Grant A7 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A7 fault code mean?
A7 on a Grant Aerona³ heat pump reports a defrost temperature sensor error: the thermistor on the outdoor coil that tells the controller when frost is forming — and when a defrost cycle has finished — has failed or is reading implausibly. Air source heat pumps frost their outdoor coil routinely in UK winters and rely on this sensor to time defrosts correctly: too few and the coil ices into a solid block, too many and efficiency drains away. With the reading untrustworthy the unit stops rather than mismanage defrosting. Grant's documented remedy is direct — a faulty sensor should be replaced — and diagnosis is the usual resistance check.
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
- Failed defrost thermistor Common
Mounted on the outdoor coil in the wettest, most freeze-thaw-cycled spot on the unit, this sensor has a hard life and fails accordingly — confirmed by resistance check.
- Connector or wiring weathering Sometimes
Water ingress and corrosion at the outdoor connectors reproduce sensor failure; checked before replacement.
- Sensor displaced from the coil Sometimes
A dislodged sensor misreports coil temperature — defrosts then mistime even before the electrical fault is total.
How to fix it
- 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.
- Glance at the outdoor coil DIY safe
If the coil is visibly caked in ice, mention it when booking — it suggests defrosting has been mistimed for a while. Do not chip ice off; the fins damage easily and the ice will melt once the unit is repaired.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will measure the defrost sensor's resistance, check its mounting position and wiring, replace the faulty sensor, and confirm a defrost cycle initiates and terminates correctly afterwards. 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
- Defrost temperature sensor (thermistor) · from £30
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–£280, depending on the underlying cause.
Frequently asked questions
What does A7 mean on my Grant Aerona³?
The sensor that manages the outdoor coil's defrost cycles has failed. Without it the unit cannot time defrosting safely, so it stops. An engineer confirms the sensor with a resistance check and replaces it — a quick, inexpensive repair as heat pump faults go.
Is frost on my heat pump normal then?
Yes — an air source heat pump frosting its coil on cold, damp days is completely normal, and it defrosts itself automatically. A coil encased in solid ice is the abnormal sign, and often means the defrost sensor or cycle has been misbehaving.
Can I pour warm water over the iced coil?
Best not — sudden thermal shock and refreezing underneath the unit cause their own problems, and the fins are delicate. Let the engineer deal with the cause; the ice clears naturally once defrosting works again.