One of four condenser-water pumps trips repeatedly while the other three operate normally. The Honeywell Building Automation System reports high condenser-water temperature, yet the measured water temperature is 29 °C, the cooling tower is running, and replacing the temperature sensor did not clear the problem. Treat the pump trip and the temperature alarm as separate signals until the initiating device, input channel, and trip logic prove that they are connected.
Diagnostic Path Comparison
Before anything else, confirm what actually stops the pump. A BAS alarm can report a process condition without commanding the motor off, while a motor protection device, variable-speed drive, starter, safety circuit, or local controller can independently remove the run output.
| Diagnostic path | Question answered | Evidence to collect | Decision |
|---|---|---|---|
| Temperature signal path | Is the BAS displaying the real water temperature? | Local reference measurement, transmitter output, controller raw input, scaled BAS value, alarm setpoint | Repair calibration, scaling, wiring, or logic if the values diverge |
| Pump trip path | Which device removes the pump run condition? | First-out indication, drive or starter status, overload state, interlocks, command and feedback sequence | Correct the initiating electrical, mechanical, or control condition |
| Hydraulic path | Does the affected pump operate at a different duty point? | Suction and discharge pressure, valve positions, strainer condition, flow indication, vibration and noise | Correct restriction, loss of prime, abnormal flow, or pump condition |
Use the pump trip path first because it identifies the device that stopped the motor. Investigate the temperature path in parallel, but do not replace more field sensors until a point-to-point comparison identifies an error. The fact that only one pump trips makes a pump-specific starter, drive, motor, valve, wiring, or hydraulic condition a higher-priority check than a common cooling-tower condition.
First-Out Trip Identification
- Capture the BAS pump command, run feedback, trip status, condenser-water temperature, and relevant interlocks over the period surrounding a trip. Do not reset the fault until its first-out indication has been recorded.
- Inspect the local motor controller immediately after the event. Record whether the stop originated from an overload relay, drive fault, protective device, emergency or safety circuit, or loss of control power. Use the exact displayed code or physical trip flag; a generic BAS
Tripindication is not the root cause. - Compare the time sequence. If the high-temperature alarm occurs before the run command is removed, inspect the shutdown logic to see whether that alarm participates in the permissive or trip chain. If the motor controller trips first and temperature rises afterward, treat the temperature increase as a consequence of lost flow.
- Check whether the BAS command remains on after run feedback disappears. Command present with feedback absent points toward the starter, drive, motor, field wiring, or mechanical load. Command removed before feedback disappears points toward BAS logic or an external interlock.
Do not move on until the stopping device and event order are known. Resetting the pump before recording first-out data destroys the most useful diagnostic information.
Temperature Measurement Chain
A replaced sensor does not validate the complete measurement loop. The sensing element, field wiring, signal conversion, controller input, engineering-unit scaling, alarm threshold, and displayed BAS point can each introduce an error. A correct 29 °C local reading can coexist with an incorrect BAS value if any downstream stage is misconfigured or drifting.
- Place a calibrated reference instrument at the same measurement location and allow both readings to stabilize under the same process condition.
- Read the value at every accessible stage: field sensor or transmitter output, controller input, converted engineering value, and Honeywell BAS display. Record all readings at the same time.
- Check input type, signal range, scaling endpoints, units, and any configured offset. Match these settings to the installed sensor or transmitter documentation rather than copying settings from an unrelated point.
- Inspect terminations for loose conductors, moisture, shielding problems, grounding errors, damaged cable, and unintended resistance. Where practical, test the input channel with an appropriate known signal and confirm the displayed value.
- Read the configured high-temperature alarm threshold, deadband, delay, and shutdown association. Compare 29 °C with the actual configured threshold and the system design requirement; the word “high” alone does not establish that 29 °C is abnormal.
If the field reference and controller value agree but the BAS display does not, correct database scaling or point mapping. If the controller raw value is wrong while the field signal is correct, investigate the input configuration, wiring, or channel. If both field devices read the same temperature, move the process investigation to flow and heat rejection.
Pump-Specific Electrical and Mechanical Checks
Three normally operating pumps provide useful comparison data. Test the affected pump under the same operating demand and compare equivalent measurements rather than relying on nameplate assumptions.
| Check | Abnormal finding | Likely diagnostic direction |
|---|---|---|
| Line current by phase | Higher current or phase imbalance on the affected unit | Supply, connection, motor, mechanical load, or hydraulic duty |
| Supply voltage | Low or imbalanced voltage at the affected controller | Feeder, fuse, contactor, terminal, or upstream supply issue |
| Overload or drive setup | Configuration differs from the motor data or peer unit | Correct configuration only after checking motor and controller documentation |
| Suction and discharge conditions | Restricted suction, unusual differential pressure, or unstable pressure | Valve position, strainer blockage, air entry, loss of prime, or hydraulic restriction |
| Mechanical condition | Excess vibration, noise, heat, or shaft resistance | Coupling, bearing, alignment, impeller, or motor condition |
Inspect isolation and control valve positions, strainers, check valves, suction availability, and evidence of air in the circuit. Confirm that the affected pump is not starting against a different valve state from the other pumps. Never raise an overload setting merely to prevent a trip; first compare measured current with motor nameplate data and identify why the protection operated.
Control Logic and Sequence Review
The BAS program may combine demand, permissives, alarms, proof timers, and run feedback. Review the actual cause-and-effect logic from the pump command to every condition capable of removing it.
- Identify the high-temperature point used by the sequence and verify that it is the intended sensor, not a duplicated, stale, or incorrectly mapped point.
- Trace every permissive and trip input for the affected pump. Compare its logic with the three pumps that remain in service, including inversion, alarm association, manual overrides, and run-feedback treatment.
- Confirm that the commanded pump number matches the physical starter or drive and its feedback. A cross-mapped command, status, or trip point can make one pump appear to fail when another device generated the indication.
- Test one condition at a time under controlled operating conditions. Confirm the command, physical output, motor response, feedback, and alarm state before proceeding to the next condition.
If all four pumps share the same temperature interlock but only one stops, compare the affected pump’s individual branch of logic and field control circuit. If the high-temperature point directly trips only that pump, validate whether that association matches the intended sequence before changing it.
Corrective Action and Verification
- Correct the identified source: measurement-loop configuration, BAS point mapping, shutdown logic, electrical defect, protection configuration, valve state, hydraulic restriction, or mechanical fault.
- Clear overrides and temporary test conditions. Restore all protection and interlocks to their documented operating state.
- Start the affected pump through the normal Honeywell BAS sequence. Confirm that the run command reaches the field controller, the pump starts, run feedback returns, and no local protection operates.
- Compare the reference water temperature, controller value, and BAS display. They must track through normal process variation within the tolerances specified for the installed instruments.
- Trend command, feedback, trip status, temperature, and available electrical or hydraulic measurements through enough operating cycles to reproduce the former failure opportunity. The repair is verified only when no first-out trip recurs and every recorded value remains within its documented operating limit.
Frequently Asked Questions
Why does one condenser-water pump trip while the other three run?
A single affected pump directs the first checks toward its starter or drive, motor, field wiring, protection settings, valves, strainer, hydraulic duty, and individual control logic. Capture the local first-out indication before resetting it.
Why does the Honeywell BAS show high temperature after replacing the sensor?
The fault can remain in the wiring, controller input type, signal scaling, point mapping, alarm threshold, or configured offset. Compare a calibrated local reading with the raw input and displayed BAS value at the same time.
Why can condenser-water temperature rise after a pump trip?
Loss of circulation reduces heat transport, so temperature may rise after the motor stops. Use timestamped command, feedback, trip, and temperature trends to determine whether temperature preceded the stop or followed lost flow.
How do I verify the condenser-water pump repair?
Run the pump through its normal BAS sequence and trend command, feedback, trip status, and temperature across repeated operating cycles. Final verification requires no recurring first-out trip and measured values that remain within the documented limits for the installed equipment.