Hydraulic Circuit Types

The three basic hydraulic circuit types — mixing circuit, injection circuit and throttle circuit — form the foundation of all heat distribution in thermal networks. The mixing circuit uses a three-port valve with constant secondary mass flow, the injection circuit requires primary-side differential pressure for two-port valve control, and the throttle circuit reduces the mass flow directly via a two-port valve. The choice of circuit type decisively influences return temperatures, control quality and the hydraulic interaction between parallel heating groups.

Mixing Circuit

In the mixing circuit, return water is blended with the hot supply water to achieve the desired supply temperature in the secondary circuit. Blending takes place via a three-port valve that continuously adjusts the ratio of network supply and return flow. The mass flow rate on the secondary side (consumer circuit) remains approximately constant, while the primary-side mass flow rate varies.

The mixing circuit requires a pressure-free or low-pressure secondary side — typically a hydraulic manifold without significant differential pressure of its own. No differential pressure is required on the primary side; a dedicated pump provides circulation in the secondary circuit.

When the temperature ratio between distribution and heating circuit temperatures becomes too large, a fixed bypass is required. As a rule of thumb, a fixed bypass is necessary when

$\frac{T_{\text{supply,distribution}} - T_{\text{return,heating circuit}}}{T_{\text{supply,heating circuit}} - T_{\text{return,heating circuit}}} > 2{.}5$

In this case, blending through the three-port valve alone is not sufficient to ensure stable temperature control.

Advantages:

• Low return temperatures, as the return flow is fully cooled
• Good controllability of supply temperature
• No primary-side differential pressure required

Disadvantages:

• With multiple heating groups on the same manifold, mutual interference can occur
• Pressure-free or low-pressure manifold required

Applications: Radiator circuits, underfloor heating, systems with condensing heat generators where low return temperatures are particularly beneficial.

Injection Circuit with Two-Port Valve

In the injection circuit, hot supply water is injected into the secondary circuit via a two-port valve. In contrast to the mixing circuit, this circuit is subject to differential pressure — the two-port valve requires a sufficient differential pressure on the primary side to drive the injection flow.

The flow rate in the consumer circuit remains constant, while the primary-side flow rate varies depending on the valve position. The secondary pump ensures uniform flow through the consumers. Since the primary side has a variable mass flow rate, a variable-speed network pump is mandatory in order to maintain stable differential pressure at varying total volume flow rates.

Advantages:

• Low return temperatures through complete cooling
• Good controllability even at pressurized manifolds
• Multiple parallel heating circuits have little mutual hydraulic influence

Disadvantages:

• At least two circulation pumps required (primary and secondary side)
• Differential pressure must be present on the primary side

Applications: District heating connections (direct and indirect), domestic hot water preparation, systems with multiple parallel heating groups.

Throttle Circuit

The throttle circuit is the simplest of the three basic configurations. A two-port valve throttles the mass flow rate through the consumer directly. In contrast to the injection circuit, the mass flow rate here is variable on both the consumer side and the producer side — when the valve is partially closed, the flow rate through the entire circuit decreases.

The throttle circuit is also subject to differential pressure and requires a variable-speed network pump to provide stable differential pressure at varying volume flow rates. For indirect connection to thermal networks (with heat exchanger), it is the preferred circuit due to its simplest design.

Advantages:

• Low return temperatures
• Simple individual room control via thermostatic valves possible
• Low equipment complexity

Disadvantages:

• Freezing risk with air heaters due to severely reduced flow rate during part-load operation
• Fluctuating mass flow rate can impair control quality if the design is unfavourable

Applications: Thermal storage charging, zone control in larger buildings, indirect district heating connections with heat exchangers.

Comparison of Circuit Types

Control Valve and Valve Authority

The function of every hydraulic circuit type depends critically on the correct sizing of the control valve. The key parameter is the $K_{VS}$ value, which specifies the volume flow rate in $\text{m}^3/\text{h}$ that passes through the fully open valve at a pressure drop of 1 bar (100 kPa):

$K_{VS} = \dot{V} \cdot \sqrt{\frac{\Delta p_0}{\Delta p_V}}$

where $\dot{V}$ is the nominal volume flow rate, $\Delta p_0 = 100 \; \text{kPa}$ is the reference pressure loss and $\Delta p_V$ is the actual pressure drop across the valve.

The valve authority $P_V$ describes the ratio of the pressure drop across the fully open valve to the total pressure drop of the controlled circuit:

$P_V = \frac{\Delta p_{\text{valve}}}{\Delta p_{\text{valve}} + \Delta p_{\text{circuit}}}$

Adequate valve authority is crucial for a stable and proportional control characteristic. If the valve authority is too low, the control characteristic becomes distorted — the valve only modulates in the last portion of its stroke, leading to unstable behaviour. The following minimum values apply:

• Three-port valves (mixing circuit): $P_V \geq 0.5$
• Two-port valves (injection/throttle circuit): $P_V \geq 0.3$, i.e. at least 30% of the maximum differential pressure drops across the valve

As a guideline for the minimum pressure drops across the control valve:

Direct and Indirect Connection

The choice of hydraulic circuit type also depends on whether a direct or indirect connection to the thermal network is used.

With a direct connection, the network water flows directly through the building-side installation. No heat exchanger is required, which simplifies the design and reduces investment costs. However, there are risks: the building installation must be designed for the pressurization level of the network, and the water quality of the network acts directly on the building components. Injection and throttle circuits are particularly common in this configuration.

With an indirect connection, hydraulic separation is achieved via a heat exchanger (typically a plate heat exchanger). The primary and secondary circuits are decoupled in terms of pressure, so that a separate, lower pressure level can be selected on the secondary side. The throttle circuit on the primary side of the heat exchanger is the preferred option for indirect connections, as it offers the simplest design while achieving low return temperatures.

Conclusion

Mixing circuit, injection circuit and throttle circuit form the foundation of all hydraulic planning in thermal networks. The mixing circuit is suitable for pressure-free manifolds with constant secondary flow rates, the injection circuit for pressurized systems with multiple parallel heating groups, and the throttle circuit for simple circuits with variable flow. In every case, correct sizing of the control valve — in particular maintaining the minimum valve authority — is crucial for stable and energy-efficient operation. In VICUS Districts, the various circuit types can be modelled directly in the network model and their hydraulic interaction simulated dynamically.

Further reading: Transfer Stations describes the complete design of the heat transfer station in which the circuit types are embedded, Hydraulic Balancing covers the coordination of hydraulic resistances for uniform supply to all consumers, and Network Temperatures explains the temperature conditions that must be considered when selecting the circuit type.

References and Standards

• VDI 2073 Part 1 — Hydraulics of water-based systems — Fundamentals
• Recknagel, H.; Sprenger, E.; Schramek, E.-R.: Taschenbuch fuer Heizung + Klimatechnik. DIV Deutscher Industrieverlag (standard reference, continuously updated).
• DIN EN 12828 — Heating systems in buildings — Design of water-based heating systems