Constant Flow with Three-Way Valve Control

A three-way mixing valve has two inlets (A and B) and one outlet (AB). In HVAC it is usually fitted on the return side of the coil, where it combines coil return water with bypass water. The three-way mixing valve is the most widely used type in HVAC. A 3 way mixing control valve is often the default choice for FCUs and AHUs.

A three-way diverting valve has one inlet (AB) and two outlets (A and B). It is fitted on the supply side and splits flow between the coil and the bypass. Diverting valves have internals designed specifically for the flow split. A 3 port diverting valve or 3 way diverting control valve is common in some heating, process and cooling tower applications.

A 3 way globe valve uses a plug that moves linearly between two seats. 3 way globe control valves offer precise modulation, good close-off and a wide size range, from threaded FCU valves to large flanged AHU and plant valves. In industrial plants, a 3 way control valve with pneumatic actuator is common where compressed air is available. On/off versions are switched by a 3 way pneumatic solenoid valve (a 3/2 air solenoid valve), while modulating versions use a positioner.

A characterized ball valve uses a ball with a T-port or L-port bore, plus a characterizing disc that shapes its flow curve. These valves are compact, have low leakage and suit FCUs and small to medium AHUs. Motorized T-port 3 way ball valves are also popular for on/off zone control.

A 3 way rotary valve uses a rotating shoe or sector inside a cylindrical body to blend or divert flow. Rotary valves are simple, robust and tolerant of dirt, which makes them popular for heating circuits, radiant systems and some process mixing duties. Their leakage is usually higher than that of seated valves, so they are less suited to applications that need tight shut-off.

Most manufacturers mark the common port AB, the control or coil port A and the bypass port B. The two end positions work like this:

• Actuator fully open: A–AB is open and B–AB is closed, so all the water goes through the coil.
• Actuator fully closed: A–AB is closed and B–AB is open, so all the water bypasses.

Some manufacturers use different markings, so we check every datasheet before installation.

A mixing valve belongs on the return side of the coil, and a diverting valve on the supply side. Return-side mixing valves are the most common arrangement in chilled water systems. Mixing valves are widely available, and on the return side the valve sits downstream of the coil and its strainer.

Flow direction matters. Globe plugs are designed for flow in one direction, so a mixing valve installed as a diverter, with flow entering the AB port, can chatter, bang or fail to control. Some ball-type valves accept either duty, but only when the manufacturer confirms it. We check flow arrows, port markings and actuator action on every valve.

On/off actuators drive the valve fully open or fully closed on a thermostat signal. A 3 way motorised valve with an on/off actuator is common on FCUs and simple zone circuits.

Floating actuators receive separate "open" and "close" signals and hold position when neither is present. They provide modulating control from simple controllers and are widely used on FCUs and small AHUs.

Modulating actuators position the valve anywhere in its stroke in response to a 0–10 V, 2–10 V or 4–20 mA signal, making it a true 3 way proportional valve. Most provide position feedback, and they are the standard choice for AHUs.

Fail-safe actuators use a spring or stored energy to return the valve to a defined position on power loss. Heating coils often fail open to the coil. Cooling applications fail to the coil or to bypass, depending on which position is safer for the process.

Smart actuators connect directly to the BMS over BACnet or Modbus, receiving commands and reporting position, alarms and diagnostics. They simplify wiring and make troubleshooting much faster.

A linear valve changes flow in direct proportion to stroke. An equal percentage valve changes flow by the same percentage for each step of stroke, giving small changes near closed and large changes near open. Coil output rises quickly at low flow, so the coil port (A–AB) is usually equal percentage. Many three-way valves pair it with a linear bypass port (B–AB) to keep total flow steadier.

Valve authority compares the valve's pressure drop with the pressure drop of the part of the circuit whose flow it varies, mainly the coil. A common target is around 0.5, which means sizing the valve for a pressure drop similar to the coil's. Total flow in a three-way circuit stays nearly constant, so the pressure across the circuit is stable and authority holds up better than in many two-way systems.

Unless the bypass is balanced, the coil path and bypass path have different resistances. When the bypass flows more easily, the circuit passes more water in full bypass than at full coil flow. This disturbs flows to other circuits and upsets the system balance.

At mid-stroke, both ports are partly open and the two paths act in parallel. Total circuit flow can then rise noticeably above design, depending on the valve characteristic and how well the bypass is balanced. With equal percentage characteristics on both ports, it can instead dip below design. An equal percentage coil port, a linear bypass port and a properly balanced bypass together keep this variation small.

A three-way valve for AHU cooling coils is typically a modulating globe or characterized ball valve controlling supply or return air temperature. In a constant flow system, the three-way control valve for AHU duty acts as the AHU chilled water control valve. It is sized on coil flow and pressure drop and paired with a balanced bypass.

A three-way valve for FCU coils is usually a compact threaded valve with an on/off, floating or modulating actuator. As the FCU chilled water control valve, it keeps each fan coil circuit at constant flow, so neighbouring FCUs are unaffected.

Three-way valves control process heat exchangers by bypassing part of the cooling water around the exchanger. Process temperatures stay steady while the supply loop sees constant flow.

A three-way diverting valve on the condenser water line can send part of the flow around the cooling tower fill. This stops condenser water from getting too cold for the chiller in cool weather or at light load.

In heating systems, three-way valves control reheat coils, preheat coils and heat exchangers. Mixing arrangements also blend return water into the supply to deliver a controlled, lower flow temperature for radiant or low-temperature circuits.

In variable flow systems, a few three-way valves at the ends of long runs keep a minimum flow moving. Water near remote terminals stays cold and the pumps never deadhead. They are a practical compromise, but their number should be limited, because every bypass reduces delta-T.

Pumps move design flow all year, even when most of it bypasses the coils. On larger systems, this is the biggest energy penalty.

Bypass water returns at supply temperature and dilutes the coil return, so return temperature falls as load drops. Three-way valve low delta-T is built into the design.

Lower return temperatures mean chillers deliver less cooling per litre pumped. In multi-chiller plants, this can keep extra chillers and pumps running at low load.

A bypass without a balancing valve, or with a wrong setting, overflows in full bypass and starves other circuits. Bypass balancing is essential.

Depending on valve characteristics and balancing, circuit flow can rise at mid-stroke. This adds to overflows near the pumps.

Energy codes generally expect larger hydronic systems with modulating valves to vary flow with load. Above certain sizes, systems with three-way valves at most terminals may not comply.

With small pumps and only a few coils, the energy penalty is modest and the simplicity is valuable.

A single chiller with constant speed pumps needs steady evaporator flow. Three-way valves provide it without any extra controls.

Where loads barely vary, three-way valves hold process temperatures precisely while the loop stays at constant flow.

Even in variable flow systems, a few end-of-line three-way valves remain a practical way to maintain minimum circulation.

Many legacy plants still run well on three-way valves. Like-for-like replacement of failed valves, rebalancing and actuator upgrades can extend their life cost-effectively.

Coil flow comes from coil load and design delta-T. Each TR of chilled water load needs about 2.4 GPM at a 10 °F delta-T, or roughly 0.55 m³/h at 5.5 °C. On that basis:

• a 50 TR AHU needs about 120 GPM (27 m³/h)
• a 1.5 TR fan coil needs about 0.8 m³/h

Valves are sized on flow and pressure drop, not pipe size:

Kv = Flow (m³/h) ÷ √ΔP (bar) Cv = Flow (GPM) ÷ √ΔP (psi), where Cv ≈ 1.156 × Kv

Take the 50 TR AHU above, carrying 27.3 m³/h, with a coil pressure drop of 0.4 bar. The target Kv is about 43. A Kvs 40 valve gives roughly 0.47 bar at design flow, equivalent to a Cv of about 46. The selected valve is often one or two sizes smaller than the pipe.

At design flow, the bypass balancing valve must create the same pressure drop as the coil path it replaces, meaning the coil plus its connecting pipework. We select a valve that reaches this pressure drop at a mid-range setting, often the same size as, or one size smaller than, the coil connection. Specifications sometimes call for a "3 way balancing valve"; in practice, a three-way circuit is balanced with one balancing valve in the circuit and a second in the bypass.

Pump head is the sum of losses along the index path:

• the chiller evaporator
• supply and return piping
• the index circuit's coil, three-way valve and balancing valve
• strainers and fittings

Circuit flow is constant, so the calculation is straightforward, but the margins must stay realistic.

Design delta-T sets the flow. A higher delta-T means less flow, smaller valves and lower pump power, provided the coils are selected to match. In three-way systems, design delta-T is achieved only at full load.

Each circuit is sized for its design flow, which is also its operating flow for every hour. Moderate velocities keep noise and erosion low at the valve and bypass, and consistent sizing makes balancing easier.

Select the valve's Kvs or Cv to meet the target pressure drop at design flow. Oversized valves control poorly near closed, while undersized valves starve the coil and add pump head.

The body rating, for example PN16, must suit system pressure, including static head in tall buildings. The close-off pressure is the differential pressure the actuator can shut against, and it must exceed the highest pressure difference the valve will see.

Small valves usually have bronze or brass bodies with threaded ends. Larger valves use cast iron or ductile iron bodies with flanges. The material must suit the water quality and temperature.

Actuator force or torque must exceed what the valve needs at close-off, and the signal type must match the controller. Stroke time must suit the control loop: fast enough for AHU response, but not so fast that it causes hunting.

Leakage is the flow that passes a closed port. Datasheets often specify leakage separately for the control port and the bypass port. Industrial valves are rated to classes such as ANSI/FCI 70-2 Class IV or Class VI. Lower leakage improves control at light load.

Belimo offers characterized control valves in 2-way and 3-way versions, globe valves for larger duties, and a wide range of actuators: on/off, floating, modulating, spring-return and communicative. A Belimo 3 way control valve is often chosen for its compact characterized ball design and low leakage.

Siemens offers three-port globe and ball valves in its Acvatix range, in threaded and flanged versions. They are paired with electromotive actuators for modulating and on/off control.

Honeywell's range includes 3-way motorised zone valves widely used on FCUs, plus 3-way globe valves and actuators for AHUs and plant duties. A Honeywell 3 way motorised valve is a familiar sight in many hotel and office FCU installations.

Danfoss supplies 3-way seated valves in threaded and flanged versions for heating, cooling and district energy applications. They are used with its AMV (3-point) and AME (modulating) actuator ranges.

Johnson Controls offers ball and globe control valves in 2-way and 3-way versions. Their electric actuators integrate with Johnson Controls' own BMS and with third-party platforms.

Other makes used on HVAC projects include Schneider Electric, Sauter, ESBE (known for rotary mixing valves) and Indian manufacturers. Whatever the brand, we check Kvs, characteristics, close-off pressure, leakage and actuator compatibility against your project.

A typical return-side mixing arrangement is laid out as follows:

• Supply side: an isolation valve and strainer ahead of the coil inlet.
• Return side: the coil outlet with a P/T point, then the three-way mixing valve. Port A comes from the coil, port B from the bypass, and port AB goes to the return. After the valve come a circuit balancing valve and an isolation valve.
• Bypass: the bypass tees off the supply line after the strainer and connects to port B through its own balancing valve.

This complete three-way valve chilled water bypass arrangement keeps every circuit at constant flow.

The bypass line carries water around the coil as the valve modulates. A balancing valve in the line, set to the coil's pressure drop, keeps circuit flow the same at full coil and full bypass. Every three-way valve with bypass needs this balancing valve, and every three-way control valve with bypass should be commissioned with it set.

Isolation valves on the supply and return let the valve, coil or strainer be serviced without draining the system. A strainer upstream protects the valve seats and coil tubes.

Test points allow pressure and flow checks across the coil and valve. Thermometers or temperature sensors on supply and return show coil performance at a glance.

Valve bodies, bypass lines and fittings all carry chilled water, so they must be insulated against condensation. Removable insulation jackets keep valves accessible, while actuators stay uninsulated.

With the valve fully open to the coil (A–AB), the circuit balancing valve is set to deliver design flow through the coil.

With the valve in full bypass (B–AB), the bypass balancing valve is set to deliver the same design flow. Both end positions then give equal circuit flow.

Once each circuit is balanced internally, circuits are balanced against each other across branches and risers. The work starts from the index circuit, so every circuit reaches design flow with the fewest adjustments.

Flows are measured through the balancing valve test points with a balancing instrument, in both full-coil and full-bypass positions. The readings are recorded in the TAB report.

FCU thermostats and AHU controllers compare room or return air temperature with setpoint and drive the three-way valve to match.

A supply air temperature loop modulates the valve for precise discharge temperatures. The supply air setpoint is often reset from room or return air temperature.

PI control is usually enough, and derivative action rarely helps on water valves. Gains are matched to actuator stroke time and coil response. A deadband stops constant small movements that wear actuators.

Position feedback shows whether each valve is actually responding. It reveals stuck valves, valves that sit at full coil (a sign of undersized coils) and circuits that rarely leave bypass (a sign of oversized coils).

Pumps start first and prove flow before a chiller starts, and they run on after the chiller stops. Three-way valves guarantee a flow path at all times, which keeps these interlocks simple.

Pump power is estimated as:

Pump power (kW) = Flow (m³/h) × Head (m) ÷ (367 × combined pump-motor efficiency)

Take a 150 TR plant circulating 81.8 m³/h at 28 m head with 68% combined efficiency. The pumps draw about 9.2 kW. Running 3,650 hours a year, that is roughly 33,500 kWh, whatever the load. In this example, every 10% cut in average pump power saves about 3,350 kWh a year.

Bypass flow wastes energy in three ways:

• the pumps move water that does no cooling
• return temperature falls, so chillers run less efficiently or more chillers run
• bypassed water picks up heat along long pipe runs

At light load, most of the pumped water may pass through bypasses rather than coils.

• Balance every bypass so no circuit overflows
• Trim oversized pump impellers instead of throttling
• Replace leaking valves that pass water through closed ports
• Retune control loops to stop hunting
• Convert most three-way valves to two-way valves or PICVs with VFD pumps
• Keep three-way valves only where minimum flow is needed

Each actuator is driven through its full stroke to confirm smooth movement, correct end positions, manual override and fail-safe action.

We check signal type, range and direction at every valve, confirming that an increasing call for cooling opens the coil port. Feedback signals are checked against actual position.

Coil and bypass paths are balanced to equal design flow, then circuits are balanced across the system. The results are recorded in the TAB report.

With the system running, we test each loop's response to setpoint changes. Coils must reach setpoint without hunting or overshoot, and final tuning values are documented.

Swapped ports or reversed flow cause reversed control action, noise or chatter. The valve must be re-piped to match its port markings and flow arrows, or its actuator action corrected where the design allows.

Scale, debris or long periods in one position can seize the plug or ball. Cleaning, exercising the valve or replacing internals restores movement, and good water treatment prevents recurrence.

Worn seats let water pass a closed port. Leakage through the coil port overcools the space, while leakage through the bypass reduces cooling at full demand. Seat replacement or a new valve fixes it.

Typical causes are power loss, wiring faults, a mismatched signal range (0–10 V against 2–10 V), reversed action or a failed motor. Checking the signal, testing the actuator and correcting the configuration resolve most cases.

Hunting comes from oversized valves, low authority, aggressive tuning or a stroke time that doesn't suit the loop. Correct valve sizing, retuning and deadband settings stabilize control.

A missing or misset bypass balancing valve lets circuits overflow in bypass mode and starve others. Rebalancing both paths restores constant circuit flow.

Low return temperatures at light load can make chillers unload and cycle frequently, especially in systems with little water volume. Balancing bypasses, reviewing setpoints, adding buffer volume where needed, or converting to variable flow reduces cycling.

Each three-way valve is swapped for a two-way valve and its bypass leg is removed or capped. The system becomes variable flow, so the plant needs pump speed control and minimum flow protection.

Replacing three-way valves with PICVs adds automatic balancing and delta-T protection in a single step. It is often the best option for buildings with many terminals or frequent tenant changes.

Beyond the valves, conversion needs VFD pumps, a differential pressure control strategy, and a minimum flow bypass or variable primary controls to protect chillers. We design the whole conversion, not just the valve swap.

Once two-way valves or PICVs are installed, VFDs and a remote DP sensor let pump speed follow demand. This is where most of the pumping savings are realized.

A few three-way valves are often kept at the ends of long runs to maintain minimum circulation. The number is kept as low as possible, and their bypasses are balanced to limit the effect on delta-T.

Where valves are in good condition but actuators are failing or basic, upgrading to modulating or smart actuators improves control. BMS integration adds position feedback, alarms and remote diagnostics.

Still running a building full of three-way valves? Ask Compress India for a valve and energy assessment.

3 way control valve price varies with:

• Valve size and Kvs, from small FCU valves to large flanged AHU valves
• Valve type: globe, characterized ball or rotary
• Body material, pressure rating and connection type
• Actuator: on/off, floating, modulating, fail-safe or smart
• Leakage class and close-off rating
• Make and country of origin
• Quantity and delivery timelines

A 3 way solenoid valve price is usually lower than that of a modulating control valve with actuator, but the two are different devices. Solenoid valves are not a substitute for coil control valves.

• Number of AHUs, FCUs and heat exchangers
• New installation versus replacement of existing valves
• Bypass piping and balancing valves
• Isolation valves, strainers and test points
• Insulation and access requirements
• Actuator wiring and BMS integration
• Testing, balancing and commissioning scope
• Shutdown windows, night work and site access
• AMC requirements