As hot water leaves the system through the faucets, shower heads, or appliances, fresh cold water from the city main is supplied to the heater where it is heated for continuing use. Thermal expansion is the term use to describe the condition that occurs when water is heated and expands in volume. If there is no demand for hot water, the expanded volume of water has no place to go. This thermal expansion of water in a closed plumbing system can create a number of annoying and potentially dangerous problems. These include: the buildup of unusually high pressure in a system (even when a pressure reducing valve is installed); pressure surges; and the chronic or continuous dripping of a temperature and pressure (T&P) relief valve.
Plumbing codes require that thermal expansion control be addressed in plumbing systems. A temperature and pressure 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. The International Plumbing Code (IPC), Uniform Plumbing Code (UPC) and Standard Plumbing Code all require thermal expansion control to be addressed.