Consider the parallel piping system shown in Figure 2-15. Assume that (initially) all three zone valves and all balancing valves are open. When the fixed-speed circulator is turned on, the flow that develops through each branch depends upon that branch’s hydraulic resistance relative to the hydraulic resistance of the other branches, as well as the hydraulic resistance of the supply and return mains and the heat source. These flow rates may or may not be appropriate to deliver the desired rate of heat to each branch.
If one zone valve is then closed, the flow rates in the other two operating zones will change. It will increase. Likewise, if the setting of any of the three manual balancing valves is changed, the flow rates in any active zone will also change.It’s possible to calculate the flow rates that will occur in any of the branches based on the status of the zone valves and the settings of the balancing valves. However, those calculations can be complex, especially for larger systems with many branches. Such calculations are typically done using computer models of the piping system.
Regardless of what the branch flow rates are, it’s important to understand that any changes to the status of the zone valves or the settings of the balancing valves will have some effect on the flows in all the branches. This characteristic is undesirable, especially if the intent of adjusting one of the balancing valves is to change the flow rate in its associated branch without affecting the flow rates in any other branch.
Still, this is the reality of attempting to establish specific flow rates in each branch that will remain at those specific values as zone valves within the system open and close. This is an inherent limitation of static balancing using any type of manual balancing valve, as it cannot react to changes in other branches of the system, and thus, cannot hold a specific flow rate in any situation other than that which existed when it was last adjusted.