Boiler shows E5 and will not fire; may occur immediately on a call for heat

Safety first: Turn off power before opening any boiler panels. Do not over‑pressurize the system.

• Check system pressure: Read the boiler pressure gauge. Most residential hydronic systems require around 12–20 psi when cold (verify with your Ruud manual). If pressure is low, the boiler will lock out.
• Verify automatic fill valve: Ensure the water supply to the boiler is open. The pressure reducing valve (auto‑fill) should maintain pressure. If the supply is closed, open it slowly and watch the gauge rise.
• Bleed air and recheck: After adding water, bleed air from radiators or air vents, then confirm pressure remains in the normal range.
• Inspect for leaks: Look around the boiler, relief valve discharge, and visible piping for signs of water leakage. Ongoing leaks will cause recurring E5.
• If pressure is normal but E5 remains: The pressure sensor or low water cutoff device may be faulty. Replacement should be done by a qualified technician, as incorrect installation can be hazardous.