Is it possible to use 3-way valve and 2-way valve together ?, i have a project with cst pump and now i want to change it to vsd pumps speed) ,

If you use VSD pump you will use a 2 way valve and if you use a constant speed pumps, use 3-way valve.  What is the benefets of using 2way and 3way together besides it is costly.

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.

origanally answered by @Mohamed Abdul Monem Abdulah