The temperature difference between supply and return is a key indicator of hydraulic performance.
Large ΔΤ
Often indicates that the flow is lower than required or that there is a restriction in the circuit.
Small ΔΤ
May be related to high flow, low load, or a bypass.
Relationship with the installation
Proper selection of piping, circulator, and valves helps the system operate close to the ΔΤ specified by the manufacturer.