Throttle circuits are sometimes referred to as constant temperature (CT) circuits. They decrease the supply flow in the throttle circuit to decrease the flow rate going further downstream.

Throttle circuits are whenever the design flow of the downstream system is equal to the designed supply flow temperature. However, they are historically less used than dividing circuits due to their highly dynamic operating principle. By throttling the flow, the heat flow towards the end units decreases.

Throttle circuits are characterized by a constant flow temperature at both the primary & secondary side and also by a variable flow rate at both sides. When operating in partial load, the return temperature of a throttle circuit decreases faster than linear with the heat flow. Because of this, throttle circuits generally perform better than dividing & mixing circuits within hydronic systems.

The main benefit of throttle circuits is their fast decrease in return temperature whenever the heat flow decreases from its designed peak load. Their main drawback is that they usually have a delay in heat supply. This can have an impact on thermal comfort during start-up periods or within systems with long distances between the distribution unit and the plant room.

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

Throttle base circuit

Underlying components

A throttle circuit usually contains the following kinds of components:

A 2 way control valve: Used to control the flow by throttling. More information on control valve components can be found here: Control valves
A balance valve: Used for static balancing the system on full load conditions. More information on balance valve components can be found here: Balance valves

Note that the throttle circuit BC itself has no parameters on its own.

Different types of throttle circuits

Currently, there are nine throttle circuits BCs. All of them are ‘passive’ blocks.

Throttle circuit

Operating principle

The throttle circuit controls the net thermal power passing through the circuit by varying the flow in the end unit. In full-load operation, the 2-way valve is completely opened whereas in partial load conditions the 2-way valve is positioned in the function of the process value (measured room temperature) and setpoint (room temperature).

System check

As this 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 circuit, it will report this as a problem while running the system check.

On-off valve

This BC operates similarly to the ‘throttle circuit’ BC, but can only switch between ‘fully open' and ‘fully closed’. It’s therefore usually not used to control the end units, but rather the production units. Whenever a boiler or heatpump is activated/deactivated, the on-off valve will be opened/closed.

Radiator valve

This BC operates similarly to the ‘throttle circuit’ BC, but has a (thermostatic) radiator valve instead of a regular 2 way control valve. More information about radiator valves can be found here: Control valves

Flow regulator with on-off valve

This BC operates similarly to the ‘on-off valve’ BC, but instead of a static balance valve, a flow regulator is used to dynamically balance the flow. More information about a flow regulator can be found here: Balance valves

Pressure independent control valve

A PICV functions as a control valve as well as a dynamic balance valve and can account for both static and dynamic imbalances. It is a relative new valve component and is generally more expensive than a regular throttle circuit. More informations of the parameters of the PICV can be found here: Balance valves

Control valve with differential pressure regulator

This BC operates similarly to the ‘pressure independent control valve’ block, but instead of a PICV, a balance valve, 2 way control valve and differential pressure regulator are used. This configuration used to be more common before the introduction of PICV valves. When the balance valve is outside, its pressure drop is not taken into account within the differential pressure measurement.

Differential pressure regulator

This BC contains two components: a differential pressure control valve (DPCV) and a balance valve. It is used to keep the differential pressure (the pressure drop at the secondary side of the BC) constant. As a DPCV cannot balance on its own, the balancer is used for static balancing. More information on a DPCV can be found here: Control valves

Reverse flow control

This BC contains four on-off valves which are controlled in pairs of two complementary sets. THis BC is used in systems designed to have bidirectional flows (like, for example, ATES/BTES systems). When the valves on the supply & return line are on, the valves on the bypasses are off. In this operation mode, the supply flow towards the secondary gate goes through the supply line and the return flow comes back through the return line. When the other way around, the flow towards the secondary gate is reversed.