Carel ALS16

What does Carel ALS16 mean?

ALS16 on the Carel Smart HP ground-source heat-pump controller (program +030220741) indicates that the humidity sensor element on serial probe 06 has failed or is reading outside its valid range. The Smart HP manages up to six serial zone probes (each capable of measuring temperature and humidity) distributed across the building zones it controls. ALS16 covers the humidity fault on the sixth serial probe specifically. The alarm resets automatically once a valid signal is restored.

Symptoms

• ALS16 displayed on the Smart HP pCO terminal.
• The humidity reading from zone 6's serial probe is absent or out-of-range.
• Any humidity-based control functions assigned to zone 6 — such as humidifier or dehumidifier scheduling — may be interrupted.
• Alarm relay activation is configurable via parameter Gfc43; it may not activate by default.
• The alarm self-resets after 60 seconds once the humidity signal returns to valid range.

Common causes

• Serial probe 06 humidity element failed or has drifted beyond calibration limits.
• Serial communication cable between the Smart HP controller and probe 06 is broken, shorted, or loose.
• Probe 06 has lost power — supply voltage absent at the probe terminals.
• Bus address conflict with another device on the serial network at the address assigned to probe 06.
• Excessive humidity or condensation inside the probe body has damaged the capacitive humidity sensor.

Diagnostic steps

1. 01

   Identify serial probe 06 location

   Refer to the Smart HP commissioning documentation or zone layout to locate the physical position of serial probe 06. It is wired to the Smart HP controller's serial field bus (FieldBus or pLAN port).

2. 02

   Inspect serial bus wiring to probe 06

   Trace the RS-485 data cable from the Smart HP controller to probe 06. Check for broken conductors, loose screw terminals at both ends, and correct polarity (A/B wiring). Verify the cable is properly terminated.

3. 03

   Verify probe 06 power supply

   Measure the supply voltage at the probe 06 terminals. No power (0 V) indicates a supply fault upstream — check the power wiring and any inline fusing for the serial probe bus.

4. 04

   Check probe 06 bus address

   Confirm that serial probe 06 is programmed to the correct bus address expected by the Smart HP controller. Address conflicts cause communication failures that trigger humidity and offline alarms simultaneously.

5. 05

   Replace probe if humidity element is failed

   If communication is confirmed good but the humidity reading is stuck, out of range, or absent only for the humidity channel (temperature may still read correctly), the humidity sensor element has failed. Replace probe 06 with a compatible Carel DPW serial probe. Allow 60 seconds after reconnection for ALS16 to auto-reset.

When to call a professional

Contact a Carel-authorised controls technician if ALS16 persists after probe replacement and communication verification. Persistent alarms alongside multiple ALS codes on different probes indicate a serial bus wiring or termination fault that requires protocol-level diagnostics. In heat-pump systems, loss of zone humidity sensing may affect comfort control and should be resolved promptly to maintain correct building conditioning.