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VPF Variable Primary Flow Control Valve Hunting — PID Re-tuning and 40% Minimum Flow

Likely causes

PID proportional (P) gain too high — valve oscillates ±20%+ around setpoint

Check: Check valve opening (%) trend in BMS for hunting waveform — look for regular up-down oscillation. Check current P gain setting (normally 0.5~2.0 range; above 3.0 is excessive)

What to do: Reduce P gain by 20~30% from current value → observe 10 minutes → if hunting amplitude decreases, extend I time by 1.5×. Reset D gain to 0 and re-tune with P and I only — add small D (0.1~0.3) after stabilization. Set minimum valve opening to 10% lower limit to prevent unstable control at low opening

Evaporator minimum flow (40% of rated) not set — over-cooling and surging risk

Check: Check evaporator minimum flow setpoint in BMS — if unset or 0, dangerous. Verify evaporator rated flow (e.g., rated 500 LPM → must maintain minimum 200 LPM)

What to do: Enter 40% of evaporator rated flow as BMS hard limit (e.g., rated 500 LPM → minimum 200 LPM). Prohibit valve opening below 5% — set lower limit at 10% minimum. If differential pressure sensor delay exceeds 1 second, replace sensor or adjust filter constant

Field tip

First action for VPF valve hunting: halve P gain and set 40% minimum flow hard limit — these two steps alone resolve 80% of cases.

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