Strainers are installed to protect items of plant and to help ensure system efficiency by keeping heat emitters (terminal units, i.e. fan coil units etc.) free from particle build-up within them.
Where should strainers be located in pipework systems ?
When used to protect items of plant (boilers / chillers / pumps etc.) strainers should be installed upstream of the item to ensure that larger particles contained in the flowing water do not enter and damage the item of plant.
Strainers can also be installed at strategic places, i.e. branches or in each terminal sub-circuit, throughout the system to ensure that smaller particles contained within the flowing water do not pass into the terminal units affecting their efficiency.
Ref - cranefs