Quick summary:
- Motorised valves in HVAC and building services range from simple on/off zone control valves to modulating control valves with pressure-independent characterisation — choosing between them requires understanding the system hydraulics and control strategy
- Two-way and three-way valve configurations have fundamentally different effects on system hydraulic behaviour — three-way valves maintain constant flow through the primary circuit while diverting between circuits, which is critical for chiller and heat pump performance
- Actuator selection, flow characteristic, and Kv sizing must be matched to the specific application — oversized valves and mismatched characteristics are among the most common causes of poor HVAC control performance
The Role of Motorised Valves in Building Services Engineering
In commercial and industrial building services engineering, motorised valves are the final control elements for heating, cooling, ventilation, and domestic hot water systems. They translate a control signal from a building management system (BMS), room thermostat, or zone controller into a physical change in flow rate or flow direction — heating a zone, cooling a space, balancing a circuit, or shutting off supply to an unoccupied area.
The scale of motorised valve applications in building services is immense. A large commercial building may contain thousands of zone control valves, fan coil unit valves, AHU heating and cooling coil valves, radiator control valves, pressure-independent balancing valves, and domestic hot water zone valves. Each represents a point where system performance depends on correct valve selection, sizing, and integration with the building control system.
Poor valve selection — oversized valves, wrong characteristic, incorrect actuator sizing — is one of the leading causes of comfort complaints, energy inefficiency, and maintenance problems in building services. The investment in correct engineering at the specification stage pays back through the service life of the building.
Two-Way vs Three-Way Valves — Fundamental Hydraulic Differences
The choice between two-way and three-way motorised valves is one of the most fundamental decisions in HVAC system design, with significant consequences for system hydraulics, energy consumption, and control stability.
Two-way valves (2-port): Control flow through a single branch — either allowing or restricting flow in the pipe in which they are installed. When a two-way zone valve closes, flow through that zone stops entirely. In a multi-zone hydronic system with multiple two-way valves, the system flow varies dynamically as zones open and close. At part-load when most zones are closed, the pump must operate against a higher system resistance (deadhead condition if all zones close simultaneously) — which wastes energy and can damage the pump if there is no pressure relief.
Two-way valves are the dominant choice in modern variable-flow systems with variable speed pump drives. The pump speed modulates in response to differential pressure — as zones close and resistance increases, the pump slows to maintain the design differential pressure setpoint. This delivers significant energy savings at part load. Two-way valves are simpler, cheaper, and smaller than three-way alternatives.
Three-way valves (3-port mixing or diverting): Have three connections — two inlet/outlet ports and one common port. They either mix two inlet streams (mixing valve) or divert a single inlet stream between two outlets (diverting valve). Critically, a three-way valve in a correctly designed system maintains approximately constant total flow through the circuit while varying the proportion through each branch.
In a chilled water system where the chiller requires constant flow for stable operation, three-way valves at each coil allow the coil flow to be reduced to minimum (through the A-B port) while diverting the balance through the bypass (B-C port) back to the primary circuit — maintaining constant chiller flow. This is critical for chillers with fixed-speed compressors and evaporators that require minimum flow to prevent freezing. Variable-speed chillers can accept variable flow, but older fixed-speed plant with three-way valves is still common in refurbished buildings.
Three-way valves are also used in mixing applications — blending hot primary water with cooler return water to control secondary circuit temperature at the mixing valve, while the secondary circuit pump provides constant flow through the secondary distribution. This is the classic “primary/secondary” or “injection circuit” arrangement widely used in radiator and underfloor heating systems.
Kv Sizing — Selecting the Right Valve Flow Coefficient
Kv (flow coefficient in SI units) is the defining parameter for valve sizing in HVAC and building services. Kv is defined as the flow in m³/h of water at 15°C that will pass through the valve at a pressure drop of 1 bar (100 kPa). It is the metric equivalent of the US Cv (which uses gpm and psi).
Correct Kv selection requires knowing the design flow rate and the design pressure drop across the valve. The pressure drop across the valve at design flow should be at least 25-50% of the total circuit pressure drop — this ensures the valve has sufficient authority to control the circuit (authority = ΔPvalve / ΔPcircuit). A valve with authority below 0.25 is considered to have poor control authority — small changes in valve position produce negligible flow changes because the circuit resistance dominates over the valve resistance.
The Kv calculation: Kv = Q / √(ΔP), where Q is flow in m³/h and ΔP is pressure drop in bar across the fully open valve at design flow. The selected valve must have a Kv at or above this calculated value in its fully open position. Do not select a valve where the calculated Kv is at the very top of the valve’s range — the valve should be approximately 70-80% open at design conditions to leave control range both above and below the design point.
Oversizing is the most common Kv error in HVAC valve selection. An oversized valve operates at a very small opening for the majority of its operating range — where control precision is poor, where the flow characteristic is typically quick-opening (not the equal percentage needed for good control), and where valve wear is concentrated on a small area of the seating surface. The result is poor control, hunting, and premature valve failure.
Pressure-Independent Control Valves (PICVs)
Pressure-independent control valves combine a variable flow control valve with an integral differential pressure regulator in a single valve body. The PICV maintains a constant differential pressure across its internal control element regardless of changes in upstream system pressure — so the flow through the PICV at any given control signal position is constant, regardless of what other valves in the system are doing.
This is a significant advantage in variable-flow systems where pump differential pressure varies with load. In a system with conventional two-way control valves, when zones close and pump pressure rises, the remaining open zones receive higher flow than their design maximum — potential overheating, loss of control, and noise. PICVs prevent this: even as system pressure rises, the integral pressure regulator closes to maintain the set flow rate through each individual PICV.
PICVs also eliminate the need for separate commissioning balancing valves. The flow setpoint is factory-set or field-adjusted on the PICV to the required design flow for each zone — self-balancing the system without additional balancing valves. This simplifies commissioning, reduces installed cost, and improves resilience to system modifications (adding or removing zones does not require rebalancing the entire system).
Limitations: PICVs are more expensive than conventional control valves (typically 3-5× the cost). They have a defined operating pressure differential range — below minimum differential, the integral regulator cannot maintain constant flow (the system becomes under-pressurised). PICVs are only effective if the system pressure differential is maintained within the PICV’s specified range — which requires the pump control system to maintain adequate differential pressure, even at low overall system load.
Actuator Selection for HVAC Motorised Valves
HVAC motorised valve actuators are predominantly electric (thermoelectric or spring-return electric), unlike process plant actuators which are often pneumatic. The actuator characteristics must be matched to the valve and the control system:
On/off (two-position) actuators: Drive the valve fully open or fully closed in response to a binary signal. Used for zone isolation, simple zone control valves, and any application where proportional control is not required. Fail-safe action (spring return to open or spring return to close on power loss) must be specified based on system fail-safe requirements — does the zone valve need to fail open (maintain heating during a power failure) or fail closed (isolate the zone)?
Floating (tri-state) actuators: Move the valve toward open or closed at a fixed speed in response to a three-position control signal (open, close, or stop). Used with older bang-bang or tri-state BMS outputs. Control precision depends on the valve transit time relative to the control cycle time — too slow and the valve overshoots before the controller can detect the change.
Modulating (0-10V or 4-20mA) actuators: Position the valve at any point between fully open and fully closed in response to an analogue signal. Suitable for AHU control applications, temperature control of heating/cooling coils, and any application requiring true proportional control. The actuator must have adequate positioning resolution (typically ≤0.5% of full travel) and repeatability (≤1% of full travel) to achieve good control performance.
Actuator torque: The actuator must provide sufficient torque to fully close the valve against the maximum differential pressure in the system. Differential pressure at valve closure is typically highest when the pump is running and all other zones are closed — the full pump head is presented across the closed zone valve. Undersized actuators fail to close under this condition, resulting in uncontrolled heating or cooling in the zone. The valve manufacturer’s maximum close-off differential pressure rating must be verified against the system maximum pump head before actuator selection.
Flow Characteristics for HVAC Applications
In HVAC systems, the flow characteristic of the control valve determines how the heat transfer in the terminal unit changes with valve position. Most heat transfer surfaces (radiators, fan coil units, AHU coils) have a non-linear relationship between flow rate and heat transfer — heat output increases rapidly at low flow rates but becomes asymptotic at high flow rates. This inherent non-linearity of the heat emitter matches well with an equal percentage valve characteristic — the equal percentage valve has low Kv change at low openings (where heat output is most sensitive to flow) and high Kv change at high openings (where heat output is least sensitive to flow). The combination of equal percentage valve and typical heat emitter characteristic produces a roughly linear relationship between valve position and heat output — which is what BMS controllers are designed to control.
Linear characteristic valves are suitable for direct flow control applications where there is no non-linear heat transfer element in series — for example, in flow balancing applications or where the valve is directly controlling temperature in a very simple circuit. For most HVAC terminal unit applications, equal percentage is the preferred characteristic.
Motorised valve engineering for building services is a discipline where getting the basics right — Kv sizing, characteristic selection, actuator torque, valve authority — produces buildings that are comfortable, energy-efficient, and easy to commission. Get it wrong, and the resulting hunting, overheating, underheating, and noise complaints will follow the building through its entire service life.