Grant FU Fault Code: Causes, Fixes & Repair Costs
What does the Grant FU fault code mean?
FU on a Grant Aerona³ heat pump (the 13kW and 17kW HPID13R32/HPID17R32 models) is a pump error: the circulating pump that drives water around your heating circuit is blocked, air-locked, or working against a restricted water circuit. Grant's documented diagnosis splits on a voltage check: if the pump is receiving normal voltage but not doing its job, the pump is replaced; if the voltage reaching it is abnormal, the fault is upstream and the main PCB should be replaced. Without circulation the heat pump cannot move its heat anywhere, so the unit stops — protecting the plate heat exchanger from freezing or overheating depending on season.
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
- Pump blocked or air-locked Common
Sludge or debris jams the rotor, or trapped air stops the pump biting — the documented first cause, common after system work or in poorly inhibited systems.
- Restricted water circuit Common
A clogged filter or strainer, a closed valve, or sludged pipework starves the pump even when it runs perfectly — the circuit is checked alongside the pump itself.
- Pump failed, or abnormal supply voltage from the board Sometimes
Grant's documented split: normal voltage at a non-performing pump condemns the pump; abnormal voltage condemns the main PCB.
How to fix it
- Check heating system pressure DIY safe
Look at the system pressure gauge (usually near the heat pump's indoor pipework). It should sit around 1.0–2.0 bar cold; below 1 bar, top up via the filling loop per your user instructions. Low pressure starves the pump and water circuit.
- Power-cycle once at the external isolator DIY safe
Switch the heat pump off at its external rotary isolator, wait about three minutes for the inverter capacitors to discharge, and switch back on. One controlled restart is legitimate; if the code returns, leave it for an engineer.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will check the pump's supply voltage (Grant's documented decision point), bleed or free the pump, clean the filter and check the circuit for restrictions, then replace the pump — or the main PCB if the voltage feeding it is abnormal. Work on the sealed refrigerant circuit legally requires an F-Gas registered engineer; your installer or a Grant-accredited (G1) installer is the right call, and Grant UK technical support (01380 736920) can assist with diagnosis.
Parts you may need
- Circulating pump · from £150
- Main PCB (inverter board) · from £300
- Magnetic filter clean/service · from £20
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 £100–£450, depending on the underlying cause.
Frequently asked questions
What does FU mean on my Grant Aerona³?
On the 13kW and 17kW models, FU means the water circulating pump is blocked, air-locked or fighting a restricted circuit. Check system pressure and restart once; the engineer's fix runs from bleeding the pump and cleaning the filter up to replacing the pump — or the main board, if the voltage feeding the pump is wrong.
How is FU different from P1?
Both are pump/circulation errors; FU is specific to the larger (13/17kW) models' pump monitoring. From a homeowner's perspective they mean the same thing — water is not circulating properly — and lead to the same checks.
Could sludge really stop a heat pump?
Easily — heat pumps move large volumes of water at modest temperatures, so they are less forgiving of sludged circuits than boilers. If the system has never been flushed or the inhibitor never topped up, expect the engineer to raise water quality alongside the immediate fix.