Substations for indirect heating have a heat exchanger separating the primary flow from the secondary flow. Indirect substations are recommended for PN16 networks and higher pressure classes.
Substations for direct heating have no heat exchanger separating the primary flow from the secondary flow. Therefore the supply from the network will flow directly into the network in the building. Direct substations are recommended for maximum PN10 or PN6 networks. The domestic hot water is heated in a cylinder by the district heating water flow in a coil within the cylinder.