Hydronic Systems Testing, Adjusting, and Balancing
Flow, pressure, and the pump curve
Hydronic balancing starts with confirming pump performance. Measure suction and discharge pressure at the pump gauge ports, convert to head, and read the resulting flow from the manufacturer's pump curve. Traps: the curve must match the actual impeller trim and speed, and gauge elevation differences and gauge error must be accounted for. In a closed loop, static head does not add to pump head; only friction does.
Individual coil and terminal flows are determined three ways: pressure drop across a balancing valve or venturi using the manufacturer's Cv/Kv relationship, gpm = Cv x sqrt(delta P), an ultrasonic clamp-on meter, or a permanently installed flow meter. Recording valve position plus differential pressure lets you verify a preset is actually achieved rather than merely set.
Use the proportional method here too: compute a balance index for each circuit, begin at the circuit with the highest index (least-favored), and adjust in sequence, rechecking earlier valves afterward. Circuit setters with memory stops and pressure-independent control valves simplify this, but PICVs still need verification of total flow.
Watch for glycol: capacity factors change, and flow-related pressure drops rise with viscosity. Air in the system produces erratic readings, so verify air elimination and proper fill pressure first.
Rule of thumb and worked example
For water, Btu/h = 500 x gpm x delta T (deg F). A coil rated 240,000 Btu/h with a 20 deg F design delta T needs 240,000 / (500 x 20) = 24 gpm. For 30 percent glycol use roughly 485, and for 50 percent roughly 450-470.