Grant L1 Fault Code: Causes, Fixes & Repair Costs
What does the Grant L1 fault code mean?
The L1 fault code on the Grant Aerona³ heat pump indicates a safety lockout triggered by the system detecting a problem with the flow water temperature. Either the water temperature has climbed too high, or there is insufficient water movement through the system. When this happens, the Aerona³ shuts itself down to prevent damage to the heat exchanger and internal components. On the controller, you will see the L1 code on the LCD display alongside a warning icon, and the red LED on the on/off switch will flash. A corresponding two-digit code will also appear 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
- Circulation pump failure or seizure Common
The circulation pump is responsible for keeping water moving continuously around the heating system. If it has seized, is running too slowly, or has failed entirely, heat builds up rapidly inside the heat exchanger with nowhere to go. The system detects this and locks out on L1 to prevent damage. This is the single most common root cause of this fault.
- Low system pressure Common
If the system pressure has dropped below around 1 bar — often after bleeding radiators or a small leak — there is not enough water volume in the circuit to circulate effectively. This can cause localised overheating near the heat exchanger sensors, triggering the L1 lockout even though the pump itself may be fine. Check the pressure gauge on your system.
- Faulty flow thermistor or temperature sensor Sometimes
The flow thermistor measures the water temperature leaving the heat pump. If it is reading incorrectly — reporting a temperature higher than the actual water temperature — the control board will lock the system out on L1 even though real conditions are normal. A faulty sensor will keep causing the same fault until it is replaced by an engineer.
- Limescale or sludge restricting water flow Sometimes
In hard water areas, scale can accumulate inside pipework and the heat exchanger, narrowing the flow path. Similarly, sludge and magnetite deposits from an unclean system can partially block flow. Both reduce circulation and cause heat to concentrate around the sensor location, which can trigger L1.
- Closed or partially closed isolation valves Sometimes
If the flow or return isolation valves on the heat pump or elsewhere in the system have been accidentally partially or fully closed, water cannot circulate at the required rate. This is easy to overlook after recent maintenance work on the system.
- Faulty main PCB Rare
The main control board processes all incoming sensor data. If the PCB itself is faulty, it may misinterpret signals from the flow thermistor and trigger an unnecessary L1 lockout. This is the least common cause but should be considered if all other components test correctly.
How to fix it
- Do not repeatedly reset the fault DIY safe
It is understandable to want to get the heating running again quickly, but repeatedly resetting an L1 lockout without addressing the underlying cause puts real stress on the heat exchanger. If the circulation pump is not moving water and the system keeps attempting to run, you risk turning a straightforward repair into a much more costly one. One reset to check whether this was a one-off event is reasonable. If the fault returns, switch the unit off and leave it off until an engineer investigates.
- Check the system pressure gauge DIY safe
Locate the pressure gauge on your heating system — it is typically on the pipework near the indoor unit or on a buffer vessel. The reading should be between 1 and 2 bar. If it has dropped below 1 bar, top the system up using the filling loop until the gauge reads around 1.5 bar, then close the filling loop fully. If you are unsure how to repressurise your specific system, consult your user guide or call your installer. Once pressure is restored, try a single reset from the Aerona³ remote controller.
- Check around the outdoor unit for obstructions DIY safe
While this is less likely to be the direct cause of L1, ensure the outdoor unit has adequate clearance and is not obstructed by leaves, debris, or snow around the casing or air intake. Restricted airflow adds load to the system, which can compound flow temperature issues.
- Check that isolation valves are fully open DIY safe
Have a look at the isolation valves on the flow and return pipework connected to the heat pump. Each valve handle should be in line with the pipe — if a handle is turned across the pipe, the valve is closed. If you find a closed valve, open it fully and attempt a single reset. If you are unsure which valves are which, leave this step to an engineer.
- Check the MCB and electrical supply DIY safe
Go to your consumer unit and confirm that the MCB for the heat pump has not tripped. If it has tripped, reset it once. If it trips again immediately, do not attempt to reset it a second time — there is an underlying electrical fault that requires a qualified engineer.
- Call an MCS-certified heat pump engineer for diagnosis heat pump engineer
If the fault persists after the homeowner checks above, the system needs professional diagnosis. An engineer will test the circulation pump for correct operation and flow, inspect the flow thermistor and measure its resistance against specification, check for sludge or scale restrictions, and assess the PCB. Any work on the refrigerant circuit, compressor, or sealed electrical components must only be carried out by a suitably qualified engineer — refrigerant work specifically requires an F-Gas registered engineer.
Parts you may need
- Circulation pump (Grundfos or equivalent) · from £120
- Flow thermistor / water temperature sensor · from £35
- Main PCB (Aerona³ control board) · from £280
- Magnetic system filter (e.g. Fernox TF1 or Magnaclean) · from £65
- System inhibitor (e.g. Fernox F1) · from £18
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–£500, depending on the underlying cause.
Frequently asked questions
Can I reset the L1 fault on my Grant Aerona³ myself?
You can attempt a single reset either from the remote controller display or by switching the heat pump off at the external isolator, waiting 30 seconds, and switching it back on. Grant's own guidance notes that some fault codes reset automatically once the underlying fault is corrected, while others require a manual reset. However, if the L1 code returns after one reset, you should not keep resetting it — contact your installer or an MCS-certified heat pump engineer instead. Repeated resets without fixing the root cause risk damaging the heat exchanger.
How much does it cost to fix a Grant L1 fault code in the UK?
For the two most common causes, expect to pay roughly £150–£280 for a circulation pump replacement (parts and labour) or £100–£200 for a flow thermistor swap. If a power flush is needed to clear sludge or limescale, costs typically run between £300 and £600 depending on system size. These figures cover most L1 repairs. In rare cases where the main PCB is at fault, parts alone can cost £250–£400 before labour, which would push the total higher — but PCB failure is an uncommon cause of this specific fault.
Why does my Grant Aerona³ keep coming back to L1 after resetting?
A recurring L1 fault almost always means the root cause has not been resolved. The most likely culprits are a circulation pump that is intermittently failing or on its way out, a flow thermistor giving drifting readings, or a build-up of sludge causing partial blockages. Each time the system reaches the temperature threshold without adequate flow, it will lock out again. An engineer should carry out a proper diagnosis rather than simply resetting the code repeatedly.
Does the Grant Aerona³ L1 fault affect my hot water as well as heating?
Yes — because L1 is a full safety lockout, the heat pump stops operating entirely. This means both space heating and domestic hot water (if your hot water cylinder is heated by the heat pump) will be unavailable until the fault is resolved. If you have an immersion heater fitted as a backup in your cylinder, you can switch this on temporarily to maintain hot water while you wait for an engineer.