ΔT is the first thing to watch: a falling ΔT is the earliest, loudest sign that a plant's health is slipping.
The line drifts toward the current ΔT. Raise the fouling slider and watch it slide from the green band down into amber and red — that slow decline over days or weeks is exactly the pattern an FDD/AI system is trained to catch early.
A ΔT below design rarely means too little cooling — it usually means the plant is moving too much water for the heat it's actually transferring. Likely causes: fouled or dirty cooling coils, stuck-open or leaking control valves, three-way valves bypassing water, or simply over-pumping at low load. The hidden cost is energy: to still meet the load the plant pushes extra flow, so pump power climbs (cube law) while the chiller sees warm-ish return water it can't use efficiently. Distinguishing a genuine fault from plain over-pumping is the key diagnostic — and the job of the symptoms we'll add in the next stages.