The 2012 International Residential Code (P2803.6.1) provides thirteen requirements for the water heater TPR discharge pipe.
1. Cannot be directly connected to the drainage system
2. Must discharge through an air gap
3. Must not be smaller than the diameter of the TPR outlet
4. Cannot serve multiple valves
5. Must discharge to the floor, water heater pan, waster receptor or outdoors
6. Discharge as to not cause personal injury or structural damage
7. Discharge to a point that is readily observable
8. No traps are allowed
9. Must flow by gravity
10. Minimum termination height of 6 inches above the floor or waste receptor
11. No threaded connection at the end of the pipe
12. No valves or fittings
13. Must be constructed of materials referenced in P2904.5* or tested, rated and approved in accordance with ASME A112.4.1
The Uniform Plumbing Code also has established requirements for the TPR drain. This information can be found in section 608.5 of the 2012 edition. Some areas that differ slightly include; language disallowing drains to terminate in the crawl space, minimum and maximum drain termination heights, and the requirement to terminate the discharge outdoors.
*Possible typographical error in the 2012 IRC. See P2905.5.
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