8.3 Controller setup and set points
The controller is the brain of the unit. It reads sensors and switches the pump, fan, compressor and valves. Setting it up means telling it what the site needs.
A set point is the target temperature, for example 55 °C hot water. A differential (also called hysteresis) is the band around the set point. The unit stops at the set point and restarts when the water falls by the differential. A fridge works the same way: the compressor stops when cold, and restarts after it warms a little.
A very small differential causes short cycling, which means frequent stops and starts. Short cycling wears the compressor and wastes energy. A very large differential lets the water get too cold before restart.
| Parameter | What it does | Typical setting |
|---|---|---|
| Hot water set point | Target water temperature | 50–55 °C for domestic and commercial hot water |
| Space heating set point | Target flow temperature | 35–45 °C (floor heating lower, radiators higher) |
| Differential | Restart band below set point | 3–5 K |
| Anti-short-cycle timer | Minimum compressor off time | 3 minutes |
| Pump overrun | Pump runs after compressor stops | 1–3 minutes |
| Defrost start | Coil temperature that triggers defrost | As per manual; depends on refrigerant and coil |
| Anti-legionella cycle | Periodic high-temperature cycle | 60 °C or above, weekly (where required) |
| Max outlet water limit | Protects against over-temperature | Below the unit's operating envelope limit |
| Clock and schedules | Sets cheap-tariff or working hours | Match site timings |
Indian practice: many commercial sites (hotels, hostels, hospitals) need hot water mainly at 5–9 am and 6–10 pm. Use schedules so the tank is hot before these peaks. In Bengaluru or Pune winters, the morning ambient can drop to 10–15 °C, so allow extra heating time.
Remember
- Set the set point to what the user needs, not the maximum the unit can reach.
- A differential of 3–5 K is typical; smaller values cause short cycling.
- Higher water temperature means lower COP — every few degrees matter.
- Record all final controller settings on the commissioning sheet.
The controller decides how the heat pump behaves; wrong set points can make a good machine look bad.
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