Summary

Grant UK is one of the best-known names in UK off-mains-gas heating, having built its reputation on the Vortex oil boiler range before expanding into air source heat pumps with the Aerona3 monobloc and the HPiD/HPiD6 hydro-box (split) ranges. The Aerona3 is a common choice on Boiler Upgrade Scheme (BUS) installations replacing oil boilers in rural off-gas-grid properties, often installed by the same OFTEC-registered heating engineers who service Grant's oil range, which makes cross-brand familiarity with "how Grant present a fault" genuinely useful — see oil boiler fault codes grant for the equivalent oil-boiler diagnostic approach from the same manufacturer.

Unlike some heat pump brands whose fault-code architecture is shared across a huge international commercial air conditioning range (as with Daikin or Samsung), Grant's heat pump documentation is comparatively compact and specific to the Aerona3/HPiD platform itself. When a fault occurs, the remote controller in the house — the interface most engineers and customers interact with day-to-day — shows a flashing red LED on the ON/OFF switch, a warning triangle icon, and an alphanumeric fault code on its LCD screen. The outdoor unit's own terminal PCB also displays a code, which is useful when the remote controller itself is suspected of a fault or communication issue.

Note on code confidence: the fault codes below are drawn from Grant's own published Aerona3 series installation and servicing instructions. Grant's documentation is less exhaustively cross-referenced across third-party sources than for the market's longest-established heat pump brands, and the precise code list can vary between the original Aerona3, later Aerona3 R32 refrigerant revisions, and the HPiD/HPiD6 hydro-box models. Codes and descriptions not directly confirmed against the specific installed model's manual are marked **** — always check the data plate and the matching manual before ordering parts or condemning a component.

Key Facts

Quick Reference Table

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Code Category Likely Cause F-Gas Required?
A0 Power supply DC voltage error No
A1 Discharge temperature Discharge sensor fault, gas leakage, or abnormal compressor operation Possibly — check sensor/wiring first, refer if refrigerant confirmed
A2 Main PCB / DC current detection Momentary power interruption (e.g. lightning), PCB fault No
A3 CT (current transformer) CT disconnected, abnormal compressor operation, excess voltage protection No, unless compressor fault confirmed
A4 AC current detection Momentary power interruption (e.g. lightning), supply fault No
P1 Pump-related Circulating pump fault or flow issue — confirm exact definition against model manual No
CF1 Fuse — power supply Blown fuse; check for underlying short before replacing No
CF3 Fuse — main PCB Blown fuse; check for underlying short before replacing No
CF4 Fuse — fan motor Blown fuse; check fan motor for locked rotor/short before replacing No
CF6 Fuse — pump Blown fuse; check circulating pump for fault before replacing No
CF7 Fuse — reactor Blown fuse; check reactor/inverter circuit before replacing No

This table reflects codes documented in Grant's published Aerona3 series installation and servicing instructions, cross-checked against field-reported troubleshooting sources where marked. Confirm the exact code list, and exact fuse ratings, against the specific model and generation's own manual before ordering parts.

Detailed Guidance

Fault-Finding Decision Tree

START: Grant Aerona3 showing fault code on remote controller
       (flashing red LED, warning icon, alphanumeric code on LCD)
│
├─ STEP 1: Read the code from the remote controller AND check
│   the outdoor unit's terminal PCB display for a matching or
│   supplementary code
│
├─ STEP 2: Which category does the code fall into?
│   ├─ A-series (A0-A4) → go to POWER/COMPRESSOR branch
│   ├─ CF-series (fuse codes) → go to FUSE branch
│   ├─ P-series → go to PUMP branch
│   │   against specific model manual]
│   └─ Not listed above → consult the specific model's
│       installation and servicing manual directly
│
├─ POWER/COMPRESSOR BRANCH (A0-A4)
│   ├─ A0 (DC voltage) → check incoming supply voltage and
│   │   wiring at the outdoor unit isolator
│   ├─ A1 (discharge temperature) → STOP before assuming
│   │   refrigerant fault:
│   │   ├─ Check outdoor unit airflow/clearance
│   │   ├─ Check discharge sensor wiring/connector
│   │   └─ If airflow and wiring check out → F-Gas engineer
│   │       to check refrigerant charge and discharge pressure
│   ├─ A2/A4 (current detection) → check for recent lightning/
│   │   power interruption event; check supply quality; if
│   │   recurring without an obvious power event, suspect PCB
│   └─ A3 (CT disconnection) → check the current-transformer
│       connection at the compressor circuit before assuming
│       compressor fault
│
├─ FUSE BRANCH (CF1, CF3, CF4, CF6, CF7)
│   ├─ Identify which circuit the specific CF code protects
│   │   (power supply / main PCB / fan / pump / reactor)
│   ├─ Isolate power, inspect the corresponding component for
│   │   an obvious short, locked rotor, or seized bearing
│   │   BEFORE replacing the fuse
│   ├─ Replace fuse with the CORRECT rating for the specific
│   │   model (ratings vary — check data plate/manual)
│   └─ If fuse blows again immediately on power-up → component
│       fault confirmed, replace the specific faulty component
│       (fan motor, pump, etc.) not just the fuse
│
└─ PUMP BRANCH (P-series)
    ├─ Check circulating pump runs and is not airlocked
    ├─ Check system water pressure and flow rate
    └─ If pump confirmed faulty → replace (no F-Gas requirement,
        water-side component)

Before Escalating: Checks Any Heating Engineer Can Do

As with every heat pump brand, a meaningful share of Grant Aerona3 fault codes that reference temperature or current protection have simple, non-refrigerant causes that should be ruled out before involving an F-Gas engineer:

  1. Outdoor unit airflow — check the coil isn't blocked by leaves, debris, snow, or nearby planting, and that manufacturer clearances are maintained
  2. Isolator and supply voltage — confirm the outdoor unit isolator is on and supply voltage is within the unit's rated tolerance; undervoltage or a recent power interruption is a genuine and recurring cause of A2/A4-type faults
  3. Water-side flow and pressure — check the circulating pump runs, the system isn't airlocked, and pressure is correct; a restricted flow can cause temperature-related protection trips that mimic a refrigerant fault
  4. Fuse circuit inspection before fuse replacement — for any CF-series code, check the specific protected component (fan motor, pump, reactor) for an obvious fault before simply fitting a new fuse and re-energising, since a fuse rarely blows without a cause
  5. Fault history — if the controller retains a fault log, check for a pattern (recurring at the same time of day, weather condition, or DHW demand) rather than diagnosing a single occurrence in isolation

Only once these are ruled out should diagnosis move to the sealed refrigerant circuit — checking pressures, superheat, subcooling, and compressor electrical characteristics — which requires F-Gas certification.

R32 Refrigerant Handling Considerations

Later Aerona3 units use R32 refrigerant, an A2L (mildly flammable, lower toxicity) classification under BS EN 378, which brings specific handling, ventilation, and recovery equipment requirements beyond those for the non-flammable A1 refrigerants used in older heat pump generations. Confirm which refrigerant is fitted from the appliance data plate before any refrigerant-related work, and ensure the attending F-Gas engineer's certification and equipment cover A2L refrigerant handling specifically, not just standard A1 F-Gas work.

Reset Procedure and When Not to Reset

Power supply and current-detection faults (A0, A2, A4) linked to a one-off power interruption event (a lightning strike, a brief supply dropout) will often clear with a single controller reset or power-cycle, and a single reset to confirm the fault clears is reasonable practice. For A1 (discharge temperature) faults and any fault where a genuine refrigerant-circuit or compressor cause is suspected, repeated resetting without addressing the underlying cause risks further compressor damage and is poor practice.

Frequently Asked Questions

Can I reset a Grant Aerona3 fault code myself without calling a specialist?

For power-supply and current-detection codes (A0, A2, A4) linked to an identifiable one-off power event, a single reset to confirm the fault clears is reasonable, provided the likely cause is also checked. For A1 (discharge temperature) or any fault where airflow, wiring, and water-side checks don't resolve it, stop and arrange diagnosis by a suitably qualified (and, where the refrigerant circuit is implicated, F-Gas certified) engineer rather than resetting repeatedly.

Where do I find the complete, authoritative fault code list for my specific Aerona3 model?

Always use the installation and servicing instructions for the exact model and generation fitted — Grant's Aerona3, Aerona3 R32, and HPiD/HPiD6 hydro-box ranges are documented separately, and code sets are not guaranteed to be identical across them. The manual, or Grant UK's own technical support line, should be the primary reference rather than general troubleshooting forums, which can carry unverified or model-mismatched information.

Does OFTEC registration cover Grant Aerona3 heat pump work?

No. OFTEC registration is specific to oil-fired heating equipment and does not itself qualify an engineer to work on a heat pump's electrical/water-side systems or its refrigerant circuit. Heat pump installation and servicing work typically requires MCS certification (for BUS grant compliance) and, for any refrigerant-circuit work, F-Gas certification appropriate to the refrigerant type fitted — an OFTEC-registered engineer moving from Grant's oil range to its heat pump range needs these additional qualifications, not a substitute for them.

Is a blown fuse (CF-series code) something I can just replace myself?

The fuse replacement itself is straightforward and doesn't require refrigerant qualification, but always isolate the unit, identify and inspect the specific component that fuse protects (fan motor, pump, reactor, etc.) for an obvious fault before fitting a new fuse, and use the exact fuse rating specified for that model — ratings differ between Grant heat pump models. If the replacement fuse blows again immediately, the underlying component is faulty and needs replacing or further diagnosis, not another fuse.

Regulations & Standards