Boiler shows E5 and will not fire, often accompanied by low or zero system pressure

Safety first: Allow the boiler to cool before adding water. Overfilling a hot system can cause thermal stress.

• Step 1 – Read system pressure: Check the boiler’s pressure gauge or digital display. If below the minimum (often 12 psi cold), the boiler will lock out.
• Step 2 – Refill system: Locate the fill valve or automatic fill assembly. Slowly open the fill valve and watch the pressure gauge rise to the recommended cold pressure (typically 12–15 psi). Close the valve securely.
• Step 3 – Bleed air: Bleed radiators and high points to remove trapped air, then recheck and top up pressure as needed.
• Step 4 – Inspect for leaks: Walk the system piping, radiators, and boiler connections looking for water drips, corrosion, or staining. Repair any leaks before refilling again.
• Step 5 – Check low water sensor: If pressure is correct but E5 persists, the low water pressure switch or sensor may be faulty. With power off, inspect its wiring and connector. Test continuity or resistance per Rheem’s service manual. Replace the sensor if defective.

Warning: Never bypass or jumper the low water cutoff or pressure switch. Operating a boiler without adequate water can destroy the heat exchanger and create a fire hazard.