Dividing distribution circuits are sometimes referred to as constant temperature (CT) circuits. They divide the supply flow over a bypass in the distribution circuit to decrease the flow rate going further downstream.

Dividing circuits are historically commonly used whenever the design flow of the downstream system is equal to the designed supply flow temperature. By dividing the flow, the heat flow towards the end units decreases.

Dividing circuits are characterized by a constant flow temperature & variable flow rate at the secondary gate and a constant flow temperature & constant flow rate at the primary gate. When operating in partial load, the return temperature of a division circuit increases quasi-linear when the heat flow decreases. Because of this, dividing circuits generally perform worse than mixing circuits and throttle circuits within hydronic systems.

The main benefit of dividing circuits is that they have no delay in heat supply. They can provide immediate thermal comfort during start-up periods or within systems with long distances between the distribution unit and the plant room. The main drawback is the increasing return temperature when operating in partial load.

A more detailed explanation on the comparison of different hydronic distribution circuits can be found here: Distribution circuits

Dividing base circuit

Underlying components

A dividing BC usually contains the following kinds of components:

A 3 way control valve in return: Used to divide the supply flow into a part going towards the secondary gate and a part that runs over the bypass back to the primary gate. More information on control valve components can be found here: Control valves
A balance valve for primary balancing: Used for static balancing the system on full load conditions. More information on balance valve components can be found here: Balance valves
A balance valve on the bypass: Used for static balancing the system on partial load conditions.

Note that the passive dividing BC itself has no parameters on its own.

Different types of dividing BCs

Currently, there are two division circuits BCs. Both of them are ‘passive’ blocks.

Passive dividing

Operating principle

The passive dividing circuit controls the net thermal power passing through the circuit by varying the flow in the end unit. In full-load operation, the right through inlet gate of the 3-way mixing valve is completely opened whereas in partial load conditions, the 3-way mixing valves are positioned in the function of the process value (measured room temperature) and setpoint (room temperature).

System check

As this dividing circuit is a so called 'passive' circuit, the circuit needs a primary pump to extract hot water from the primary side. If the software encounters a missing pump on the primary side of the passive dividing circuit, it will report this as a problem while running the system check.

Mixing injection with primary dividing

This BC operates similarly to the ‘passive mixing’ BC, but is now a source of constant flow at the primary gate due to the primary dividing circuit. Combining the mixing & dividing circuits is mainly done to combine the advantages of the mixing circuit (constant flow at the secondary side & stable controllability) with the advantages of the dividing circuit (no delay on heat supply). The downside is an increased return temperature in partial load.