Carel AL012

What does Carel AL012 mean?

AL012 on the Carel Heos system controller indicates a flow switch alarm on the MT (medium-temperature) pump circuit. The flow switch in the MT water loop has opened, signaling insufficient or absent water flow in the circuit. The alarm is delayed according to parameter Dbb07 before activating. Automatic reset is applied and the MT analogue pump output is cut off.

Symptoms

• AL012 displayed on the Heos system controller after the Dbb07 delay.
• MT analogue pump output is de-energized or modulation stops.
• MT water circuit flow has fallen below the flow switch trip point.
• Automatic reset is attempted by the controller.

Common causes

• MT pump has lost prime or is not developing adequate flow to keep the flow switch closed.
• Air lock in the MT water circuit causing flow interruption.
• Flow switch set point or position is incorrect, tripping prematurely.
• MT pump motor thermal overload has tripped, stopping the pump.
• Blockage in the MT circuit strainer or piping reducing flow below the switch trip point.

Diagnostic steps

1. 01

   Check MT pump operation

   Verify the MT pump is running and developing flow. Listen for motor noise, check pump pressure gauges, and confirm the pump is not cavitating or air-locked.

2. 02

   Bleed air from the MT circuit

   Open air vent valves at circuit high points to remove trapped air. Air locks in the water loop are a common cause of momentary flow loss that trips the flow switch.

3. 03

   Inspect the MT circuit strainer

   Remove and clean the suction strainer upstream of the MT pump. Debris blockage reduces flow and can cause the flow switch to open.

4. 04

   Check flow switch position and setpoint

   Verify the flow switch is correctly positioned in the pipe and set to the appropriate trip flow rate for the MT circuit design. A mispositioned switch may trip at normal design flow.

5. 05

   Verify MT pump thermal overload status

   Check the pump motor starter for a tripped thermal overload. If tripped, reset after confirming motor current is within nameplate ratings.

When to call a professional

If the MT pump requires replacement or the MT hydronic circuit has a persistent flow deficiency despite clearing air locks and cleaning strainers, contact a licensed mechanical engineer or hydronic systems technician. Do not operate the MT circuit without confirmed adequate flow — low flow in a chilled water circuit causes freeze-up risk and heat exchanger damage.