Expansion and purge kits (EPK)

Source: Radiant Floor heating company - Filling the closed system

Expansion and purge kits are necessary for the heated fluid entering the hot water tank from the solar collectors and the boiler. Closed loops circulate the same fluid around and around, completely separate from the domestic water supply in the Stiebel Eltron tank. The expansion tank allows the fluid to expand with heat. Fluid is added when the system is installed through valves, and can be purged and replaced as needed. Each kit consists of an expansion tank, valves, and pressure gauges that allow fluid to enter and leave the circulation piping.

Normally, hot water from the boiler or antifreeze from the solar collectors enter from the left, travel through the Expansion and Purge Kit (EPK) and into the hot water tank.