What Is PID Control?

The Shower Analogy

You step into the shower. Too cold. You turn the hot water knob — a lot, because it is very cold. Water gets warmer. Slightly too warm now. You turn back — a little. Perfect.

Then someone flushes a toilet in the other bathroom. Pressure drops. Water goes cold again. You adjust again. You keep making small corrections continuously until you find the perfect balance.

You just performed PID control. Your brain was the controller. Water temperature was the measured value. Your comfort was the setpoint. Your hand movements were the output.

P — Proportional — How far am I from target?

Big gap between current temp and setpoint → Big correction
Small gap → Small correction
Turn the knob a lot when water is very cold
Turn just a little when almost right

I — Integral — How long have I been off target?

Even small persistent error → Integral pushes harder to eliminate it
Has memory — accumulates error over time
Shower slightly cold for 2 minutes → give a bit bigger turn than current gap alone suggests
Error eliminated → Integral relaxes

D — Derivative — How fast is it changing?

Temperature rising very fast → Apply correction early before overshooting
You see water heating rapidly → ease off the hot knob before it becomes too hot
Value stable → Derivative does nothing

Why Not Just ON/OFF Control?

If your shower could only be full cold or full hot — no middle ground — you would swing from freezing to boiling forever, never finding comfort. Same for BMS. On/off control causes continuous oscillation. PID finds exact balance and holds it.

Why AHUs Oscillate:

When P value is set too high — controller overcorrects every cycle. Like someone turning the shower knob too aggressively — always overshooting both ways, never settling. Correct PID tuning finds the values where the system reaches setpoint quickly, without overshooting, and holds it stably.

Related Topics

• What is BMS integration? — how a BMS connects with VFDs, energy meters, BACnet/Modbus devices and other building systems
• How to design a BMS system step by step — the complete BMS design methodology covering site survey, IO list, controller selection, sequence of operations
• What is a Building Management System (BMS)? — fundamentals of BMS controls and architecture for HVAC, lighting, energy and access
• What is BMS commissioning? — the disciplined commissioning process that turns a BMS install into a working building brain
• Browse all Control Loops & FBD topics — more from this section of the EnSmart BMS Library