Two-Way Control Valves

The body contains the flow path and connects to the pipework. It is made in brass, bronze, cast iron, ductile iron or stainless steel depending on size and application.

The regulating element shapes the valve’s flow characteristic. Globe valves use a plug; ball valves use a characterized ball or disc; butterfly valves use a rotating disc.

The seat and seals determine shut-off tightness and leakage class – important for energy efficiency and preventing passing valves.

The stem transfers actuator movement to the plug or ball. Packing and O-rings seal the stem to prevent gland leakage.

The actuator drives the valve. A linkage or direct-coupling adapter connects it to the stem, and must match the valve’s stroke or rotation.

A two-way globe control valve uses a linear-stroke plug and seat. It offers excellent control resolution and a well-defined characteristic, and is common on AHUs, heat exchangers and larger coils.

A two-way control ball valve uses a quarter-turn ball with a characterizing disc to give an equal percentage characteristic. It is compact, has tight shut-off and high close-off pressure, and is widely used on FCUs and small to medium AHUs.

Motorized butterfly valves suit large pipe sizes, typically DN65 and above. They are used mainly for isolation and two-position duties at chillers, cooling towers and headers, and for some large modulating applications.

Compact zone valves with thermoelectric or small motorized actuators provide simple on/off control for FCUs, chilled beams and radiators – the most economical fan coil control valve option.

A standard two-way valve is pressure dependent: its flow changes whenever system pressure changes. A 2 way pressure independent control valve (PICV) adds a built-in DP regulator and flow limiter, so flow depends only on valve position. Standard valves cost less; PICVs control better at part load.

Normally open valves open when power is lost; normally closed valves close. The choice depends on the application – for example, heating coils in cold climates may be normally open for freeze protection, while most chilled water FCU valves are normally closed.

A two-way on off control valve is either fully open or fully closed. It is simple and economical, and suits FCUs, chilled beams and zones where some temperature swing is acceptable.

Floating control uses open and close signals to move the valve incrementally. It provides modulating-type control with simple wiring and is common in many FCU and AHU controllers.

A two-way modulating control valve positions the valve proportionally to a 0-10V control valve or 4–20mA signal. It gives smooth, accurate temperature control and is the standard for AHUs, critical areas and high-specification FCUs.

An equal percentage control valve increases flow by the same percentage for each equal step of opening. Because coil heat output rises steeply at low flow, this characteristic helps give a roughly linear relationship between valve position and heat output – ideal for most HVAC coils.

Flow is directly proportional to opening. Linear valves suit applications where the controlled output is proportional to flow, and are often used with PICVs and some heat exchanger applications.

Most flow is delivered at small openings. This suits on/off duties rather than modulating control.

The inherent characteristic is measured at constant DP. In a real system, DP across the valve changes as it opens, distorting the characteristic – this is the installed characteristic. Good authority keeps the installed characteristic close to the inherent one.

Control valve authority is the ratio of the valve’s pressure drop when fully open to the pressure drop across the valve when fully closed (the available circuit DP). An authority of around 0.25–0.5 or higher is typically targeted for stable modulating control. Low authority – often caused by oversizing – leads to hunting.

Rangeability is the ratio between maximum and minimum controllable flow. High rangeability allows stable control at low loads, which is important in buildings that spend most of the year at part load.

Kvs is the flow in m³/h through the fully open valve at 1 bar pressure drop. Cv is the imperial equivalent (US GPM at 1 psi), where Cv ≈ 1.16 × Kv. This is the key value for sizing.

Most HVAC control valves are PN16 or PN25. High-rise buildings and district cooling networks often require PN25.

The maximum DP against which the valve and actuator can close tightly. It must exceed the highest DP the valve will see, especially near the pump at low load.

Leakage class defines how much water passes when the valve is closed. Tight shut-off prevents energy waste and passing valves.

Valves are rated for chilled and hot water within specified temperatures, and most accept glycol mixtures within the manufacturer’s limits.

Globe valves have a defined stroke length; ball valves typically rotate 90°. The actuator must match exactly for full-range control.

Silent, compact and economical, thermoelectric actuators provide on/off control for FCUs, chilled beams and radiators. They open or close slowly using a heated wax element.

A two-way control valve with a modulating actuator positions the valve accurately in proportion to the control signal – the standard for AHUs and precise temperature control.

Floating actuators respond to open and close pulses, giving modulating-type control with simple controller outputs.

Spring return actuators drive the valve to a safe position – open or closed – on power loss. They are used for freeze protection, critical AHUs and process safety.

Communicating actuators report position, alarms and diagnostics over BACnet or Modbus, simplifying BMS integration and supporting optimisation strategies.

Pneumatic actuators are used in some industrial and process plants with instrument air, and in older buildings with pneumatic controls. Two-way pneumatic control valves provide high force and fast response where compressed air is available.

The actuator must provide enough force (globe valves) or torque (ball and butterfly valves) to achieve the required close-off pressure. Undersized actuators cause passing valves; oversized ones add unnecessary cost. Every HVAC valve actuator we supply is checked against the valve’s close-off data.

Standard for small threaded valves, typically DN15–DN50, used on FCUs, small AHUs and heating circuits.

Used for larger flanged valves on AHUs, heat exchangers and plant equipment. Ductile iron suits higher pressure ratings.

Stainless steel valves are used for process water, aggressive fluids or hygienic applications in pharmaceutical and food facilities.

• Threaded ends – FCU and small AHU valves
• Flanged ends – larger AHU, heat exchanger and plant valves
• Grooved ends – faster installation in grooved piping systems

A two-way control valve for FCU circuits – the classic FCU 2 way valve – is usually a DN15–DN20 zone valve or characterized ball valve with on/off, floating or modulating control. It is the most common FCU chilled water control valve in offices, hotels and hospitals.

A two-way control valve for AHU cooling coils is typically a globe or characterized ball valve with a modulating actuator. As an AHU chilled water control valve and AHU cooling coil control valve, it controls supply air temperature and, in many cases, humidity.

Small two-way valves with thermoelectric or modulating actuators give precise control of chilled beams and radiant panels.

On plate heat exchangers, two-way valves control primary-side flow to maintain secondary supply temperature.

A two-way hot water control valve or heating coil control valve controls reheat coils, AHU heating coils and radiators. Temperature ratings and fail-safe position are confirmed for each application.

Motorized butterfly or ball valves isolate chillers, cooling tower cells and pumps when they are off, preventing flow through idle equipment and supporting correct plant sequencing.

Pumps with VFDs respond to changing flow, usually controlled by a chilled water differential pressure sensor at a remote point in the system.

Chillers need a minimum evaporator flow. A modulating bypass valve maintains this flow when most two-way valves are closed.

If every valve closes, pumps could run against a closed system. VFD minimum speeds, bypasses or a small number of three-way valves prevent this.

Standard two-way valves are pressure dependent. Balancing valves set design flow at each coil, while DPCVs on risers or branches stabilise DP and protect control valve differential pressure at part load.

Some designs keep a small flow at the ends of long runs to maintain supply temperature at remote terminals, especially where response time matters.

Flow through a standard two-way valve changes with system pressure, not just valve position.

Balancing valves or presettable valves are needed to set design flow at each coil.

Valves selected by pipe size are often oversized, giving low authority and unstable control.

Valves near the pump may see very high DP at part load, causing noise, erosion and difficulty closing.

Oversizing, poor authority and untuned controllers cause temperatures to swing and actuators to wear.

Without minimum-flow provisions, pumps can deadhead when most valves close. Correct system design prevents this.

Any two-way control valve for a variable flow HVAC system is the natural choice – they are what make variable flow possible.

Where DPCVs stabilise DP on risers or branches, standard two-way valves at terminals perform well at a lower cost than PICVs.

Standard two-way valves deliver the energy benefits of variable flow at the lowest valve cost.

For hotels, apartments and offices where simple on/off control is acceptable, two-way zone valves with thermoelectric actuators are economical and reliable.

Valves are sized on design flow and a target pressure drop. Line-size valves are usually oversized, giving poor authority.

Control valve Kv calculation uses the standard relationship:

K_v = \frac{Q}{\sqrt{\Delta P}}

where Q is flow in m³/h and ΔP is the valve pressure drop in bar. For control valve Cv calculation, Cv ≈ 1.16 × Kv. Select the nearest standard Kvs, then recheck the actual pressure drop.

For modulating control, the valve’s pressure drop at design flow should typically be similar to the pressure drop of the coil and branch it controls, giving an authority of around 0.5.

Choose a valve and actuator combination whose close-off pressure exceeds the maximum DP expected at the valve, including low-load conditions near the pump.

Equal percentage suits most HVAC coils; linear suits some heat exchanger and PICV applications; quick opening suits on/off duty only.

A 2 TR FCU (about 7.0 kW) at a 5.5 K delta-T needs about 1.1 m³/h. Targeting a valve pressure drop of about 20 kPa (0.2 bar) gives a required Kv of roughly 2.5. A DN15 valve with Kvs 2.5 is typically selected, with an on/off or modulating actuator to suit the controller. These figures are illustrative; final selection follows the manufacturer’s data.

A 25 TR AHU coil (about 88 kW) at a 5.5 K delta-T needs about 13.7 m³/h. If the coil and branch pressure drop is about 40 kPa, targeting a similar valve pressure drop gives a required Kv of roughly 22. The nearest standard Kvs is 25, typically a DN40 globe or characterized ball valve, giving an actual valve drop of about 30 kPa and an authority of around 0.43. A modulating actuator with adequate close-off is then selected. These figures are illustrative; final selection follows the manufacturer’s data.

Belimo two-way control valves are widely used characterized ball and globe valves with a broad range of matched actuators.

Siemens offers globe and ball control valves with actuators designed to integrate with its building automation systems.

Honeywell supplies zone valves, two-way motorised valves and control valves with matching actuators for FCU and AHU applications.

Danfoss provides 2 way control valves and actuators alongside its balancing and PICV ranges.

Johnson Controls offers control valves and actuators often specified with its BMS platforms.

We also supply reputable Indian and international brands to suit your specifications, budget and delivery schedule. Share your approved make list and we will propose matched valve and actuator options.

Valves are installed in the marked flow direction, usually on the coil return. Reversed installation can cause noise, poor control or failure to close.

Actuators are mounted within the manufacturer’s allowed orientations – generally not hanging below the valve on chilled water, where condensation could drip into the actuator.

Strainers protect the valve seat from debris; isolation valves allow valve or actuator replacement without draining the system.

A balancing valve in series with each two-way valve sets design flow and provides a measuring point, unless a PICV or DPCV-controlled branch makes it unnecessary.

Actuators need clear access for wiring checks, manual override and replacement through ceiling panels or FCU access covers.

Chilled water valve bodies are insulated to prevent condensation, while keeping actuators, manual overrides and test points accessible.

Each valve is driven from fully closed to fully open to confirm full stroke and smooth movement.

We confirm that the valve responds correctly to the controller signal and that position feedback matches actual position.

Fail positions are tested by removing power and checking the valve moves to its intended safe state.

Valves are checked for tight shut-off under operating DP, catching passing valves before handover.

Controller gains and time constants are tuned so temperatures settle smoothly without hunting.

Balancing valves in series with two-way valves are set so every coil receives design flow at full load.

Correct signal types (on/off, floating, 0–10V or 4–20mA) and supply voltages are verified to prevent control errors.

Position feedback lets the BMS see how far each valve is open – essential for diagnostics and pump optimisation.

The BMS lowers the pump DP setpoint until the most-open valve is nearly fully open. This strategy can deliver significant pump energy savings and is a key tool for HVAC flow optimization.

By reducing flow at part load, two-way valve systems with variable speed pumps cut pump energy substantially compared with constant flow three-way systems.

With no bypassing at coils, return water temperature and delta-T stay high, allowing chillers to run at better efficiency.

• Converting three-way valves to two-way valves
• Adding VFDs and remote DP control to pumps
• Resizing oversized valves to restore authority
• Adding DPCVs or upgrading critical valves to PICVs
• Implementing critical valve reset through the BMS
• Repairing passing valves that waste chilled water

Worn seats, debris or insufficient actuator close-off let water pass when the valve should be shut. We repair, replace or upgrade actuators as needed.

Scale, debris or a seized stem can lock the valve. Cleaning, exercising or replacement restores operation.

Power, wiring, signal or internal faults are diagnosed systematically, and failed actuators are replaced with compatible units.

Oversized valves with low authority cause temperature swings. Resizing the valve or adding DP control usually solves it.

High DP across valves near the pump causes noise and erosion. DPCVs, lower pump setpoints or valve resizing reduce it.

Worn packing or O-rings cause leaks at the stem. Packing replacement or valve replacement fixes the problem.

Poorly insulated chilled water valves or wrong actuator orientation allow condensation into actuators. We correct insulation and orientation, and replace damaged actuators.

Converting three-way valves to two-way valves turns a constant flow system into a variable flow system, cutting pump energy and raising delta-T.

Where control or balancing problems persist, upgrading critical valves to PICVs gives pressure-independent control at every load.

Old or failed actuators can be replaced with modern modulating or communicating units, often without changing the valve body.

DPCVs on risers or branches stabilise DP across existing two-way valves, improving authority and reducing noise.

A complete conversion combines two-way valves, pump VFDs, DP sensors, chiller minimum-flow bypasses and updated BMS sequences – planned and executed as one package by our team.

Small on/off zone valves for FCUs are the most economical. Characterized ball valves for FCUs and small AHUs sit in the middle, while large flanged globe valves and motorized butterfly valves for AHUs and plant cost significantly more. Two-way motorised valve price is driven as much by the actuator as by the valve body.

• Thermoelectric on/off actuators – lowest cost
• Floating and modulating 0–10V / 4–20mA actuators – moderate cost
• Communicating BACnet / Modbus actuators – higher cost
• Spring return fail-safe actuators – highest cost
• Higher force or torque for larger valves and higher close-off – adds cost

• Number, size and type of valves
• Actuator type, signals and fail-safe requirements
• Balancing valves, strainers and isolation valves
• New build or retrofit in an occupied building
• BMS wiring, integration and point testing
• Commissioning, loop tuning and flow balancing
• Documentation and witness testing requirements
• AMC and lifecycle support