HVAC Valve Selection: Balancing Valves, PICVs and Butterfly Valves

What a valve has to carry in a chilled water circuit is flow, not kilowatts, and capacity alone does not fix that flow. In Q = m × cp × ΔT the specific heat of water is constant, so halving the design temperature difference doubles the flow for the same load. In working numbers: at a 5 K design difference, 1 kW of cooling load calls for about 0.17 m³/h of water.

Typical water-cooled plants are designed around 7/12 °C on the evaporator side and 30/35 °C on the condenser side, a 5 K difference on both circuits. That figure lives in the calculation sheet, but whether it survives in the field is decided by valve type, balancing solution and control strategy. Let the difference drop to 3 K and the flow required for the same load rises by roughly 67 percent.

So an HVAC specification should start from a circuit schedule rather than an equipment list: design flow, design temperature difference, available differential pressure and mode of operation written out circuit by circuit. The product families under valve and valve control systems can then be mapped onto that schedule instead of the other way round.

On the backbone of the system a valve has one job: to shut fully for maintenance. Chiller inlets and outlets, pump suction and discharge, header take-offs and risers are on/off duties where no throttling is expected. What matters is low resistance when open and reliable shut-off years later, and the tightness expectation can be written into the specification with reference to EN 12266-1.

Velocity drives the sizing. Mains distribution is commonly kept at 1.5-3.0 m/s and terminal branches at 0.6-1.2 m/s; above roughly 4 m/s flow noise becomes audible in occupied spaces. Isolation valves are selected line size, and a separate control element takes the throttling duty. A strainer with a 0.5-1 mm mesh ahead of control valves and PICVs appears in almost every specification.

At the terminal the picture changes, because the valve on a fan coil or air handling unit coil handles both isolation and flow adjustment. From DN15 to DN50 a ball valve plus a separate control element remains the economical answer, while from DN65 upwards the isolation duty starts moving to butterfly valves. On air handling unit coils that crossover usually happens somewhere around DN80 to DN100.

The advantage of a butterfly valve grows with size. At DN100 and above, a wafer or lug body is far lighter than a flanged ball valve of the same pressure class and its face-to-face length is much shorter, which pays off directly in tight plant rooms around pump sets and headers. Below DN50 the balance tips the other way and a ball valve is both cheaper and less restrictive.

For throttling, the usable band is narrow. Below 20 degrees and above 70 degrees of disc opening the relationship between flow and position becomes effectively uncontrollable, so the practical modulating band is 30 to 70 degrees. Outside that band local velocity at the disc edge rises sharply and brings noise and erosion with it. Where continuous modulation is required, plan the butterfly valve as an isolation device and give the modulating duty to a dedicated control valve.

Operation has a threshold of its own: lever operation is common up to DN200, and above that a gearbox becomes a practical necessity. Because the disc stays in the flow path even when fully open, the resistance coefficient of a butterfly valve is several times that of a full-bore ball valve; the two types are compared in detail in the article on butterfly valves versus ball valves.

A static, manually set balancing valve brings a circuit to its design flow through a preset stage and a pair of measuring ports. For the reading to be trustworthy, at least around 3 kPa of signal pressure has to be left across the valve; below that the flow measurement disappears into its own error band. More importantly, the setting is only valid at the operating point where it was taken.

That is exactly where variable flow systems break down. As terminal valves close, the available differential pressure across the circuit rises and the terminals still open draw more than their design flow. A pressure independent control valve combines a differential pressure regulator, a preset flow limiter and the control valve itself in one body, which removes that drift; depending on the model it holds the set flow across a working range that typically starts somewhere between 15 and 35 kPa and extends to several hundred kPa.

The second gain from a PICV is valve authority. A conventional two-port control valve is expected to keep authority above 0.5, and that authority is pressure deliberately burned in the circuit; in a PICV the regulator fixes the differential across the control element, so authority is practically 1 and no separate calculation is needed. Against that, a PICV sits in a higher first-cost band and tolerates dirty water far less well than a simple balancing valve.

Low delta-T syndrome is the condition where return water never reaches its design temperature and the system circulates ever more water to move the same load. In a circuit designed for 5 K, letting the difference fall to 3 K raises the flow requirement by roughly 67 percent, and since pump shaft power scales close to the cube of flow on a fixed system curve, the energy penalty is far steeper than the flow penalty.

The causes rarely come alone. Bypass circuits built around three-port valves hold primary flow constant even as load falls; oversized coils fail to warm the water enough at part load; fouled surfaces and trapped air cut heat transfer; and in a primary-secondary arrangement, secondary flow exceeding primary reverses flow in the decoupler, lifting supply temperature by 1 to 2 K.

The order of correction is well established: replace three-port bypasses with two-port valves or PICVs, rebalance the circuits, then move the pump to variable speed and lower the differential pressure setpoint to what the index circuit actually needs. Applied together, these three steps improve both pump energy and chiller loading; the process-side version of the same argument is set out in the article on energy efficiency in process lines.

Thermal inertia and comfort tolerance decide the control type. On/off valves driven by thermal actuators typically have a stroke time in the 180-300 s band and leave roughly ±0.5-1.0 K of swing in room temperature. On underfloor heating manifolds and small fan coils, where thermal inertia is high, that swing is not perceived and modulating control adds cost without adding comfort.

On air handling unit coils, in humidity-controlled spaces and on process cooling circuits, modulating control is genuinely required. Three-point floating drive builds position by chopping a 30-120 s stroke with open and close relays, while a 0-10 V or 4-20 mA drive commands position directly and confirms it with a feedback signal. In modulating service the actuator deadband should stay below 1 to 2 percent, or the loop will hunt.

Where an on/off valve has to imitate modulation, pulse width modulation is used, normally with a cycle time of 10 to 20 minutes; shorter cycles wear both the actuator and the seating surface. Behaviour on loss of power belongs to the same decision: if the valve must drive closed, specify a spring return or capacitor-backed actuated valve rather than relying on the control system to hold position.

On a chilled water circuit the valve body sits at 6-8 °C, while a space at 26 °C and 60 percent relative humidity has a dew point of roughly 17.6 °C. The body surface is therefore about ten degrees below dew point and will condense continuously unless it is insulated. On fan coil valves above a suspended ceiling, that dripping turns into permanent damage to the ceiling finish within a season.

Continuity matters more than thickness. Closed-cell elastomeric insulation of 13-19 mm is a common band for indoor chilled water lines, rising to 25-32 mm in humid spaces and outdoors. But if the vapour barrier is broken around a valve body, a flange or a measuring port, moisture migrates into the insulation and corrosion starts unseen on the pipe surface. This is why removable insulation jackets for valves and balancing valves belong in the specification.

Two details are routinely missed on the actuator side. First, an extended neck body or a stem extension keeps the actuator outside the insulated envelope; mounted straight onto a cold body, an actuator condenses inside its own enclosure. Second is ingress protection: IP54 suffices in dry areas, IP65 is expected in humid plant rooms and IP67 where flooding is possible. Wherever the layout allows, mount the actuator above the valve rather than below the horizontal axis of the stem.

Vanera is an industrial valve, actuator and control equipment supplier based in Umraniye, Istanbul, and supports HVAC projects in building a valve and balancing schedule circuit by circuit. Chiller and pump set isolation, header and riser valves, fan coil and air handling unit coil control elements and the actuator control type are all settled together at the quotation stage.

In existing buildings a field audit makes the source of low delta-T visible: the three-port bypasses still in place, the oversized control valves and the risers that were never balanced. Matching valve and actuator families to those circuits takes only three inputs: design flow, design temperature difference (typically 5 K on chilled water circuits) and the current pump curve, which can be sent through the contact page.