How a Water Heater Explodes
Most residential single-family dwellings, condominiums, and townhouses come equipped with a storage-tank style water heater. Kinetic Molecular Theory of matter tells us that as matter heats up, it expands. This applies to solids and fluids (gasses, liquids). As water heats up, water expands. If that water is contained in a confined space with nowhere to go, it exerts increased pressure on the system. If pressure increases beyond the structural integrity of the vessel, failure of materials can occur, sometimes in spectacular fashion. In other words, things go boom.
Three main systems are in place to prevent that boom: The thermostat controls, the thermal expansion tank, and the Temperature & Pressure Relief Valve (TPR Valve).
The Thermostat
The Thermostat controls the temperature of the water in the tank and uses a negative feedback loop to monitor and regulate that temperature. As the consumer, you set the thermostat to your desired temperature (the set point), the gas ignites or the electrical elements get hot until it reaches the desired temperature. Once the temperature has reached it set point, the thermostat senses this, and tells the heating elements to turn off. With the heating element now off, the water begins to cool slowly. After the water has cooled to a certain point (or someone took a shower), the thermostat will tell the heating elements to turn on and begin the heating cycle again until the temperature of the water reaches its set point again. This cycle of heating and cooling repeats to maintain the water temperature of the tank. A malfunctioning thermostat may increase the water temperature to unsafe levels. If you notice sudden increases is temperature from your water heater, the first thing to check is the thermostat.
How to adjust the temperature on a gas water heater
How to adjust the temperature on an electric water heater
The Thermal Expansion Tank
Thermal Expansion Tanks should be installed on the water heaters cold water supply line. These tanks have a rubber diaphragm inside which allows a place for the hot water to safely expand into and reduce pressure on the entire plumbing system. One one side of the internal rubber diaphragm is the hot water, and on the other side is pressurized air. You should be able to see or feel a Schrader valve on the top of the expansion tank which allows the tank to be pumped up with air. This air pressure provides resistance against the expanding water from tearing the rubber diaphragm. Over time, the tanks can lose air and need re-pressurizing. Test your pressure tank: Take a pen and push in the Schrader valve briefly. If water squirts out, the rubber diaphragm inside as failed and the thermal expansion tank is completely non-functional and must be replaced with a new one.
How to test and maintain the thermal expansion tank
Temperature & Pressure Relief Valve (TPR Valve)
The TPR Valve (sometimes called a T&P Valve) is a special valve installed on your water heater which is designed to prevent explosion by opening and discharging water if the temperature or pressure reach dangerously high levels. TPR valves should be installed by a licensed plumber according to manufacturers instructions. If the valve can not open to release the pressure, the water heater can explode like a bomb or take off like a rocket. There are several things that can be wrong about a TPR valve installation, either the valve itself or the discharge piping (look for a future blog post!). Make sure your tank has a TPR valve installed and test it periodically.
Don’t let your water heater become a bomb and/or rocket!
Fortunately, water heater explosions are rare, because these safety systems are in place. Sometimes, however, they fail or are installed incorrectly. It happened in Phoenix, AZ. We always recommend plumbing work be done by a licensed and qualified plumber and always recommend you have your systems inspected by a licensed home inspector to ensure proper function and safe installation. We here at Kulshan Home Inspection Services know what to look for when it comes to water heaters and we strive to keep our clients educated and safe.
