Commissioned a 6-pipe system to design flows, signed off, but as soon as the building loads up the balance wanders and some terminals starve. Is this a fundamental of variable-flow or did we balance it wrong?
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View the First Principles section of this entryIn variable flow, the ΔP across each branch shifts as two-port valves modulate elsewhere — so a fixed manual balance is only correct at one operating point. Pressure-independent control valves (PICVs) hold the flow regardless.
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This is fundamental, not a balancing error. In a variable-flow system the differential pressure across any branch changes as two-port valves elsewhere modulate — so a fixed-position manual balance is only correct at one operating point. The first-principles fix is pressure-independent control valves (PICVs) at the terminals: they hold the flow regardless of the system ΔP, so the balance survives load change. Manual double-regulating valves only truly work on constant-flow systems.
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