Thermal Expansion occurs when cold water is heated in the hot water heater. As the water heats, it expands and increases the pressure of your system. Utah Plumbing Code 607.3.2 states that a water system with a backflow device that creates thermal expansion must be equipped with a device for controlling pressure.
Installed on most water heaters there is a high-pressure relief valve that will discharge when excessive water pressure is achieved. This is why hot water heaters are to be installed near a floor drain that will allow the discharged water to flow to the sewer system. If the relief valve on your water heater has been removed or has not been tested annually by yourself or a plumber, trapped water between the water heater and the backflow device can build up excessive pressure. This will cause thermal expansion in the water heater which can cause a rupture to occur. The water heater relief valves should be checked on a regular basis to prevent this from happening.
Alternative protection is the installation of an expansion tank. This tank has a bladder in it that expands as the pressure increases and prevents the water pressure from developing in the first place.