- First, since water expands when heated, the water pressure in the tank will increase as the water is superheated. If the pressure exceeds the maximum pressure threshold (approximately 300 psi) the tank could rupture or even explode. I have seen pictures where a house has been severely damaged or destroyed by exploding pressure vessels.
- Second, the release of superheated water (now significantly above 212°F) would immediately return to atmospheric pressure and flash into steam, causing a sudden increase in volume and release of energy. The force of the flash steam would exceed the explosion of one pound of (TNT)!
How TPR Valves Work:
If the water reaches a temperature of 210°F (before superheating occurs) or the water pressure exceeds 150 PSI, the valve will open and release the overheated water. This will allow cold water to enter, lowering the temperature of the water in the tank and consequently the water pressure. The valve will remain open as long as the temperature or pressure exceeds the preset limit. The valve must be located on the tank at the top or on the side near the top, where the water is generally the hottest. TPR valves are not included with the water heater, so be sure that the TPR valve matches the BTU (heat rating) rating of the water heater. A discharge tube must be attached to all TPR valves. A word about discharge tubes:
- The pipe itself must be made of a material that is rated for both high temperature and pressure, which includes most rigid wall copper or iron.
- The size of the pipe must match the opening size of the TPR valve discharge (usually ¾ inch).
- The tube must terminate no more than 6 inches from the floor or be directed to the exterior of the home.
Herein lay the issues that I find with plumbers or home handymen - laziness or lack of knowledge. Many do not install drain tubes, under size them, or install materials not rated for high temperatures. Water discharging at 150°F can cause serious burns on a person’s torso. Additionally, PVC pipe is rated for 140°F and will become soft and pliable in the event of a discharge. PVC pipe needs to be replaced with a metallic tubing or CPVC which is rated for higher temperatures. If you haven’t done so check to see if your water heater has a drain tube installed and what material is being used. If it is missing, undersized, does not extend to within 6” of the floor, or uses inappropriate materials such as PVC, call your local plumbing contractor. Michael Thornton – complete-home-inspections.com
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