General Air Conditioning | VRF Technical Guide
Auto Changeover Operation VRF
Function, setup and thermostat operating logic
Purpose: Auto Changeover automatically changes between cooling and heating according to room temperature, the set temperature (Ts) and the selected mode-change tolerance (α).
1. System Type Requirements
| System type | Required setup | How Auto Changeover operates |
|---|---|---|
| Heat-recovery VRF | Enable the function individually on every indoor unit that requires Auto Changeover. | Only indoor units with the function enabled perform automatic mode change. |
| Heat-pump VRF | Configure the master indoor unit. | Indoor units on the same refrigerant system follow the master indoor unit set temperature and mode decision. |
Important: Confirm the refrigerant-system layout and master indoor unit assignment before changing function settings. Incorrect master/slave configuration can produce unexpected system operation.
2. Function Setting No. 65
Mode Change Criteria Temperature Setting
Function Setting No. 65 determines the temperature difference (α) used to initiate the change between cooling and heating.
| Set value | Mode-change criteria α | Application guidance |
|---|---|---|
| 00 | 2.0°C | Factory default. |
| 01 | 1.0°C | Tighter changeover band; greater risk of frequent mode changes. |
| 02 | 1.5°C | Moderately tight control. |
| 03 | 2.0°C | Alternative value providing the same α as the factory default. |
| 04 | 2.5°C | Wider band to reduce changeover frequency. |
| 05 | 3.0°C | Widest available band; suitable where mode hunting must be minimised. |
Caution: Select the value to suit the installation, building load and operating conditions. An unsuitable value can cause frequent thermostat cycling or repeated heating/cooling changeover.
3. Operating Principle
The controller uses the set temperature (Ts) and the selected α value to create a changeover band. Room temperature is detected in 0.5°C increments.
| Threshold | Thermostat action |
|---|---|
| Room temperature reaches Ts + α | Cooling thermostat turns ON. |
| Room temperature falls to Ts - 0.5°C | Cooling thermostat turns OFF. |
| Room temperature falls to Ts - α | Heating thermostat turns ON. |
| Room temperature rises to Ts + 0.5°C | Heating thermostat turns OFF. |
Why the thresholds differ: The 0.5°C OFF thresholds create hysteresis around Ts. This prevents immediate reversal between heating and cooling and reduces mode hunting.
4. Worked Example
With a set temperature of 22°C and α set to 2.0°C:
| Action | Calculated threshold |
|---|---|
| Cooling thermostat ON | 24.0°C (Ts + α) |
| Cooling thermostat OFF | 21.5°C (Ts - 0.5°C) |
| Heating thermostat ON | 20.0°C (Ts - α) |
| Heating thermostat OFF | 22.5°C (Ts + 0.5°C) |
Interpretation: Cooling can remain active as temperature falls through the set point and turns off at Ts - 0.5°C. Heating can remain active as temperature rises through the set point and turns off at Ts + 0.5°C. This deliberate overlap provides stable thermostat operation.
5. Setup and Commissioning Checks
- Confirm whether the system is heat-recovery or heat-pump type.
- Verify refrigerant-system addresses and master indoor unit assignment.
- Apply Function 65 to the required indoor units or master indoor unit, as applicable.
- Record the selected value and calculated thresholds in the commissioning report.
- Confirm the room temperature sensing location is suitable and not affected by drafts, direct sunlight or local heat sources.
- Test both heating and cooling thermostat responses where site conditions allow.
- Monitor for repeated mode changes, short cycling or conflicting zone demands.
6. Quick Reference
| Function number | 65 |
| Factory default | Set value 00 = 2.0°C |
| Available α range | 1.0°C to 3.0°C |
| Room temperature resolution | 0.5°C |
| Heat-recovery setup | Enable per indoor unit. |
| Heat-pump setup | Configure the master indoor unit. |
Technical support: For assistance with VRF function settings or system behaviour, contact Assist Technical Support through the current internal support channel.