Carel AL20

What does Carel AL20 mean?

AL20 on Carel µChiller and pCO chiller controllers indicates a fault with probe B1. On chiller platforms, B1 is typically the primary water inlet or ambient temperature probe. When B1 fails, the controller cannot read the temperature it uses for primary regulation — such as chilled water supply temperature control — and may shut down the unit completely if the probe drives the main control loop.

Symptoms

• AL20 displayed on the Carel controller.
• Primary temperature control based on B1 is unavailable or operating in fallback.
• Unit may shut down completely if B1 is the ambient or inlet water probe driving unit enable.
• Manual reset required after probe repair.

Common causes

• B1 probe wiring open-circuit or disconnected at the probe or controller terminal.
• NTC sensor element failed.
• Corrosion or moisture ingress at the probe connection.
• Incorrect probe type installed.

Diagnostic steps

1. 01

   Identify B1 probe location and function

   Refer to the controller wiring diagram to confirm which physical probe is designated B1 and what temperature it measures in your specific chiller configuration.

2. 02

   Inspect wiring at the B1 terminal

   Check the B1 terminal on the controller. Re-seat conductors and tighten screws. Look for corroded wires or broken conductors.

3. 03

   Measure B1 probe resistance

   Disconnect B1 and measure resistance across the probe leads. An NTC at 20 °C reads approximately 10 kΩ; open or short confirms failure.

4. 04

   Replace the B1 probe

   Install a compatible Carel NTC replacement and confirm the probe is correctly mounted in its immersion well or clip location.

5. 05

   Reset and verify

   Clear AL20 at the controller and confirm the B1 temperature reading is plausible before restarting the unit in normal operation.

When to call a professional

If B1 is an immersion probe in a pressurized water circuit, its replacement requires the water circuit to be isolated, drained at that section, and re-pressurized after fitting — a task requiring a qualified engineer. Also escalate if AL20 persists after probe replacement, indicating a controller analog input fault.