Hydraulic Circuit Types

The three basic hydraulic circuit types, the mixing circuit, the injection circuit and the throttle circuit, form the foundation of all heat distribution in thermal networks. The mixing circuit uses a three-port valve with a constant secondary mass flow. The injection circuit needs primary-side differential pressure to drive its two-port valve. The throttle circuit reduces the mass flow directly through a two-port valve. Which circuit is chosen has a strong effect on return temperatures, on control quality and on the hydraulic interaction between parallel heating groups.

Mixing Circuit

In the mixing circuit, return water is blended with the hot supply water to reach the desired supply temperature in the secondary circuit. A three-port valve does the blending, continuously adjusting the ratio of network supply to return flow. The mass flow rate on the secondary side (consumer circuit) stays roughly constant, while the primary-side mass flow rate varies.

A mixing circuit needs 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, and a dedicated pump provides the circulation in the secondary circuit.

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

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

In that case, blending through the three-port valve alone can no longer hold the supply temperature steady.

The mixing circuit cools the return flow fully, giving low return temperatures and good control of the supply temperature, all without any primary-side differential pressure. On the debit side, it requires a pressure-free or low-pressure manifold, and where several heating groups share one manifold they can interfere with each other. Typical applications are radiator circuits, underfloor heating and systems with condensing heat generators, where low return temperatures pay off particularly well.

Injection Circuit with Two-Port Valve

In the injection circuit, hot supply water is injected into the secondary circuit through a two-port valve. Unlike the mixing circuit, this arrangement is subject to differential pressure: the two-port valve needs sufficient differential pressure on the primary side to drive the injection flow.

The flow rate in the consumer circuit stays constant while the primary-side flow rate varies with the valve position, and the secondary pump keeps the flow through the consumers uniform. Because the primary side sees a variable mass flow rate, a variable-speed network pump is mandatory to hold a stable differential pressure at changing total volume flow rates.

The injection circuit cools the return flow completely for low return temperatures and controls well even on pressurized manifolds, and multiple parallel heating circuits barely influence one another hydraulically. It does require at least two circulation pumps, one on the primary and one on the secondary side, and differential pressure must be present on the primary side. Typical applications are district heating connections, whether direct or indirect, domestic hot water preparation, and 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. Unlike the injection circuit, the mass flow rate here varies on both the consumer and the producer side, so partially closing the valve reduces the flow through the whole circuit.

The throttle circuit is likewise subject to differential pressure and needs a variable-speed network pump to hold a stable differential pressure at varying volume flow rates. For indirect connection to thermal networks through a heat exchanger, its simple design makes it the preferred circuit.

Advantages are low return temperatures, straightforward individual room control via thermostatic valves and low equipment complexity. The drawbacks are a freezing risk with air heaters when the flow rate drops sharply at part load, and a fluctuating mass flow rate that can impair control quality if the design is unfavourable. Typical applications are thermal storage charging, zone control in larger buildings and indirect district heating connections with heat exchangers.

Comparison of Circuit Types

Control Valve and Valve Authority

Every hydraulic circuit type depends on the control valve being sized correctly. The key parameter is the $K_{VS}$ value, which gives 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 what gives a stable, proportional control characteristic. If it is too low, the characteristic becomes distorted and the valve modulates only in the last portion of its stroke, which leads 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 the thermal network is connected directly or indirectly.

With a direct connection, the network water flows straight through the building-side installation. No heat exchanger is needed, which simplifies the design and reduces investment costs. The trade-off is that the building installation must withstand the pressurization level of the network, and the network water quality acts directly on the building components. Injection and throttle circuits are especially common here.

An indirect connection separates the hydraulics with a heat exchanger, usually a plate type. Primary and secondary circuits are decoupled in pressure, so the secondary side can run at a separate, lower pressure level. On the primary side of the heat exchanger, the throttle circuit is the preferred option, since it combines the simplest design with low return temperatures.

Matching circuit to application

Mixing circuit, injection circuit and throttle circuit form the foundation of all hydraulic planning in thermal networks. The mixing circuit suits pressure-free manifolds with constant secondary flow rates. The injection circuit suits pressurized systems with several parallel heating groups. The throttle circuit suits simple circuits with variable flow. In every case, sizing the control valve correctly, above all keeping the minimum valve authority, is what makes operation stable and energy-efficient. In VICUS Districts, the 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