PID values are a starting point, not the end of commissioning
An HVAC controller works with a real process: a valve, pump, heat exchanger, sensor, delays and often other interacting loops. A formula can provide a useful starting point, but it does not know about mechanical limits, dead bands, an incorrectly sized valve or process changes between summer and winter.
SIMC PI is useful when the process can be approximated by a gain, time constant and dead time. Choosing a larger τc generally gives a slower and more robust response. Ziegler–Nichols is historically important but may be aggressive for HVAC, so its result should be treated as guidance rather than a setting to apply blindly.
Before changing a live loop, check
- controller and actuator action direction;
- output limits, minimum times and safety interlocks;
- whether the sensor is fast enough and correctly located;
- whether oscillation is caused by tuning or by a mechanical problem.