Temperature Pressure Relief valve and Discharge Tube
Before we talk about maintaining water heaters, it’s important to know about pressure and temperature.
Recommended pressure in water heaters is 150 PSI (pounds per square inch), now assume if the pressure rises to 300 or more PSI, what will happen (the temperature is under limits)? The tank will not hold the pressure and rupture. Once the tank rupture, the water will come to the atmospheric pressure, some water will leak, and the rest will stay in the tank. In the worst-case scenario, you would have a lot of water on the floor. As we all know, the water starts boiling at 100°C, but inside the tank, the boiling water could not turn into steam because the steam needs space. (FYI: Water expands 1700 times when it turns into steam) Now assume the same 300 PSI situation with 150 Degree Celsius temperature.
The recommended temperature in hot water tanks is 49°C (120°F). Due to the high pressure and temperature situation, the tank will rupture, and as soon as the water comes to the atmospheric pressure, it will turn into steam. Just assume 40 or 60 gallons of water turning into steam (need 1700 times more space). There would be an enormous amount of energy release, it can lift your house from the foundation, or it can blow the tank several 100 feet high in the air like a rocket.
If I scared you, I am sorry that’s not what I want to do. I just want to clear the concept, so you know the importance of TPR valves.
Let’s discuss TPR valves and what as a homeowner you should never do?
TPR or Temperature Pressure Relief Valves are designed to release the extra pressure (>150 PSI) and temperature (>99°C) that builds up into the water tanks. There is a discharge tube attached to that valve, which should always be discharged within 6 inches of the floor (If it releases from a high point, the hot water can scald someone nearby) One big mistake that homeowners make when they see the discharge pipe is leaking all the time, they plug the pipe or the valve. Now you know the concept, you can assume what will happen if the high pressure and temperature do not get access to go out.
As a homeowner, you should:
1. Never Ever plug the TPR valve or the discharge tube.
2. Always make sure the discharge tube is attached, and it’s releasing within 6 inches of the floor.
If you notice continuous leaking through the discharge pipe, the pressure and temperatures are continuously rising above the limits, which means the internal mechanism is not working correctly. It’s time to change the tank. In Quebec, insurance companies ask to change the Hot Water Tanks every 10 years (manufacturing date)
Reference: Carson Dunlop, InterNACHI, Mythbusters and other sources
Written by: Amandeep Singh (16/11/2020, 15:00)
Certified Building Inspector
M.Eng. Civil Engineering (Concordia University)