Grant P1 Fault Code: Causes, Fixes & Repair Costs
What does the Grant P1 fault code mean?
P1 on a Grant Aerona³ heat pump is a pump error: the controller has detected that water is not circulating properly through the heating circuit — Grant's documentation attributes it to a clogged water circuit or a pump malfunction, and directs the remedy accordingly: check the pump and the water circuit. Circulation is the heat pump's lifeline; without adequate flow the plate heat exchanger cannot shed the heat (or in defrost, draw it), and protection stops the unit. In practice P1 is one of the most commonly reported Aerona³ codes, and its causes are usually system-side and unglamorous: low pressure, a dirty filter, an airlock, a seized or failing pump, or sludged pipework restricting flow.
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
- Clogged filter or restricted water circuit Common
The magnetic filter or strainer loads up with sludge and chokes flow — Grant's documented first suspect and the most frequent real-world finding.
- Low system pressure or airlock Common
Pressure below about 1 bar, or air trapped after work on the system, starves the pump; both are quick checks and quick fixes.
- Circulating pump failing Sometimes
A worn or seized pump no longer achieves the required flow even against a clean circuit — replaced once the circuit itself is proven clear.
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 clean the filter and strainer, bleed the circuit, verify flow rate against the commissioning figure, and test the pump — cleaning or replacing as found. Persistent sludge means a system flush and fresh inhibitor belong in the conversation. 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
- 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 £90–£400, depending on the underlying cause.
Frequently asked questions
What does P1 mean on my Grant Aerona³?
Water is not circulating properly — Grant's documentation says to check the pump and the water circuit. Check your system pressure (top up if under 1 bar) and restart once; beyond that an engineer cleans the filter, bleeds the circuit, verifies the flow rate and tests the pump.
P1 keeps coming back every few weeks — why?
Recurring P1 almost always means the underlying restriction was never cleared: a filter that reloads with sludge quickly, or a circuit that needs a proper flush and inhibitor rather than another filter clean. Ask the engineer to measure the actual flow rate against the commissioning figure — that number settles the argument.
Is this expensive?
Usually not — a filter clean and re-pressurise sits at the bottom of the range, a new pump toward £400. The costly version is ignoring it: repeated circulation failures work the compressor hard for nothing.