Central plant faults
Chillers, boilers, towers and pumps.
Eighteen rules for the central plant: low chilled-water and hot-water ΔT, condenser and tower approach drift, short-cycling, chillers idling at low load, missing supply-temperature and pressure resets, decoupler reverse flow and condensing boilers that never condense. Plant faults are fewer than air-side faults but each one is usually worth more.
How to read it. Three days at the chiller. Blue is the chilled water leaving the chiller, teal is the water coming back. The gap between them (the ΔT) should stay wide when the plant is loaded. Here it narrows day by day; the red area marks where it has collapsed, which means pumps are moving far more water than the load needs.
The rules
- PLANT-01
Chiller can't make chilled-water supply setpoint
The chiller is on but CHW supply temperature is above its setpoint by more than the tolerance.
Diagnostic · reliabilityBAS trends - PLANT-02
Low chilled-water ΔT syndrome
The chilled-water loop's supply-to-return delta is below design when the chiller is loaded.
Capacity-basedBAS trends - PLANT-03
Condenser approach drifting up — likely tube fouling
The gap between condenser leaving water temp and refrigerant condensing temp has grown over time when normalized by load.
Capacity-basedBAS trends - PLANT-04
Boiler and chiller running at the same time
The building is calling for both heating and cooling at the central plant simultaneously.
Capacity-basedBAS trends - PLANT-05
Pump running with no flow
Pump status reads on but flow is at or near zero — the pump is deadheading against closed valves.
Capacity-basedBAS trends - PLANT-06
Cooling-tower fan short-cycling
The cooling tower's fan is starting and stopping more frequently than its minimum-runtime allows.
Capacity-basedBAS trends - PLANT-07
Chiller short-cycling
The chiller is starting and stopping more frequently than its minimum-runtime safety limit allows.
Capacity-basedBAS trends - PLANT-07B
Boiler short-cycling
The boiler is starting and stopping more frequently than its minimum-runtime safety limit allows.
Capacity-basedBAS trends - PLANT-07D
Chiller efficiency drifting
The chiller uses more kW for the same cooling than it did at the start of the data.
Capacity-basedBAS trends - PLANT-08
Chiller running at very low load
The chiller idles below a quarter of its rated draw for hours.
Capacity-basedBAS trends - PLANT-09
Chilled-water supply reset absent
The chilled-water supply setpoint stays flat for weeks while the weather swings.
Capacity-basedBAS trends - PLANT-10
Condenser-water reset absent
The condenser-water setpoint never follows the wet-bulb.
Capacity-basedBAS trends - PLANT-11
Cooling-tower approach high
Tower leaving water is far above the outdoor wet-bulb with the fans working.
Capacity-basedBAS trends - PLANT-12
Tower fans overshooting setpoint
Tower fans at full speed while the leaving water is already colder than setpoint.
Capacity-basedBAS trends - PLANT-13
Decoupler reverse flow
The secondary loop pulls more water than the primary supplies.
Capacity-basedBAS trends - PLANT-14
Condensing boiler not condensing
Hot-water return to a condensing boiler is warm enough that the flue gas cannot condense, so the boiler runs as a conventional one.
Capacity-basedBAS trends - PLANT-15
Hot-water loop low ΔT
The hot-water loop ΔT collapses while the boiler runs.
Capacity-basedBAS trends - PLANT-16
Pump ΔP reset absent
The secondary pump differential-pressure setpoint is flat for weeks.
Capacity-basedBAS trends
What is low ΔT costing in pumping?
Estimate the extra pump energy from moving more water than the load needs.
Enter positive numbers to see an estimate.
Pump power rises with the cube of flow. At design ΔT the loop needs only (actual ÷ design) of today’s flow, so pump power at design would be that ratio cubed. Energy Agent measures ΔT and pump power from the plant trends.
Screening estimate. Energy Agent measures the real fault hours and load from your trends and prices them at your tariff.
See these rules run on your building.
Send twelve months of interval data and a BAS trend export. Energy Agent returns a ranked list of what is wasting money, what each item is worth per year at your tariff, and what to do about it.