How It Works
When water is heated, its density decreases and volume expands. Because water is not compressible, the extra volume created by expansion needs somewhere to go. During no-flow periods in a system, pressure reducing valves, backflow preventers, and other one-way valves are closed, thus eliminating a path for expanded water to flow back to the system supply. That is when system pressure increases, causing a number of undesirable and potentially dangerous problems.
Plumbing Code Requirements
Both the International Plumbing Code (IPC) and Uniform Plumbing Code (UPC) require thermal expansion control to be addressed in plumbing systems. A temperature and pressure (T&P) relief valve is not considered a thermal expansion device. This is because when water is allowed to continuously drip from the T&P relief valve, minerals from the water can build up on the valve, eventually blocking it. This blockage can render the T&P valve useless and potentially lead to hot water heater explosions.