Heating, cooling and domestic hot water circuits have an associated fluid which carries thermal energy via the pipes to the various components in the circuit.
These components have blue ducts:
Different fluids have different thermal properties – they can carry different amounts of heat per given volume.
To calculate how the fluid transfers energy, you can specify the density, specific heat capacity and freezing point of the fluid:
Fluids can be assigned to a circuit by dragging the flid from the treeview onto the pipes in a circuit.
To determine which fluid is applied to a circuit, click on the pip and examine the properties window: