Boiler display shows E04 and shuts down, often after running for a short period

Safety: Turn off power and allow the boiler to cool. Do not bypass safety limits.

Steps:

• 1. Check system pressure and temperature: Look at the boiler gauge. Normal residential hydronic systems typically run around 12–20 psi when cold and under 30 psi hot. If temperature is very high, wait for cooling.
• 2. Verify circulator operation: When there is a call for heat, feel the circulator pump body. It should vibrate slightly and get warm. If it is cold and silent, check the circulator relay and wiring. A seized circulator can cause rapid temperature rise.
• 3. Inspect isolation valves: Ensure supply and return valves near the boiler and on main loops are open. A closed valve will trap hot water in the boiler and trip the high limit.
• 4. Bleed air from system: Air-bound zones can prevent flow. Use zone purge valves or manual bleeders on radiators/baseboards to remove trapped air, following Slant/Fin system piping diagrams.
• 5. High limit control: If the boiler repeatedly hits high limit at normal load and flow is confirmed, the high limit aquastat or sensor may be out of calibration and require replacement.

Call a technician: If you suspect scale buildup in the heat exchanger or cannot confirm circulator operation, have a professional evaluate. Overheating can damage the boiler and piping.