Playbook: Hydronic Heat-Exchanger Faults

Step 1 — Prove the topology and evidence

1. Identify one physical exchanger and trace both inlet/outlet pairs from drawings and a walk-down. Keep primary and secondary identities fixed.
2. Confirm every temperature and flow has that same equipment scope, timestamp basis, and unit. Reject common-header or fleet totals.
3. Record each side’s fluid, glycol concentration where applicable, density and heat-capacity source, and validity range.
4. Verify derived effectiveness/heat transfer inputs, energy-balance tolerance, expected-model version/readiness/domain, and training/commissioning period. Never fit a clean expectation on the episode being judged.
5. Confirm automatic mode, stable setpoints/flows/valves, maintenance state, safety/protective sequences, and the commissioned re-warm interval after a direction, pump, valve, or setpoint change.

Step 2 — Separate the signatures

1. For HX-0002, verify exchange_cmd is the final state that means both branches should flow. Availability or an upstream plant request is not enough. Compare the missing side with its pump command/proof, isolation valves, strainer DP, air/pressure state, and meter quality.
2. For HX-0001, compare actual and expected effectiveness only inside the frozen model’s domain. Check the two independently calculated side heat rates before blaming the exchanger.
3. For HX-0003, confirm the named valve is intended to isolate the whole path and its final command is closed. Allow commissioned transport/thermal soak, then inspect actual position if available, residual branch flow, bypasses, check valves, and gravity circulation.

Step 3 — Remote triage

1. Release only documented BAS overrides. Do not defeat freeze protection, minimum-flow, pressure, or other protective sequences.
2. Trend all four temperatures, both flows, final command/valve command, signed heat transfer, actual/expected effectiveness, and readiness/domain flags at a common fixed cadence.
3. Compare sensor offsets during a legitimate no-transfer equalization period only when the system can be placed there safely.
4. Inspect model inputs and fluid-property configuration. A bad density, glycol concentration, point sign, or connection swap can manufacture both low effectiveness and false energy imbalance.

Step 4 — Qualified on-site service

Only qualified hydronic/HVAC personnel following site lockout/tagout, pressure/temperature isolation, drain/fill, chemical-handling, and manufacturer procedures may open equipment, stroke valves locally, clean plates/tubes, or service pumps. Never isolate a required safety path or open a hot/pressurized system to test a diagnostic.

1. Inspect the missing-flow side for pump/coupling failure, closed isolation, actuator/linkage failure, clogged strainer, air lock, fouling, failed check valve, low system pressure, or a bad meter.
2. Inspect low effectiveness for plate/tube fouling, scaling, blocked channels, gasket/internal bypass leakage, incorrect piping, degraded glycol, and sensor placement/calibration.
3. Inspect unintended transfer for a passing valve seat, undersized actuator or insufficient close-off rating, linkage failure, manual bypass, parallel open path, failed check valve, or thermosiphon flow.
4. Use chemical or mechanical cleaning only under the exchanger and water- treatment manufacturer’s procedures; preserve corrosion and freeze control.

Step 5 — Confirm resolution

1. Revalidate all point scaling, side identity, timestamps, and energy balance.
2. Observe settled automatic operation in the applicable heating and/or cooling direction and confirm both branch flows when exchange is finally commanded.
3. Re-establish or deliberately preserve the clean baseline under a documented change-control policy after cleaning/replacement; do not let an online model silently learn a fault.
4. With the isolation valve legitimately closed and soak expired, confirm transfer remains within the commissioned no-load uncertainty envelope.
5. Observe for at least one full persistence interval after settling and verify the finding does not reassert.