Grant E8 Fault Code: Causes, Fixes & Repair Costs
What does the Grant E8 fault code mean?
The E8 fault code on the Grant Aerona³ air source heat pump indicates that the unit has entered a protective shutdown due to unreasonable or overload operating conditions. This is most commonly triggered by restricted airflow at the outdoor unit's air inlet or outlet, or by an incorrect refrigerant charge. When the system detects it is operating outside safe parameters — unable to move or exchange heat effectively — it shuts down to prevent damage to the compressor and other components. The fault is displayed as E8 on the Aerona Remote Controller (accompanied by a flashing red LED) and on the terminal PCB LED display on the outdoor unit itself.
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 air inlet or outlet on the outdoor unit Common
Leaves, garden debris, overgrown vegetation, compacted snow or ice, or objects placed too close to the outdoor unit can obstruct airflow through the heat exchanger coil. Without sufficient airflow, the system cannot dissipate heat properly and trips into overload protection. This is the first thing to check and the most frequently identified cause of the E8 fault.
- Dirty or fouled outdoor heat exchanger coil Common
Over time, dust, dirt, pollen and fine debris accumulate on the coil fins of the outdoor unit. A heavily soiled coil significantly reduces heat transfer efficiency, causing the system to work harder than intended. The resulting strain can push operating pressures and temperatures beyond acceptable limits, triggering the E8 overload protection.
- Incorrect refrigerant charge (overcharge or undercharge) Sometimes
If the sealed refrigerant circuit contains too much or too little refrigerant — whether due to a previous top-up error, a leak, or work carried out without proper recovery and recharge — the system's heat exchange process is disrupted. Both overcharge and undercharge scenarios can cause abnormal high-side pressures or compressor overheating, leading to an overload shutdown. The Grant installation manual explicitly lists excess refrigerant as a cause to investigate for this code.
- Fan motor fault or reduced fan speed Sometimes
The outdoor fan draws air across the heat exchanger coil. If the fan motor is running slowly, intermittently or not at all due to an electrical fault or bearing wear, airflow is compromised even when the unit itself is unobstructed. This has essentially the same effect as a physical blockage and can trigger the E8 overload condition.
- Temperature sensor fault causing false overload detection Rare
The Aerona³ uses multiple temperature sensors — including the discharge temperature sensor, defrost/suction sensor and outdoor ambient sensor — to monitor operating conditions. If one of these sensors has drifted out of calibration or developed a fault, it may report readings that cause the controller to incorrectly conclude the system is in an overload state, even when physical conditions are normal.
- Main PCB fault Rare
In a small number of cases, the E8 fault can be caused by a fault on the main control PCB itself rather than any external or refrigerant-circuit cause. If the code persists after the unit has been fully powered down for at least three minutes and restarted, and no other cause can be identified, PCB replacement may be required.
How to fix it
- Inspect the area around the outdoor unit and clear any obstructions DIY safe
Walk around the outdoor unit and remove any leaves, garden debris, plant growth or other objects blocking the air inlet grille or the outlet at the top or sides. Grant and most manufacturers recommend at least 300mm of clear space around the unit. In winter, check for compacted snow or ice around the base and grilles — carefully clear these by hand or with lukewarm water (never boiling water or a sharp implement that could damage the fins).
- Check that the outdoor unit fan is visible and not physically jammed DIY safe
With the unit powered off at the isolator, visually inspect the fan blade through the outlet grille. Confirm there is no debris wedged in the fan. Do not reach into the unit or attempt to spin the blade manually. If you can see the blade is visibly bent or damaged, note this for the engineer.
- Power the unit down at the external isolator and wait at least three minutes before restarting DIY safe
Switch off the outdoor unit at the dedicated isolator (usually a red or grey weatherproof switch mounted on the wall near the unit). Wait a full three minutes to allow the control board to fully reset. Then switch the isolator back on and observe the controller. If the E8 code clears and the unit returns to normal operation, monitor it closely over the next few hours. Note that Grant advises homeowners against manually resetting fault codes on the Aerona Remote Controller — if the unit does not restart cleanly on its own after power restoration, leave the reset to your engineer.
- Check your system pressure and top up if below the recommended range DIY safe
While the E8 code is not directly caused by low system water pressure, confirming that your heating circuit is within the correct pressure range (typically 1.0–2.0 bar when cold, as shown on your system pressure gauge) rules out any secondary issues. If pressure is low, top up via the filling loop in the usual way. If you are unsure how to do this safely, leave it to your engineer.
- If the fault persists, contact an MCS-certified heat pump engineer heat pump engineer
If the E8 code returns after clearing obstructions and a clean power cycle, the underlying cause is likely to require professional investigation. An MCS-certified engineer will inspect and clean the outdoor coil, test the fan motor electrically, check operating pressures and — if a refrigerant-circuit fault is confirmed — carry out recovery and recharge work. Any refrigerant handling must legally be carried out by an F-Gas registered engineer. The engineer will also test all relevant temperature sensors against the resistance tables in the Aerona³ service manual and, if required, replace the main PCB.
Parts you may need
- Outdoor temperature / ambient sensor · from £35
- Discharge temperature sensor · from £40
- Defrost / suction sensor · from £38
- Outdoor fan motor (Aerona³) · from £180
- Main PCB (terminal / control board) · from £320
- R32 refrigerant recharge (labour and refrigerant) · from £250
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 £120–£450, depending on the underlying cause.
Frequently asked questions
Can I reset the E8 fault code on my Grant Aerona³ myself?
Grant's own guidance is clear that homeowners should not attempt to manually reset fault codes on the Aerona Remote Controller. Some codes clear automatically once the underlying issue is resolved, but others require a manual reset by a trained engineer. The safest approach is to switch the unit off at the external isolator, wait three minutes, and switch it back on. If the E8 code does not clear on its own after that, call your installer or an MCS-certified engineer rather than attempting a controller-level reset yourself.
Why does my Grant Aerona³ show E8 in winter — could it be snow or ice?
Yes, this is one of the more common winter triggers for the E8 fault. Snow accumulation around the base of the unit, or ice forming over the air inlet grilles, can restrict airflow enough to push the system into overload protection. Carefully clear any snow or ice from around the unit (do not use sharp tools near the fins) and ensure the area immediately around the outdoor unit is free from obstruction. If the fault clears after this and does not return, no further action is needed beyond keeping the area clear during cold spells.
How much does it cost to fix an E8 fault on a Grant Aerona³?
For the most common causes — a professional coil clean, airflow check and service visit — most homeowners pay in the region of £120 to £300. If the refrigerant charge needs to be corrected (recovered and recharged by an F-Gas registered engineer), expect costs of roughly £200 to £450 depending on the amount of refrigerant required and travel time. More involved repairs such as a fan motor replacement typically fall in the £250–£500 range including parts and labour. A main PCB replacement is less common but can cost £500–£900 or more — if your engineer suspects this, it is worth getting a written quote before proceeding.
How can I prevent the E8 fault from happening again?
Annual servicing by an MCS-certified engineer is the most effective preventive measure — the engineer will clean the outdoor coil, check refrigerant pressures, test sensors and inspect the fan, catching any developing issues early. Between services, keep the area around the outdoor unit clear of vegetation and debris, and check after heavy snowfall in winter. Keeping your service schedule up to date also protects your manufacturer warranty with Grant.