pipe is a common defect when a replacement water heater’s TPR valve is lower than the original one, and the installer connects to the exisitng discharge piping at the wall. 

8) Termination of the TPR drain pipe cannot be more than 6-inches, or less than two pipe diameters, above the floor, ground, or waster receptor flood rim level. Again, for safety of anyone nearby when it goes off. The example shown below, with the pipe almost touching the floor, will impede the water flow and cause it to splash upwards.

9) Must not have a threaded connection at the end of the piping. Multiple instances of water heater explosions have been caused by a handyman capping the end of a nuisance drip from TPR piping. Threading at the end appears to make it acceptable.

10) No valves or tee-fittings in the piping. Nothing should impede or split the direction of the flow. For some reason, the installer of the water heater shown below decided that the TPR drain piping needed a vent pipe.

11) Pipe should be of approved materials. Copper, glavanized steel, CPVC, and PEX or PE-RT are all approved. PVC is not approved, because it is not rated for transporting hot water.

12) If the discharge piping is PEX or PE-RT tubing, then it must be one nominal size larger than the relief valve outlet. Also, the flexible tubing should be fastened in place. 

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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