Grant A5 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A5 fault code mean?
The A5 code appears on the Grant Aerona³ remote controller and terminal PCB display when the heat pump's control system detects that the unit is operating outside its safe working limits. This is broadly described in the Aerona³ installation manual as abnormal or unreasonable operation under an overload condition. In practice it means the heat pump has triggered a protective shutdown — it will not simply recover on its own and requires investigation before normal heating or hot water production can resume.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Blocked or restricted airflow at the outdoor unit Common
Leaves, garden debris, vegetation, snow or ice packed against the air inlet grille or evaporator coil restricts the flow of air that the heat pump depends on. The unit works progressively harder to compensate, pressures climb and the overload protection trips. This is the first thing to check because it costs nothing to fix and is surprisingly common — especially after autumn leaf-fall or an overnight frost.
- Incorrect refrigerant charge (overcharge or undercharge) Common
If the refrigerant circuit contains too much or too little refrigerant — whether because of a previous service error or a slow leak — the operating pressures will move outside the design envelope. An overcharged system sees abnormally high discharge pressures; an undercharged system causes the compressor to work harder and overheat. Both conditions can trigger A5. This is confirmed in the Aerona³ installation manual as a primary cause, and only an F-Gas registered engineer can check and correct the charge.
- Compressor fault or failure Sometimes
The scroll or rotary compressor at the heart of the refrigerant circuit can develop mechanical wear, winding faults or starting problems over time. When the compressor cannot spin up correctly or draws excessive current, the inverter detects the overload and shuts the system down. Field reports from Aerona³ installations indicate that a failing compressor is a realistic cause of A5, particularly on units that have not been serviced regularly or that have accumulated several years of operation.
- Faulty inverter or main PCB Sometimes
The inverter drive board controls the variable-speed compressor motor. If the PCB develops a fault — perhaps due to a voltage spike, component ageing or moisture ingress — it may misread operating conditions or fail to control the compressor correctly, producing an A5 lockout. The Aerona³ manual suggests that if the error reappears immediately after a power-cycle, PCB replacement should be considered.
How to fix it
- Inspect the outdoor unit for obstructions and clear them DIY safe
Walk around the outdoor unit and look carefully at all four sides. Remove any leaves, plant growth, garden furniture, mud or accumulated debris from the air inlet grille and the area immediately around the unit. In winter, check whether ice or compacted snow is covering the grille — if so, allow it to thaw naturally or gently pour lukewarm (not boiling) water over it. The unit needs at least 30–50 cm of clear space on the intake side and clear space above. Do not use sharp tools near the fins.
- Check and correct system water pressure if low DIY safe
Look at the pressure gauge on your heating system (often on the buffer vessel, the cylinder or the heat pump itself). A typical heat pump system should read between 1.0 and 1.5 bar when cold. If the reading is below 0.8 bar, top up via the filling loop following your installer's instructions. A low-pressure condition alone will not cause A5, but confirming the system is otherwise healthy is a useful first check before calling an engineer.
- Do not attempt a manual fault reset yourself heat pump engineer
Grant UK advises that some Aerona³ fault codes require a manual reset that must only be carried out by a qualified heat pump installer or service engineer. Resetting A5 without first identifying and resolving the root cause risks damaging the compressor or inverter board further, potentially turning a recoverable fault into a much more expensive one. Leave the controller as it is.
- Contact a Grant-accredited or MCS-certified heat pump engineer to diagnose the fault heat pump engineer
An engineer will check both the fault code on the Aerona remote controller and the two-digit code on the terminal PCB inside the unit to confirm the A5 reading. Their diagnostic process will typically cover: measuring refrigerant pressures on the high and low sides; checking the compressor's running current and electrical continuity; inspecting the inverter/main PCB for signs of damage; and confirming whether the refrigerant charge needs to be recovered, weighed and corrected. All refrigerant work must be carried out by an F-Gas registered engineer. If the fault reappears immediately after a controlled power-cycle, the main PCB is likely to need replacement.
- Follow the engineer's recommended repair and arrange a post-repair test heat pump engineer
Once the root cause has been addressed — whether that is a refrigerant recharge, PCB swap, or compressor replacement — ask the engineer to carry out a full system performance check before signing off. Confirm that the heat pump returns to normal operation, that the fault code has been properly cleared and does not reappear, and that flow temperatures and system pressures are back within normal range. If your unit is covered by a Grant G1 installer warranty, check whether the repair affects your warranty terms before any parts are replaced.
Parts you may need
- Grant Aerona³ Main PCB / Inverter Board · from £450
- Refrigerant R32 (recharge, labour included) · from £280
- Compressor (unit-size dependent) · from £900
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£550, depending on the underlying cause.
Frequently asked questions
Can I reset the A5 fault code myself on a Grant Aerona³?
Grant UK specifically advises against homeowners resetting fault codes on the Aerona³ remote controller themselves. Some codes — including A5 — require a manual reset that should only be performed by a qualified heat pump installer or service engineer, and only after the underlying cause has been identified and resolved. Resetting without fixing the root problem can lead to repeat shutdowns or further damage to the compressor or inverter board.
Could the A5 fault just be caused by snow or leaves blocking the outdoor unit?
Yes, and this is the first thing worth checking. Blocked airflow is one of the primary causes listed in the Aerona³ installation manual for the A5 overload condition. Clear any debris, ice or vegetation from around the outdoor unit and then call an engineer. Even if the blockage was the sole trigger, the unit will still need the fault code professionally reset — it will not simply restart on its own.
How much does it cost to fix a Grant Aerona³ A5 fault?
For the most common causes — a refrigerant recharge or a PCB replacement — most UK homeowners pay somewhere between £150 and £550 including the engineer's call-out and labour. If the compressor itself has failed, the cost rises considerably: compressor replacement on an Aerona³ typically runs to £1,500–£3,000 or more depending on the unit size and whether additional components were damaged. It is worth checking your warranty status first — units installed and annually serviced by Grant G1 engineers benefit from a 7-year warranty that may cover some repair costs.
Does an A5 fault on a Grant Aerona³ always mean the compressor has failed?
Not necessarily. While a failed compressor is one possible cause, A5 more broadly indicates the heat pump has detected an overload condition. In many cases the cause turns out to be something less serious — a blocked outdoor unit or an out-of-tolerance refrigerant charge. A qualified engineer will carry out pressure and current checks to narrow down the cause before any parts are condemned. Jumping straight to a compressor replacement without proper diagnosis is both unnecessary and expensive.