Hydraulic Balancing

Hydraulic balancing ensures that every consumer in a district heating network receives exactly the required volume flow rate — without over- or under-supply. Four systems are available: static balancing with balancing valves, automatic differential pressure controllers, combination valves (with valve authority of 1.0) and electronic pressure-independent valves with ultrasonic flow measurement. Without correct balancing, return temperatures rise, pump energy consumption increases and even an optimally sized pump cannot guarantee uniform supply.

Static Hydraulic Balancing

In static hydraulic balancing, each consumer is assigned a control valve that regulates the volume flow rate according to the current heat demand. In addition, a balancing valve is used that limits the maximum volume flow rate in the respective branch via a fixed presetting. The objective is to provide exactly the design volume flow rate at every consumer under full-load conditions.

The balancing valve is set to a fixed resistance during commissioning. It throttles the differential pressure in the respective branch so that no consumer is over- or under-supplied under design conditions. The setting values are derived from the pressure loss calculation of the network.

Problems During Part-Load Operation

Static balancing only works reliably under design conditions. As soon as individual consumers go into part load and their control valves close, the pressure distribution in the network changes:

• The available differential pressure at the remaining open consumers increases.
• Balancing valves with fixed presettings cannot compensate for this pressure increase.
• Consumers in full-load branches are over-supplied — return temperatures rise and network efficiency decreases.
• Consumers in hydraulically unfavourable branches may still be under-supplied.

With a constant pump control curve — i.e. a fixed differential pressure at the pump — this problem is further aggravated. During part load, when the total volume flow rate decreases, the available differential pressure in the network rises further. The consequences are increased flow rates at consumers with open valves, and as a result:

• Excessively high return temperatures due to insufficient heat transfer (the volume flow rate is higher than necessary for heat transfer)
• Increased pump power consumption
• Noise at heavily throttled valves

Static balancing is therefore only conditionally suitable for small networks with few consumers and limited part-load variation.

Automatic Differential Pressure Controller

The automatic differential pressure controller solves the central problem of static balancing by keeping the differential pressure across a system section — e.g. a branch or a transfer station — constant regardless of the load conditions in the rest of the network. The setpoint is typically in the range of 10 to 100 kPa and is set during commissioning.

Operating Principle

The differential pressure controller is a self-acting valve with a spring-loaded diaphragm. The differential pressure across the protected section is sensed via impulse lines. If the differential pressure rises above the setpoint — for example because neighbouring consumers go into part load — the controller closes and throttles the excess pressure. If the differential pressure falls below the setpoint, it opens accordingly.

Advantages

• No over- or under-supply: The differential pressure at the control valve remains constant, regardless of the load in other branches.
• Stable control: The control valve always operates under defined pressure conditions, enabling stable control behaviour without oscillation tendency.
• Supply temperature and efficiency independent of balancing: Since each branch receives the intended differential pressure, return temperatures and heat transfer remain in the optimal range.

Pump Control Curve

When using automatic differential pressure controllers, a proportional pump control curve is recommended. In this case, the pump’s differential pressure setpoint decreases with decreasing volume flow rate. This significantly reduces pump power consumption during part-load operation. It is important to select the minimum delivery pressure during part load such that a sufficient differential pressure is still available at the hydraulically most unfavourable consumer for proper operation of the differential pressure controller and the control valve.

Limitations

When using automatic differential pressure controllers, the minimum valve authority of the downstream control valves must be maintained. The valve authority $P_V$ describes the ratio of the pressure drop across the control valve to the total pressure drop of the controlled circuit:

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

For good control quality, $P_V \geq 0.5$ should be achieved. If the valve authority is too low, the control valve reacts disproportionately to small stroke changes, which can lead to unstable control behaviour.

Combination Valve (Pressure-Independent Control Valve)

The combination valve combines a differential pressure controller and control valve in a single housing. The pressure differential across the control valve section is factory-set to a fixed value — typically 20 kPa. Changes in network pressure are compensated by the integrated differential pressure controller without affecting the control characteristic of the valve.

Valve Authority

The decisive advantage of the combination valve is its valve authority of $P_V = 1.0$. Since the differential pressure controller absorbs all external pressure changes, the entire usable differential pressure always drops across the control valve section. The variable pressure drop across the controlled section is irrelevant for the control function. The valve therefore operates with optimal control characteristics under all operating conditions.

Advantages

• All advantages of the automatic differential pressure controller are retained.
• No mutual interference: Mass flow rate changes in one branch do not affect the control quality of neighbouring branches, since each combination valve regulates its own differential pressure.
• Simplified planning, as valve authority and differential pressure are factory-defined.
• Compact design compared to the combination of a separate differential pressure controller and control valve.

Pump Control Curve

As with the separate differential pressure controller, a proportional pump control curve with minimum delivery pressure is recommended. The minimum delivery pressure must be selected so that the input differential pressure required by the combination valve is reliably provided at the hydraulically most unfavourable consumer.

Electronic Pressure-Independent Control Valve

Electronic pressure-independent control valves represent the latest development in the field of hydraulic balancing. Instead of a mechanical differential pressure controller, they use real-time ultrasonic flow measurement. The current flow rate is continuously measured and compared with the setpoint. In case of deviations, an electronic actuator automatically corrects the valve opening.

Operating Principle

The valve receives a flow setpoint from the supervisory controller (e.g. a building automation system). The integrated ultrasonic sensor measures the actual flow rate without moving parts and without pressure loss. If the actual value deviates from the setpoint — for example due to pressure changes in the network — the actuator adjusts the valve position until setpoint and actual value match.

Advantages

• Lower differential pressure: Since no mechanical differential pressure controller with its own pressure loss is present, the total differential pressure in the network can be designed lower. This saves pump energy.
• Flow rate as a readable value: The current flow rate is available as a digital measurement value and can be transmitted to the building automation or network control system via bus systems (e.g. BACnet, Modbus). This simplifies monitoring and fault diagnosis.
• Pump optimisation: The real-time data from the flow valves can be used for dynamic adjustment of the pump speed. Instead of a rigid proportional curve, the pump can react to the actual conditions in the network on a demand-driven basis.
• Easy flushing: The valve can be fully opened for flushing operations without a mechanical differential pressure controller limiting the flow rate. This eliminates a common practical problem during commissioning and maintenance.

Pump Control Curve

A proportional pump control curve with minimum delivery pressure is also suitable for electronic pressure-independent valves. However, thanks to the real-time flow data from the valves, the minimum delivery pressure can be determined more precisely than with purely mechanical systems.

Comparison of Systems

The following table summarises the key differences between the four balancing systems:

*Observe minimum delivery pressure during part load.

The three dynamic systems (automatic differential pressure controller, combination valve, electronic pressure-independent valve) offer significantly higher supply reliability and energy efficiency compared to static balancing. The choice between them depends on the project requirements: the combination valve offers the simplest planning and commissioning, while the electronic pressure-independent valve provides additional benefits for monitoring and pump optimisation — at higher investment cost.

Conclusion

Hydraulic balancing is not a one-time commissioning step but a central planning aspect that affects the efficiency of a district heating network over its entire service life. Static balancing systems quickly reach their limits in larger networks with variable load profiles. Automatic differential pressure controllers, combination valves and electronic pressure-independent valves ensure supply quality even during part-load operation and enable energy-efficient pump control. Thermo-hydraulic simulation with tools such as VICUS Districts allows the effects of different balancing systems on volume flow distribution, return temperatures and pump power consumption to be quantitatively evaluated as early as the planning phase.

Further reading: Transfer Stations describes the design of heat transfer stations where hydraulic balancing is applied, Return Temperature Optimisation shows how correct balancing reduces return temperatures and improves network efficiency, and Pressure Loss Calculation explains the fundamentals of pressure loss determination required for balancing calculations.

References and Standards

• VDI 2073 Part 1 — Hydraulics of water-based systems — Fundamentals
• DIN EN 14336 — Heating systems in buildings — Installation and commissioning of water-based heating systems
• VDI 2035 Part 1 — Prevention of damage in water heating systems — Scale formation and waterside corrosion