Thermal expansion control
In the connection of a water distribution system to water-heating appliances, there is the potential for the migration of heated water into the water distribution piping. In a typical water distribution system, the water will expand into the public water service. If the expansion of water is not accommodated in the system, dangerously high pressures can develop that can cause damage to piping, components and the water heater.
For water system sizes up to and including 2 inches (51 mm), a device for controlling pressure should be installed where the pressure on the downstream-side of a pressure-reducing valve exceeds the pressure-reducing valve setting. This is because of thermal expansion. Where a backflow-prevention device, check valve, or other device is installed on a water supply system using storage water heating equipment (such that thermal expansion causes an increase in pressure), a device for controlling pressure should be installed. This device can be an expansion tank with a flexible diaphragm inside it. www.apollohome.com