Understanding the Difference Between 2-Way and 3-Way Valves

For HVAC installers and building automation technicians, selecting the right control valve is fundamental to system stability. While the industry has largely shifted toward variable flow systems, we still see plenty of 3-way valves in older Australian plant rooms.

Understanding the physics and hydraulic impact of 2-way vs. 3-way valves is critical—whether you are commissioning a modern VAV system or retrofitting a legacy constant-volume chiller.

At Controls Traders, we stock a wide range of Control Valves from brands like Belimo and Siemens. Here is a technical breakdown of the differences and when to use each.

What is a 2-Way Valve?

A 2-way valve has two ports: an inlet (A) and an outlet (AB). Its primary function is to throttle the flow of water through a coil to control temperature.

How it works: As the demand for cooling decreases, the valve closes, restricting the flow of chilled water to the coil.
System Impact: Because the valve stops flow, it creates a Variable Flow system. When the valve closes, the system flow rate drops, allowing Variable Speed Drives (VSDs) on the pumps to ramp down, saving significant electrical energy.
Typical Application: Modern energy-efficient buildings using variable speed pumping, AHUs, and Fan Coil Units (FCUs).

What is a 3-Way Valve?

A 3-way valve has three ports: an inlet (A), a bypass (B), and a common outlet (AB).

How it works: Instead of stopping the water, a 3-way valve diverts it. When the coil doesn't need cooling, the valve directs the water around the coil (bypass) back to the return line.
System Impact: This creates a Constant Flow system. The pump works at the same speed regardless of the cooling load because the total volume of water circulating remains constant—it just skips the coil.
Typical Application: Older constant-volume systems, or specific "end-of-line" bypass points to ensure minimum flow for chiller protection.

Key Differences: Flow, Energy, and Design

Feature	2-Way Valve	3-Way Valve
Flow Characteristic	Variable Flow (Varies with load)	Constant Flow (Always 100%)
Pump Energy	High Efficiency: VSD pumps ramp down at part load.	Low Efficiency: Pumps run at 100% speed continuously.
Water Temperature	Return water temp rises at part load (High ΔT).	Return water temp drops at part load (Low ΔT).
System Cost	Lower piping cost (2 pipes), higher control complexity (requires VSDs).	Higher piping cost (requires bypass piping), simpler pumping (no VSDs).

When to Use Which?

1. New Builds & VAV Systems (Use 2-Way)

Almost all modern Green Star or NABERS-rated buildings in Australia utilize 2-way valves. By using 2-way valves paired with Belimo Valves or Pressure Independent Control Valves (PICVs), you maximize the efficiency of variable speed pumps.

Why? The energy savings from slowing down large pumps at part-load are massive compared to running them full speed 24/7.

2. Retrofits & Constant Volume (Use 3-Way)

If you are replacing a valve in an old system (pre-2000s) that utilizes constant speed pumps without VSDs, you generally must replace like-for-like with a 3-way valve.

Why? If you install a 2-way valve in a constant speed system, closing the valve will "dead-head" the pump, causing pressure spikes that can burst pipes or damage the pump seals.

3. The "End-of-Line" Exception

Even in modern variable flow systems, you will often see a single 3-way valve installed at the furthest FCU from the pump.

Why? This ensures a small amount of water always circulates to keep the loop active and prevent the pump from dead-heading if all other 2-way valves close simultaneously.

Common Installer Mistakes

Mixing Up Mixing vs. Diverting: 3-way valves come in two types. A Mixing valve (two inlets, one outlet) blends return and supply water. A Diverting valve (one inlet, two outlets) directs flow. Installing a mixing valve in a diverting application can cause valve chatter and premature failure.
Oversizing the Actuator: Putting a massive high-torque actuator on a small valve can snap the stem. Always match the torque (Nm) to the valve body. (See our guide on [Actuators] sizing).
Ignoring ΔP (Differential Pressure): In 2-way systems, when valves close, system pressure rises. If the Actuators aren't rated for the high close-off pressure, they will be forced open, leading to "ghost flows" and energy waste.

Conclusion

Choosing between 2-way and 3-way valves isn't just about plumbing; it's about the entire hydraulic strategy of the building.

Go 2-Way for energy efficiency and VSD systems.
Go 3-Way for constant volume legacy systems or pump protection.

At Controls Traders, we warehouse a massive stock of both valve types from trusted brands like Belimo, Siemens, and Honeywell. If you are retrofitting an old plant room and need to cross-reference a part number, our team has over 40 years of experience to help you get it right.

Read the full guide on our website for diagrams and selection tips.