Closed-loop systems can be further classified as direct return or reverse return.
Direct Return
The direct return system allows piping to be run in the most direct path to optimize piping costs.
The disadvantage is that the flow at each fan coil unit or air handler usually needs to be balanced using a balancing valve.
The length of the water circuit through the supply and return piping to each fan coil or air handler is different in direct return piping. Fan coils close to the pump receive greater flow rate than those further away unless balancing is accomplished.
Open-loop systems such as the condenser water system with a cooling tower are always direct return since individual terminals don’t exist and balancing is relatively simple.
Reverse Return
The reverse return system is piped so that the length of the water circuit through the supply and return piping to each fan coil or air handler is essentially the same. Therefore, pressure drops are basically equal. Buildings such as hotels with multiple identical fan coil units with identical flows are excellent candidates for reverse return systems.
Reverse return has greater pipe lengths and cost. However, the cost of adding a balancing valve for each fan coil using a direct return system could offset the additional costs of the added reverse return piping.
If the individual fan coil or air handling unit water pressure drops are not reasonably close to each other, engineers will often specify balancing valves anyway, regardless of the piping arrangement.
