PID Loops Explained for BAS Techs — The Math Behind the Modulation

Your VAV’s discharge air temp trace looks like a cardiogram. Heat is running full bore, the zone is still cold, and the control signal is either pinned or oscillating. Before you touch the valve, you need to understand what the controller is actually doing behind the scenes — and that means PID. In this episode we break down Proportional, Integral, and Derivative with light math, a real VAV reheat walkthrough (setpoint 90°F, error 18°F, Kp×Ki worked out live), and field-tested tuning heuristics a Ziegler-Nichols textbook won’t give you. We cover: • What a control loop actually is: setpoint, process variable, error, output • P (Proportional) — reacts NOW, but always leaves a steady-state offset. Formula, intuition, field behavior. • I (Integral) — remembers the past, pulls you to setpoint, but wind-up causes overshoot. • D (Derivative) — anticipates the future, looks great on paper, amplifies sensor noise in the real world. Why most BAS loops leave D at zero. • The full PID equation — output = Kp·e + Ki·∫e·dt + Kd·de/dt — and why three numbers control 90% of what your building does. • A VAV reheat worked example: real setpoint, real error, real output percentage. • Four field symptoms of a broken loop: hunting, sluggish, overshoot, pinned-at-limits. What to change and why. • A seven-step field tuning recipe (the “Ziegler-Nichols lite” that actually works when you’re standing on a ladder). • G36 trim & respond — what it is, why it’s different from PID, and when to use which. This is the math a controls tech actually uses, the way a field tech actually explains it. Timestamps 0:00 The hunting trace everyone has seen 0:34 What a control loop actually is 1:07 P — Proportional (react to error now) 1:51 I — Integral (remember what happened) 2:32 D — Derivative (anticipate what’s coming) 3:06 The PID equation in plain English 3:33 VAV reheat walkthrough — live math 4:21 Four ways a loop can go wrong 5:16 A seven-step field tuning recipe 5:53 G36 trim & respond: React · Remember · Anticipate 6:21 Next episode — TAB × Controls Series: BAS Fundamentals • Ep 1: Protocols (BACnet, Modbus, LON) • Ep 2: MS/TP Troubleshooting • Ep 3: MS/TP Network Architecture • Ep 4: Analog vs Binary I/O Troubleshooting • Ep 5: Economizer Sequencing • Ep 6: VAV Box Operation • Ep 7: PID Loops — The Math Behind the Modulation ← you are here • Ep 8 (next): Air Balancing × Controls — how TAB contractors and controls techs can actually work together