20 on a Grant Aerona³ R32: Circuit 4 temperature sensor error (not used)
Short answer
On a Grant heat pump, 20 means: Circuit 4 temperature sensor error (not used). The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manuals for this range word it in different ways:
- Circuit 4 temperature sensor error (not used).
- No communication with heat pump module.
Part involved: unused circuit; Modbus link wiring centre - heat pump.
When it shows: Alert passed from the Smart Controller to the wired room thermostat display; also shown in the Smart Controller alarm list Alert passed from the Smart Controller to the wireless room thermostat display; also shown in the Smart Controller alarm list.
Likely causes:
- Heat pump has no power / isolator OFF.
- Modbus wiring connection or polarity incorrect.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: Not stated.
Manual reset: Not stated.
What you can check
- Note the exact code shown on the controller display and tell your installer or Grant-trained engineer.
- Tap the alarm icon on the Smart Controller touchscreen to view the current alarm list.
- Check the heat pump isolator switch and the consumer-unit breaker/RCD for the heat pump are ON (do not repeatedly reset a tripping breaker).
What the engineer will check
- Check the heat pump has power and isolator is ON.
- Check connection and polarity of Modbus wires between wiring centre and heat pump (s3.12).
Who to ask for: Heating controls installer / Grant-trained engineer.
What you might notice
What owners and engineers report
18 owner and engineer reports mention this.
- Controller / thermostat problem on the Aerona³ R32: 9 reports
- No heating on the Aerona³ R32: 5 reports
- No hot water on the Aerona³ R32: 4 reports
Sources
- Grant Aerona Smart Heat Pump System Controller - Installation & Operating Instructions (UK DOC 0203) ↗ · Rev 2.2 · pages 53, 56 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026