Meaning: No active faults stored.
Likely causes: Normal operation.
Checks: —
Actions: —
Meaning: Flow NTC sensor is open circuit or shorted.
Likely causes: Damaged sensor, loose/broken wiring, water ingress in connector.
Checks: Visual check of wiring and plug, measure sensor resistance against Vaillant tables.
Actions: Repair/replace wiring; replace flow sensor if readings are out of spec.
Meaning: Return NTC sensor is open circuit or shorted.
Likely causes: Broken cable, poor connection at PCB, failed NTC.
Checks: Inspect cable and connector, measure resistance compared to temperature.
Actions: Repair wiring or replace return sensor as required.
Meaning: Flow temperature has exceeded the safety limit.
Likely causes: No circulation, airlock, closed valves, heavily blocked system.
Checks: Check pump operation, verify all valves are open, bleed air, inspect filters/strainers.
Actions: Restore flow and remove blockages, reset STB (if manual) and controller once issue is resolved.
Meaning: Water pressure is below the minimum threshold.
Likely causes: System leak, PRV passing, low expansion vessel charge.
Checks: Check pressure gauge, inspect for leaks, test EV pre-charge, check discharge pipe.
Actions: Refill to 1.2–1.5 bar when cold; fix leaks and recharge/replace expansion vessel as needed.
Meaning: Circulation pump is called but flow/temperature change is not detected.
Likely causes: Seized or air-locked pump, closed isolation valves, blocked strainers.
Checks: Confirm pump is powered and spinning, check valves, clean strainers and dirt separators, bleed air.
Actions: Restore/replace pump, open valves, flush or clean blocked components.
Meaning: Flow rate through the heat pump is below the minimum required.
Likely causes: Pump speed too low, TRVs/zone valves mostly shut, partial blockage or air.
Checks: Increase pump speed, confirm enough emitters are open, check for air and debris in system.
Actions: Balance system, bleed, clean filters; if problem persists, investigate flow sensor or hydraulic design.
Meaning: The flow sensor signal is implausible or missing.
Likely causes: Air, sludge or limescale at sensor, damaged sensor, wiring fault.
Checks: Clean sensor/strainer, inspect cabling, compare readings with measured flow/ΔT.
Actions: Clean or replace flow sensor, repair wiring.
Meaning: No or faulty communication between indoor controller and outdoor PCB.
Likely causes: Reversed eBUS polarity, broken cable, outdoor PCB not powered.
Checks: Check supply to outdoor unit, verify bus wiring polarity & continuity, inspect terminals.
Actions: Correct wiring, restore power, replace cable/PCB if required.
Meaning: Temperature difference between flow and return is too small or reading is implausible.
Likely causes: Very low load, bypass too open, sensor mis-read, incorrect hydraulic layout.
Checks: Check ΔT across system, verify bypass valves, compare with independent temperature readings.
Actions: Adjust flow rates and bypass, correct system design issues, replace sensor if faulty.
Meaning: Domestic hot water NTC sensor fault.
Likely causes: NTC failure, damaged cable, poor terminal connection.
Checks: Measure resistance, compare to temperature; inspect cable and clamp position in cylinder pocket.
Actions: Reseat or replace NTC, repair wiring.
Meaning: Flow temperature is not increasing as expected.
Likely causes: Oversized system volume, high heat loss, low compressor output, very cold conditions.
Checks: Check operating conditions, compare output to design, verify refrigerant circuit performance.
Actions: Adjust weather compensation, review design and emitter capacity; engineer to check refrigerant side if poor output suspected.
Meaning: Return temperature does not rise in line with demand.
Likely causes: Large system volume, very high load, incorrect hydraulic configuration.
Checks: As for F.100 – system volume, ΔT, output.
Actions: Optimise system settings or design; check for undersized emitters.
Meaning: Flow temperature has exceeded limit during operation.
Likely causes: Low flow rate, blocked or closed circuits, incorrect setpoint, faulty sensor.
Checks: Confirm pump performance, open all valves, check set flow temp and sensor readings.
Actions: Restore adequate flow, correct settings, replace sensor if proven faulty.
Meaning: Internal compressor protection function has intervened.
Likely causes: High discharge temperature, high or low pressure issues, poor heat transfer.
Checks: Engineer: check pressures, superheat/subcool, coil condition, fans.
Actions: Rectify airflow or hydraulic issues, carry out full refrigerant-side diagnostics.
Meaning: Outdoor air NTC open or short circuit.
Likely causes: Cable damage, water ingress, failed NTC.
Checks: Measure resistance, inspect mounting position and cable routing.
Actions: Replace outdoor sensor, repair wiring.
Meaning: Supply frequency or voltage detected outside allowed range.
Likely causes: Grid issues, long cable runs, local supply instability.
Checks: Measure voltage and frequency at terminals under load.
Actions: Consult electrician if values are out of spec; consider surge or voltage protection.
Meaning: High-pressure switch or sensor has reached the trip limit.
Likely causes: Blocked or dirty outdoor coil, fan not running, excessive water temperature, incorrect refrigerant charge.
Checks: Check fan operation and airflow, inspect coil for dirt/ice, check water temperatures and flow.
Actions: Clean coil, restore airflow & flow; F-gas engineer to check charge and operation if fault recurs.
Meaning: Internal eBUS or module communication problem.
Likely causes: Bus wiring issues, duplicate addresses, noise on bus.
Checks: Confirm correct bus topology, check polarity, reduce unnecessary parallel devices.
Actions: Correct bus wiring, separate noisy cables, re-address modules if necessary.
Meaning: Supply voltage has fallen outside the controller’s safe range.
Likely causes: Undersized cables, grid dips, loose connections.
Checks: Voltage at heat pump during compressor start, check terminals and cable sizing.
Actions: Tighten/renew connections, upgrade cabling, involve electrician if supply is unstable.
Meaning: Internal sensor (e.g. in room controller) faulty.
Likely causes: Damaged sensor or incorrect mounting.
Checks: Compare displayed temperature to a reference thermometer, inspect wiring.
Actions: Reposition or replace sensor/controller.
Meaning: Control condition or sensor problem is blocking cylinder charging.
Likely causes: DHW sensor errors, configuration settings, 3-port valve wiring.
Checks: Check DHW NTC, valve position, control parameters and schedules.
Actions: Correct wiring/valve operation, adjust settings, replace sensor if needed.
Meaning: Controller is intentionally not calling for heating/cooling (weather comp or schedule).
Likely causes: Room temperature above setpoint, warm weather shutdown, time schedule.
Checks: Review setpoints, weather compensation curve, and time programs.
Actions: Adjust schedules or curve if the building is not comfortable.
Meaning: Low-pressure switch/sensor has triggered.
Likely causes: Refrigerant loss, blocked evaporator, expansion valve fault.
Checks: Engineer only – check pressures, leak test, inspect evaporator for ice/blockage.
Actions: Repair leaks, evacuate and recharge system, replace faulty components.
Meaning: An external safety device (e.g. flow switch, safety loop) has opened.
Likely causes: Flow switch operation, external lockout or wiring break.
Checks: Trace external safety wiring, confirm all safety devices are closed and functioning.
Actions: Reset/repair the external device, fix cable faults, then reset the controller.