Three-Way Control Valves

Most three-way valves use this convention:

• Port A – the controlled port, usually connected to the coil
• Port B – the bypass port
• Port AB – the common port, always open

In a mixing valve, water enters through A and B and leaves through AB. In a diverting valve, water enters through AB and leaves through A and B.

The coil port (A) usually has an equal percentage characteristic to suit coil heat output; the bypass port (B) often has a linear or complementary characteristic to keep total flow steady.

Ports and flow direction are marked on the body. Following these markings is essential – a reversed or mis-piped three-way valve is one of the most common causes of poor control.

The body contains the three ports and internal flow paths. It is made in brass, bronze, cast iron, ductile iron or stainless steel depending on size and duty.

Globe-type valves use a single or twin plug moving between two seats. Ball-type valves use a characterized ball – often a T-port or L-port design – to proportion flow between ports.

Seats and seals define leakage between ports (A–AB and B–AB), which affects energy efficiency and control accuracy.

The stem transfers actuator movement to the plug or ball, with packing or O-rings to prevent gland leakage.

The actuator drives the valve through a linkage or direct coupling that must match the valve’s stroke or rotation.

A three-way mixing valve has two inlets (A and B) and one outlet (AB). It blends two streams – for example coil return and bypass water, or hot supply and cooler return – to deliver the required flow or temperature. Mixing valves are the most common type in HVAC.

A three-way diverting valve has one inlet (AB) and two outlets (A and B). It splits one stream between two paths, such as a coil and a bypass, or a cooling tower and a bypass line. Three-way diverting control valves need internals specifically designed for diverting duty.

A three-way globe control valve uses a plug moving between two seats, giving precise characteristics and good control resolution. Globe valves are common on AHUs, heat exchangers and process circuits.

A three-way control ball valve uses a characterized T-port or L-port ball driven by a rotary actuator. It is compact, economical and widely used on FCUs and small AHUs.

Rotary mixing valves use a rotating sector to blend flows. They are common in heating and boiler circuits, where smooth mixing and low pressure drop matter more than tight shut-off.

A 3 way zone valve or 3 port motorised valve with a thermoelectric or small motorized actuator provides simple on/off control for FCUs and heating zones, sending water through the coil or the bypass.

The valve sends all water either through the coil or the bypass. It is simple and economical for FCUs and small zones.

Open and close signals move the valve incrementally, giving modulating-type control with simple controller outputs.

A three-way modulating control valve follows a 0-10V three-way valve or 4–20mA signal for smooth, accurate temperature control – the standard for AHUs, heat exchangers and process duty.

The coil port usually has an equal percentage characteristic, compensating for the non-linear heat output of coils so the valve position relates more evenly to heat transfer.

The bypass port often has a linear or complementary characteristic. The combination is designed to keep total flow through the valve more constant across the stroke.

Even with complementary characteristics, total flow can vary at mid-stroke – especially if the bypass is not balanced to match coil resistance. An unbalanced bypass can cause significant overflow when the valve is in bypass or mid-position.

In a three-way circuit, authority compares the valve’s pressure drop to the pressure drop of the coil branch it controls. Selecting the valve pressure drop to be similar to the coil branch pressure drop gives good, stable control.

High rangeability lets the valve control accurately at low loads, when only a small flow is needed through the coil.

Kvs is the flow in m³/h through the fully open port at 1 bar pressure drop. Cv ≈ 1.16 × Kv. This is the key sizing value.

Most HVAC three-way valves are PN16 or PN25, with PN25 often required in high-rise and high-pressure systems.

The maximum differential pressure against which the actuator can close each port. It must exceed the highest DP the valve will see in service.

Leakage is specified separately for the controlled path (A–AB) and the bypass path (B–AB). Low A–AB leakage prevents unwanted coil flow when the valve is in full bypass.

Valves are rated for specified medium temperatures – important for boiler and hot water mixing duties – and most accept glycol mixtures within the manufacturer’s limits.

Globe valves have a defined stroke; ball and rotary valves have a defined rotation. The actuator must match exactly.

Silent and economical, thermoelectric actuators provide on/off control for small three-way FCU and zone valves.

Modulating actuators position the valve precisely in proportion to the control signal – the standard for AHUs, heat exchangers and process duty.

Floating actuators respond to open and close pulses for modulating-type control with simple wiring.

Spring return actuators move the valve to a safe position on power loss – for example full flow through a heating coil for freeze protection, or full bypass on a process circuit.

Communicating actuators report position, alarms and diagnostics, simplifying BMS integration.

In plants with instrument air, a three-way control valve with a pneumatic actuator provides high force and fast response for process and industrial temperature control.

The actuator must deliver enough force or torque to close each port against the maximum DP. We check every valve-actuator pair against the manufacturer’s close-off data.

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

Used for larger flanged valves on AHUs, heat exchangers, cooling tower bypasses and plant circuits.

Stainless steel valves suit process water, food processing and hygienic applications.

• 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 three-way control valve for FCU circuits – often called a fan coil three-way valve – is a small DN15–DN20 zone valve or ball valve that sends water through the coil or the bypass. It is common in constant flow systems and hotel FCU installations.

A three-way control valve for AHU cooling coils – a three-way modulating valve for an AHU chilled water coil – controls supply air temperature while keeping chiller flow constant. It is a common AHU chilled water control valve in existing constant flow plants.

Three-way valves control the primary side of heat exchangers, keeping primary flow steady while holding secondary temperature.

A three-way valve at the cooling tower diverts condenser water through the tower or a bypass to maintain minimum condenser water temperature in cool weather or at low load.

A boiler three-way mixing valve blends hot supply water into the return to keep boiler return temperature above its minimum, preventing condensation and corrosion in non-condensing boilers.

In process loops, a three-way temperature control valve blends or diverts water to hold machine, mould or product temperature precisely.

In variable flow systems, a few three-way valves at the ends of long runs keep a minimum flow circulating, maintaining supply temperature at remote terminals and helping protect pumps.

The most common HVAC arrangement puts a mixing valve on the coil return. Coil return water enters port A, bypass water enters port B, and the combined flow leaves through AB. This gives diverting action across the coil while using a standard mixing valve.

A diverting valve on the coil supply takes flow in through AB and sends it to the coil (A) or bypass (B). This arrangement suits cooling tower bypass and some process duties.

Mixing valves are generally not suitable for use as diverting valves by simply reversing flow – the internals are designed for flow in one direction, and reversal can cause noise, poor control and damage. Where diverting action is needed at a coil, a mixing valve on the return is the usual solution; where true diverting is needed, a purpose-designed diverting valve is used. Our team confirms the correct three-way mixing vs diverting valve arrangement for every application.

Pumps circulate full flow even at low load, so pump energy stays high all year.

Bypassed water returns to the chiller unused, lowering return temperature and delta-T and reducing chiller efficiency.

Each bypass needs a balancing valve set to match coil resistance, otherwise flow increases when the valve bypasses.

Without careful balancing and matched characteristics, total flow can rise at mid-stroke, disturbing neighbouring circuits.

Three-way valves and their bypass pipework cost more and take more space than equivalent two-way valves.

Energy codes such as ASHRAE 90.1 and India’s ECBC favour variable flow for larger hydronic systems, limiting the use of three-way valves at every coil in new designs. Exact requirements depend on code edition and system size.

In existing constant flow hydronic systems, a three-way control valve for a constant flow HVAC system keeps chillers and pumps operating as designed.

Three-way valves on cooling tower bypasses maintain minimum condenser water temperature for stable chiller operation.

Mixing valves protect boilers and control heating circuit temperatures.

Industrial processes that need steady flow and precise temperature rely on three-way mixing or diverting valves.

A small number of end-of-line three-way valves maintain circulation in variable flow systems.

Size on design flow and target pressure drop, not pipe size. Line-size valves are usually oversized.

Control valve Kv calculation uses:

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.

The bypass balancing valve is sized and set so bypass resistance matches the coil circuit resistance at design flow. This keeps total flow steady as the valve moves between coil and bypass.

Select the valve pressure drop to be similar to the coil branch pressure drop, giving good authority and stable modulation.

The valve-actuator combination must close each port against the maximum DP expected.

For coil control, a mixing valve on the return is the standard choice. True diverting duties, such as some cooling tower bypasses, use purpose-designed diverting valves.

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. A mixing valve with Kvs 25 – typically DN40 – is selected for return-side installation, with a modulating actuator. The bypass balancing valve is then set to about 40 kPa at 13.7 m³/h to match the coil. These figures are illustrative; final selection follows the manufacturer’s data.

A 500 TR chiller at about 3 US GPM per TR needs roughly 340 m³/h of condenser water. Allowing a valve pressure drop of around 25 kPa gives a required Kv of roughly 680, typically a DN200-range valve. At these sizes, a three-way butterfly assembly or two linked motorized butterfly valves are commonly used instead of a globe valve. These figures are illustrative; final selection follows the manufacturer’s data and the cooling tower arrangement.

Belimo three-way control valves include characterized ball and globe ranges with a broad choice of matched actuators, including three-way diverting valves for specific duties.

Siemens offers three-way globe and ball valves with actuators designed for integration with its building automation systems.

Honeywell supplies three-way zone valves, 3 way motorised valves and control valves for FCU, heating and AHU applications.

Danfoss provides three-way control valves and actuators for heating and cooling systems.

Johnson Controls offers three-way 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.

Ports A, B and AB are piped exactly as marked – coil to A, bypass to B, common to AB – with flow direction matching the valve design.

Mixing valves are installed on the coil return; diverting valves on the supply. We confirm the arrangement matches the valve type before installation.

Actuators are mounted within the manufacturer’s allowed positions and kept above chilled water valves, away from condensation.

Every three-way valve with bypass includes a balancing valve in the bypass line, set to match coil resistance. This three-way bypass control valve arrangement keeps total flow stable.

Strainers protect valve seats; isolation valves allow maintenance without draining the system.

Actuators and manual overrides need safe access through ceiling panels or AHU and FCU access doors.

Chilled water valves and bypass pipework are insulated, while keeping actuators, overrides and test points accessible.

Each valve is driven through its full stroke to confirm smooth, complete movement.

We confirm correct response to control signals and accurate position feedback.

We check that full signal gives full coil flow and zero signal gives full bypass – catching reversed ports before handover.

Coil flow is set to design, and the bypass balancing valve is adjusted so full-bypass flow matches full-coil flow.

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

Signal types, voltages and fail-safe positions are verified end to end with the BMS.

Position feedback lets operators see valve status and diagnose control problems quickly.

Trending supply temperatures and valve positions helps identify hunting, passing valves and unbalanced bypasses before they become complaints.

In constant flow systems, pumps run at full flow regardless of load, so pump energy stays high throughout the year.

Bypassed chilled water returns to the plant unused, lowering delta-T and forcing chillers to run less efficiently – often with more machines online than the load requires.

• Balancing bypasses correctly to stop overflow
• Converting coil three-way valves to two-way valves or PICVs
• Adding VFDs and DP control to pumps
• Retaining a few end-of-line three-way valves for minimum flow
• Optimising cooling tower bypass control to improve chiller efficiency
• Repairing passing valves that waste chilled water

Reversed A and B ports cause inverted control – the coil gets more water as load falls. We re-pipe or reconfigure to correct it.

Worn seats let water pass through the coil when the valve should be in full bypass, wasting energy. Repair or replacement restores control.

Scale, debris or long periods without movement can seize a valve. Cleaning, exercising or replacement solves it.

Power, wiring or signal faults and failed actuators are diagnosed and corrected with compatible replacements.

Oversized valves, poor authority or untuned loops cause hunting. Resizing, bypass balancing and retuning fix it.

Excess DP across the valve causes noise. Rebalancing, pump adjustments or valve resizing reduce it.

Worn packing or O-rings cause stem leaks, fixed by packing replacement or valve replacement.

An unset bypass balancing valve lets excessive water bypass the coil, raising system flow and lowering delta-T. We set bypass flow to match coil design.

Like-for-like replacement with correctly sized modern valves restores control and stops energy waste from passing valves.

Closing bypasses and converting coil valves to two-way operation turns a constant flow system into variable flow, cutting pump energy and raising delta-T – provided pumps get VFDs and chillers have minimum-flow protection.

Replacing three-way valves with PICVs delivers variable flow and automatic balancing in one step.

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

In conversions to variable flow, a few three-way valves at the ends of long runs are often retained to maintain minimum circulation and supply temperature.

Small three-way zone valves and ball valves for FCUs are the most economical. Globe-type three-way valves for AHUs and heat exchangers cost more, and large flanged valves or butterfly assemblies for cooling tower bypasses are priced significantly higher. Purpose-designed diverting valves typically cost more than standard mixing valves of the same size.

• 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
• Pneumatic actuators – depend on size and accessories such as positioners

• Number, size and type of valves
• Actuator type, signals and fail-safe requirements
• Bypass pipework and balancing valves
• Strainers and isolation valves
• New build or retrofit in an occupied building
• BMS wiring, integration and point testing
• Commissioning, bypass balancing and loop tuning
• AMC and lifecycle support