Balancing with Flow

Aspects of system balancing in hydronic systems

Hydraulic balancing involves adjusting the flow rates in the system to ensure that the distribution of water among different branches, circuits, or zones is balanced. This ensures that each part of the system receives the intended flow of water, and helps in optimizing the performance and efficiency of the HVAC system.

Balancing is crucial for ensuring that multiple pumps and components in a system work together harmoniously.

Many flow distribution problems, either permanent or transient, are attributed to control loops, when in fact the causes are often hydraulic. The source of a problem is often tied to water flow disparities in relation to design values. Most of these problems can be detected and, in general, resolved, if balancing valves are placed in strategic locations.

Here is a brief list of hydraulic system equipment and components that require balancing and/or an adjustment:

Large HVAC installations require multiple adjustments to flow, using a throttling valve on the pump discharge so that the design flow is reached. HVAC systems are often oversized, resulting in pumps functioning to right of the system curve. To bring the pump back to the design flow, a TAB contractor would typically open all valves and manually adjust the pump speed of each VFD until all coils are satisfied.

During the pump start up process, the Test & Balance or TAB contractors follow ASHRAE Standard 111 whereby they need to determine the flow at the pump with all the system control valves open. And they adjust the pump flow to within +/-10% of the design flow. Normally the contractor reads the pressure gauge, looks up the impeller trim or the pump speed, and gets a copy of the pump curve. From there the contractor would plot the pressure on the Y-axis, look up the corresponding trim (or speed depending the curve) and then find out whether the flow rate from the curve matches what is seen on-site.

One of the features available in the DE pump is Auto-Flow Balancing. Used during commissioning, Auto-Flow Balancing automatically determines control curve, and minimum head at zero flow, for energy savings.

A Design Envelope pump will operate in a constant flow mode and stop at the actual on-site head. It then re-plots the control curve between design flow at on-site system head, and setpoint for energy savings. This feature is especially useful where buildings come online in stages. For example, design flow may be 500 gpm to start, then 700 gpm after 3 months, and then 850 gpm a few months later.

Balancers may forget to rebalance to the new flow levels as the system demands change. The balancing process can be made much easier if the flow rate is accurately displayed on the pump interface itself. More importantly, the balancing process can be made more accurate by using design envelope technology. The measurement and calculation of system flows based on impeller rotations provides an accuracy level of +/-5% which a helpful improvement on the industry standard of +/- 10%.