When to use 2-way valve and 3-way valve in HVAC system?

A three way control valve is used for constant flow systems while a two way control valve is used for a variable flow system.

This is how we look to the the story:

1. 2Way Valve:
   • 2Way (Or2 Ports) Valve is passing the water in one direction only. so if the valve is fully close it will trap the water before it.
   • this will lead to a pressure increase in this branch
2. 3 Way Valve:

• 3Way (Or3 ports) Valve is passing the water in2 directions.
• so if the valve is fully open the full amount of water will be moving in one direction, if it closed the water will pass to the other direction, if the valve is partially open then percent of the water will flow through direction1 and the remaining will pass through the other (for Diverting Valve which installed in the supply line).
• In another cases if we install the valve in the return line so if the valve is open water will flow through the unit (Cooling coil as example) then pass through main direction. if the valve is close the water will by pass the unit and then flow through the other direction through the valve (Mixing Valve which installed in the Return line).
• this will not cause the pressure rising.  

Why we use the2 way valve? when you use the2 way valves in HVAC system in all the equipments in your building this means that you will not need all the chilled water to go through your system all the time if you don't need much cooling. so you will be able to reduce the speed of the secondary pumps of your system. this will lead to a huge energy saving in the running cost of your building (depend on the number of Pumps and thier sizes). Also you will be able to reduce the size of your pumps. But in this case you have to use Variable Speed Pumps. Also you should have2 sets of Pumps, One set constant with Speed for Chillers and another set to serve the building. ans also By Pass Valve to gurantee the min flow of the chillers. Otherwise you can use one set with a variable flow chillers. This has to be a decision in the mechanical design stage depend on the cost calculation of the project.

Bottom line is:

use the2 way Valve in the system that can withstand the variable water flow

use the3 way valve in the systems that needs a constant water flow.

2 way and 3 way valves are installed for controlling the fluids...either Gas or chilled water.....some times its works for modulating the fluids as per requirements and some times works as shut of valves...

In Chilled water line Criteria to use2 or3-way valve depends on the application type: 1.  2-way valves are recommended in Chilled water piping with primary const. flow and secondary Variable speed pump. 3-way valves are not suitable for this system. 2.  2-way valves are recommended in Primary only Variable speed pump. to maintain a minimum flow across chiller, a motorised bypass valve is used across the headers in this system  3.  When there is a primary only constant speed pumping system is used,3-way valve is recommended. for this application2-way valves are not best suitable.

A 2-way valve is having an inlet and an outlet port. It is used for control flow as on/off or having more complex variable flow applications with pumps and VFDs.

But  a 3-way valves are commonly found in constant flow/volume pumping systems and can be either mixing or diverting valves.

Three way valves (modulating or ON/OFF) are used to bypass the flowing media. For example in AHUs to bypass the chilled water flowing through it as the set point temperature has been achieved.

On the other hand Two way valves (modulating or ON/OFF) are used for the same purpose above but the chilled water circulating pumps must have variable speed drive in order to avoid liquid hammering at the valve.

If we install Fan Coil Unit, After coil we install 2 way valve (Motor riser), and 3 way valve is same with 2 way valve 3 way valve for balancing cold water system

1. A 2 way valve is used in a chiller system with variable speed pump set. When the load reduces inside the room the sensor  Pass signal to the actuator, though the valve closes/partialy close. which will create a pressure in the pipeline. transducer installed in the piping system sences this pressure change and send information/signal to the variable pump to reduce the pump speed. Which result in the low runnung cost/Power consumption.

2. A 3 way valve is used in a chiller system with constant pump set.  Similarly when the load reduce inside the room the sensor pass signal to the actuator,result in opening/partial opening of the bypass opening(Third inlet in the 3way valve). Whic result in the flow of chilled water back to the piping system without entering the AHU/FCU.