Boiler display shows A51; unit may not fire and indicates low water level or safety lockout.

Safety: Turn OFF power and allow the boiler to cool. Never add cold water to a very hot boiler; thermal shock can damage the heat exchanger.

Steps:

• 1. Check system pressure: Read the pressure gauge on the Raypak boiler. For hydronic systems, typical cold fill pressure is 12–20 psi (check your system specs). If pressure is very low, the system may need to be refilled and leaks investigated.
• 2. Inspect for visible leaks: Look around the boiler, piping, and relief valve discharge for signs of water leakage.
• 3. Check LWCO device: Locate the low water cutoff (probe or float type). Inspect wiring connections. For probe types, mineral buildup on the probe can cause false low-water readings.
• 4. Clean LWCO probe (if accessible): Isolate and drain the boiler to below the probe level. Remove the probe and clean it with a non-abrasive pad to remove scale. Reinstall with new gasket if required.

If A51 remains after restoring proper water level and cleaning, the LWCO or control board may be faulty and should be serviced by a professional.