HVAC Automation: On-site PID Parameter Tuning

During a commissioning job, the AHU supply air temperature was set to 18°C, but the actual value kept oscillating between 16°C and 21°C. Every time it overshot, it took several minutes to recover. The engineer tried increasing the proportional gain, which only made the oscillation worse; decreasing it made the response sluggish, as if it wasn't reacting at all. After struggling for a while, the parameters were a complete mess.

01 Before Tuning—If these two things aren't right, tuning parameters is a waste of time

When PID tuning fails, most people's first instinct is to tweak the parameters.

Prerequisite 1: Is the sensor in the right place?

Where the temperature sensor is installed dictates what temperature the PID is actually controlling. If the AHU supply air temperature sensor is mounted on the wall opposite the supply air outlet, it's measuring the temperature of the air already mixed with the room air, not the true supply air temperature. The lag will be severe, and the PID will inevitably oscillate.

Correct location: Inside the supply air duct, on a straight pipe section 0.5 to 1 meter away from the supply air outlet. The same goes for differential pressure sensors; if placed incorrectly, the readings won't represent the actual demand at the terminal units.

Prerequisite 2: Know the time constant of the controlled object

HVAC systems are slow to respond. From the moment a chilled water valve actuates to when the supply air temperature actually changes, there is usually a lag of 30 seconds to 3 minutes, depending on the coil size, air volume, and water flow. Just because the valve moves doesn't mean the temperature changes immediately; and just because the temperature hasn't changed doesn't mean the valve isn't doing its job.

Before tuning, manually apply a step signal (e.g., jump the valve from 20% directly to 80%) and observe how long it takes for the supply air temperature to start changing and eventually stabilize.

02 The "Feel" of P, I, and D parameters

P controls speed, I eliminates error, and D is basically useless.

P - Proportional: The larger the error, the harder the action. P dictates how much force to apply based on how far the current temperature is from the setpoint. High P means fast response but prone to overshoot and oscillation; low P means slow but stable response.

I - Integral: The longer the error accumulates, the more it slowly eliminates the difference. Relying solely on P will leave a steady-state error (the temperature will always be slightly off from the setpoint). I's job is to slowly accumulate this small deviation into an action until the steady-state error disappears. If I is too strong, it easily leads to integral windup and overshoot.

D - Derivative: Anticipates and brakes based on the trend of the error. It looks at the rate of change of the error to proactively reduce the output. Theoretically, it can reduce overshoot, but HVAC signals are noisy, and D tends to amplify this noise, causing the valves to jitter frequently.

On-site recommendation: Set D to 0 first. Only consider adding it in very specific situations.

03 On-site Observation Tuning Method—Start with P, one step at a time

The golden rule of tuning is: change only one parameter at a time, and wait for it to stabilize before moving on to the next. If you change two parameters at once, you won't know which one caused the problem when things go wrong.

Step 1: Turn off I and D, tune P alone

Set the integral time to its maximum (or set the I gain to 0), and set the derivative to 0. The system becomes pure proportional control. This way, the effect of P is the clearest.

Start with a relatively small P value (e.g., P gain = 0.5, or proportional band = 200%). Introduce a disturbance (e.g., manually change the setpoint by 2°C) and observe the temperature curve:

If the response is too slow and it takes forever to get near the setpoint → P is too small, increase it slightly.

If the temperature overshoots the setpoint and keeps oscillating → P is too large, decrease it.

Goal: The temperature should converge near the setpoint within 1 to 2 overshoots. A slight steady-state error is normal.

Step 2: Once P is stable, slowly add I to eliminate the steady-state error

Under pure proportional control, the temperature will stabilize near the setpoint but with a slight offset (steady-state error). At this point, adjust the integral time from large to small (or increase the I gain from small to large) to let I slowly eliminate the offset.

Don't set the integral time too small all at once. Decrease it by 20% to 30% each time, and observe a complete response cycle:

If the steady-state error is gone but the temperature starts oscillating slowly (low-frequency, large-amplitude swings) → I is too strong, increase the integral time slightly.

If you see integral windup (the temperature significantly exceeds the setpoint for a long time before coming back) → Check if there is an anti-windup setting, or increase the integral time.

Goal: The temperature stabilizes within ±0.5°C of the setpoint without continuous oscillation.

Step 3: Leave D alone in HVAC scenarios

If the system is stable after tuning P and I, just leave D at 0. HVAC sensor signals have noise, and D will amplify this noise, causing the valve to jitter slightly and frequently, which accelerates valve wear and isn't worth the trade-off.

The only time to consider adding D is: the system has an extremely short response time (e.g., high-precision pressure differential control in certain cleanrooms), and there is still obvious rapid oscillation after tuning P+I. Even then, start with an extremely small value and only increase it slightly each time.

Step 4: Verification—Test it under a different operating condition

Parameters tuned for one operating condition don't guarantee good performance in others. HVAC system loads change with the seasons. After tuning, you must verify the parameters under at least two typical conditions: high load and low load.

High load condition: High outdoor temperature, all terminal units fully open, high system pressure.

Low load condition: Nighttime or shoulder season, a large number of terminal units closed.

Only when the parameters perform acceptably under both conditions can the tuning be considered truly complete.

Suzhou Pharma Machinery Co.,Ltd.

2026/07/31

Gino