Grant's Aerona3 R32 manual has no dedicated freeze code, so an iced-up outdoor unit usually shows as a fan motor error (C1 or C3) or a defrost temperature sensor error (A7). These codes tell you the fan or the defrost sensing has a problem, which ice can cause or make worse.
If a code keeps coming back after one reset, book an engineer rather than trying again. You can book a heat pump repair online or call 0333 041 5130, and our full Grant Aerona error codes guide covers every other code.
Please read first: this guide is here to help you understand what your Grant Aerona heat pump is telling you. Every effort has been made to keep the information accurate, but manufacturers update their manuals and codes can differ between models and software versions, so always check the official Grant Aerona manual for changes before acting on anything here. Do not attempt anything that could be dangerous, and avoid any work that could void your warranty. If in doubt, call a qualified heat pump engineer.
What do C1, C3 and A7 mean on a Grant Aerona heat pump?
C1 and C3 are fan motor errors, and A7 is a defrost temperature sensor error. They come from the outdoor unit code list in Grant's Aerona3 R32 installer manual, which covers the HPID6R32, HPID10R32, HPID13R32 and HPID17R32 models.
| Code | Meaning in Grant's manual | Reset type |
|---|---|---|
| C3 | Fan motor error on the HPID6R32 and HPID10R32, lower fan on the HPID17R32 | Manual |
| C1 | Upper fan motor error on the HPID17R32 | Automatic |
| A7 | Defrost temperature sensor error | Automatic |
Grant does not list ice as a cause of any of these codes. In practice, though, a fan blocked by ice or a sensor that stops defrost working properly are common reasons for an outdoor unit to freeze up.
Is ice on a heat pump normal?
Yes, a thin layer of frost on the outdoor coil is normal in cold, damp weather, and the unit clears it with regular defrost cycles. During a defrost the fan may pause, you may see steam and water will drain from the base. Our guide to the heat pump defrost cycle explains what normal looks like.
It becomes a fault when ice builds into a solid block, the fan is jammed or encased, or a C1, C3 or A7 code appears. Our guide to a frozen outdoor unit covers what to do when the ice will not clear.
Why do C1, C3 or A7 appear in cold weather?
These codes appear in cold weather because the fan and defrost sensor work hardest then, and ice can jam the fan or stop defrost cycles finishing.
From Grant's manual: C1 and C3 are linked to blown fuses (CF6 or CF7), a faulty fan motor or a faulty main board. A7 is a failed defrost sensor, which an engineer checks by testing its resistance against Grant's table. Grant's separate "water not getting warm" fault, which has no code, also lists the defrost heater, the 4-way valve, short cycling from poor air circulation and a clogged water circuit.
General engineering knowledge: other common underlying causes are:
- Snow, ice or leaves jamming the fan or blocking airflow
- Low flow from sludge, a blocked strainer or magnetic filter, or air locks
- Low system pressure
- A frozen or blocked condensate drain, letting ice build up under the coil
- A unit sitting too low, so snow and meltwater collect around it
- Low refrigerant charge, which only an F-Gas engineer can check
What can you safely check yourself?
You can make a few simple checks from outside the unit, without tools and without opening any panels.
- Keep the power on. Frost protection needs electricity, so do not switch off at the isolator; Grant also says turning the mains off and on to reset is not recommended.
- Clear snow, leaves and debris from around and under the outdoor unit, working from outside the casing.
- Check the condensate drain path is clear and not frozen. If it is iced, thaw it with lukewarm water only, never boiling.
- Check the pressure gauge is in its normal range and that radiator and zone valves are open. If pressure keeps dropping, see our guide to a heat pump losing pressure.
- If you are going away, use holiday or frost settings rather than turning the system off. Our guide to heat pump holiday settings explains how.
- Do one reset only. C1 and A7 clear themselves once the fault goes; for C3, press the minus and plus buttons together for three seconds on the remote controller.
What not to do:
- Never chip or scrape ice off the coil or fan, as the fins and blades damage easily
- Never use boiling water or a heat gun on the coil
- Never remove panels or covers to reach the fan
- Never touch the refrigerant circuit, which is F-Gas certified engineer work only
When should you call an engineer?
Call an engineer if C3 returns after one reset, or if C1 or A7 do not clear once the unit is free of ice. Other clear triggers are:
- The fan stays still with nothing blocking it
- The outdoor unit stays a solid block of ice after a defrost cycle
- Heating or hot water has stopped
- Pressure keeps falling or you can see a leak
- Other codes such as A2, A4, FU or P1 appear at the same time
Fuse, fan voltage and sensor resistance tests need a qualified engineer, and anything on the refrigerant side needs an F-Gas certified engineer. For wider symptoms, see our guide to common air source heat pump problems.
How can you stop it happening again?
Regular maintenance prevents most cold-weather faults. Grant's annual service schedule covers clearing debris from the evaporator, checking the condensate drain, flexible hoses and insulation, the expansion vessel and system pressure, the antifreeze inhibitor concentration, the magnetic filter and electrical connections.
On top of that, these general engineering points help:
- Annual service: book it before winter. Our annual service checklist shows what is covered.
- Clean water: heavy sludge that keeps blocking the filter may need a heat pump powerflush.
- Freeze protection: keep the right antifreeze or inhibitor level, or fit freeze-protection valves where the design calls for them.
- Raised feet and drainage: keep the unit clear of the ground on a free-draining base.
- Location: leave space around the unit and avoid spots where snow drifts or roof water drips onto it.
Our heat pump winter tips and guide to running a heat pump in cold weather cover more.
Other Grant Aerona codes you might see in a cold snap
These codes from Grant's Aerona3 R32 list can appear in the same conditions. Each links to our full Grant error code guide.
| Code | Meaning | First step |
|---|---|---|
| A2, A4 | Excess DC or AC current, often from a blocked air inlet or outlet | Clear the airflow, then one manual reset |
| A0 | DC voltage error, with the fan motor among the causes | Call an engineer |
| C2 | Outdoor temperature sensor error | Call an engineer |
| FU | High pressure switch operating | Check valves and pressure, then one reset |
| P1 | Pump error | Check pressure and valves, then call an engineer |
Need help with a frozen Grant Aerona heat pump?
If your Grant Aerona has iced up or keeps showing C1, C3 or A7, we can help. Our F-Gas certified engineers repair Grant heat pumps across the UK, and our emergency heat pump repair service offers same-day visits where possible.
We test the fan, fuses and defrost sensor to find the real cause, then carry out heat pump repairs with a 12-month guarantee. An annual heat pump service follows Grant's own checklist, and our cover plans combine servicing with repairs.
See the Grant heat pumps we work on, contact us or call 0333 041 5130.
Frequently asked questions
Is a C3 code always caused by ice?
No. Grant's manual links C3 to blown fuses, a faulty fan motor or a faulty main board, so it can appear in any weather. Ice is simply a common trigger in winter.
Does my Grant Aerona keep a record of these codes?
Yes. The unit stores its last 10 error codes, newest first, and an engineer can read them from the terminal PCB. That history shows whether C3 or A7 has been repeating.
How does an engineer test the defrost sensor?
They disconnect the sensor, measure its resistance with a meter and compare it with the table in Grant's manual at a known temperature. A reading far from the expected value, or an open circuit, means a faulty sensor or cable.
Can I cover my heat pump in winter?
No, do not wrap or cover the outdoor unit, because it needs free airflow to work and to defrost. A purpose-made shelter can help with snow, but only if it keeps the clearances in Grant's installation manual.
Why is my house cold when there is no code?
Grant lists a "water not getting warm" fault with no code, linked to a blown fuse, the 4-way valve, the defrost heater, sensors, gas leakage, short cycling and a clogged water circuit. An engineer needs to work through these checks.
