A4 alarm, unit stops, no heating output

Safety: Isolate electrical power before touching pumps or wiring. Brine can be corrosive; wear gloves and eye protection.

• Check brine pump operation: With power on and the unit calling for heat, feel the brine circulation pump on the Nibe unit or in the brine circuit. It should vibrate and be warm. If silent and cold, check its isolator switch and fuses.
• Inspect brine level and pressure: Check the brine expansion vessel gauge. If pressure is low, contact your installer; topping up brine requires the correct antifreeze mixture.
• Clean brine filter: Many Nibe ground‑source systems have a strainer on the brine circuit. With power off and valves closed, remove and clean the filter screen, then restore flow and bleed air.
• Reset alarm: After restoring flow, reset the A4 alarm via the Nibe control panel and monitor brine inlet temperature.

Technician required if brine temperature remains very low or the alarm recurs. The ground loop may be undersized or partially frozen, requiring professional assessment.