Grant U1 Fault Code: Causes, Fixes & Repair Costs
What does the Grant U1 fault code mean?
U1 on a Grant Aerona³ heat pump (the 13kW and 17kW HPID13R32/HPID17R32 models) is compressor overheat protection: the compressor's thermal protection has operated because it ran hotter than its safe limit. Grant's documentation names the underlying causes — refrigerant gas leakage, or the overheat relay operating — and its remedy is telling: recover the refrigerant and recharge with the prescribed mass. A compressor short of refrigerant loses the cooling effect the refrigerant flow itself provides and overheats; that makes U1 one of the codes most likely to involve the sealed circuit, which only an F-Gas registered engineer may legally open, leak-test and recharge.
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
- Refrigerant loss Common
A leak reduces the charge; the compressor loses its refrigerant cooling and runs hot until protection trips — Grant's documented cause, remedied by leak repair, recovery and a weighed recharge.
- Overheat relay operation under sustained hard running Sometimes
Prolonged heavy load with marginal airflow or water flow can drive compressor temperature to the limit even with a correct charge.
- Compressor wear Rare
An ageing compressor generates more heat for the same work; recurring U1 with a proven charge points here, and the repair conversation becomes a bigger one.
How to fix it
- Let it cool; restart once at most DIY safe
After the compressor cools, one restart is reasonable. If U1 returns, stop — repeatedly running an overheating compressor is how a repair becomes a replacement.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will check compressor temperatures and running conditions and, if refrigerant loss is suspected, leak-test the circuit, repair, recover and recharge with the prescribed mass — the documented remedy, and strictly F-Gas work. 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
- Refrigerant leak repair and recharge · from £250
- Compressor · from £600
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–£700, depending on the underlying cause.
Frequently asked questions
What does U1 mean on my Grant Aerona³?
On the 13kW and 17kW models, the compressor overheated and its protection shut it down. The documented cause is usually refrigerant loss — the compressor relies on refrigerant flow to keep itself cool — and the remedy is a leak-test, repair and weighed recharge by an F-Gas registered engineer.
Can refrigerant just be topped up?
Not properly — the charge is a prescribed mass, and 'topping up' a leaking system without finding the leak is both poor practice and environmentally regulated. The documented remedy is recover, repair, recharge to specification.
Is this the beginning of the end for my heat pump?
Not necessarily — a found-and-fixed leak with a fresh charge often restores years of service. But recurring U1 with a proven charge suggests compressor wear, and at that point an honest engineer will price the repair against the unit's age and warranty position for you.