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.