The paddle of water under the tank-type water heater, either gas-powered or electric, is often related to water leaking and condensation. What if there is dripping from the relief valve or a fixture, during the unit’s recovery cycle, when no hot or cold water is used? Find out what the cause of the dripping is and how to fix it.
Before we tackle the problem, let’s see what the relief valve, expansion tank, and thermal expansion are, how these elements are related to water dripping issue, and what it can be done to help in troubleshooting.
The temperature and pressure relief valve or short TPR is the safety feature that is designed to reduce the buildup pressure inside the tank by releasing hot water out. The buildup pressure is the result of the thermal expansion inside the system – especially when the temperature and pressure go beyond allowed levels.
The TPR valve is used then to release that pressure so the water heater tank and elements do not get too much stress, which can lead to creaking metal sound, flue distortion and exhaust gas leak, premature element failure, deformation, bulging, even rupture.
Occasional dripping from the TPR valve is the reason why you will see some wet spots at the end of the discharge pipe.
Thermal expansion followed by the extreme pressure and temperature problem, as explained, happens when water is heated in the closed system – a system that has one-way valves such as back-flow valves, pressure reducing valve, check valve or even water softeners.
When water is heated in such a system, it expands and increases its volume putting pressure on the heater and plumbing system. Therefore, an expansion tank or vessel must be installed to compensate for an increase in the water volume.
Before installing an expansion tank, make sure to confirm the thermal expansion in the water heater, by performing the following test:
If the system is closed, the pressure will start increasing fast and steady.
Once the pressure reaches the maximum value set on the TPR valve, the valve will open, release some water followed by the pressure drop. The valve will remain closed if the pressure inside the system does not exceed this value – otherwise, you will see dripping.
One of the solutions for hot water dripping from the TPR valve or faucet is simple - install a pressure relief valve - PRV and the expansion tank on the supply line.
The PRV valve is needed to reduce the water pressure from the municipality pipeline system if it is high. The water heater is operating with the maximum pressure between 40 and 60 psi while the maximum pressure of the incoming water can reach 80 psi – so it must be reduced.
The expansion tank is usually installed on the cold-water side, on the incoming water pipe, between the check valve and water heater. It is designed as a small tank that has two chambers, one side that contains air under pressure, and the other side filled with water - connected to plumbing, and the rubber diaphragm that divides these two chambers.
The rubber diaphragm can move, depending on the inside pressure – so when water pressure increases the diaphragm moves toward the air chamber, compressing the air, taking the extra water and reducing the pressure. For the residential use, the device is designed to deal with the pressure up to 150 psi.
With these elements installed, and if you repeat the above test, you will see that the pressure on the gauge will increase slightly (not sharp and high) and will keep that way throughout the heating process. The portion of water that has expanded and the pressure will not put any stress on the tank or element, as it has been released into the expansion tank.
A dripping pressure relief valve is a good indicator of the thermal expansion – do not replace the element right away, perform the test as the valve might not be broken.
Thermal expansion, followed by the extreme pressure and temperature can be very dangerous; it can affect normal water heater operation and even cause irreparable damages.
Therefore, even if the building code doesn't require installation of the expansion tank it is recommended to install one. Talk to your plumber to install the right size and pressurize it correctly. Also, once a year check out the elements to confirm its functionality.