430 High or low system water pressure

What the control is telling you

System pressure is outside the normal 13–87 psi band. Between 7 and 13 psi the boiler runs derated (warning); at 7 psi or less, or 87 psi and above, it shuts down.

Applies to: I-Series Plus boilers — 150K BTU and smaller and 175K BTU and higher (Combi and Solo)

Likely causes

• Low system fill pressure or leaks in the CH system
• Undersized, undercharged or waterlogged expansion tank
• Faulty or leaking pressure relief valve
• Malfunctioning third-party auto fill valve
• Debris on or failure of the pressure sensor
• Over-pressurized system

Test and repair steps

1. Read system pressure in performance data 01 and compare it with an external pressure gauge.
2. If pressure is low, add water until at least 18 psi is reached (see the Installation and Operation Manual for filling).
3. After adding water, perform the deaeration process described in the Installation and Operation Manual.
4. Check the expansion tank is correctly sized for the system volume, properly charged, holding pressure and not waterlogged.
5. Look for leaks anywhere in the central heating system.
6. Confirm a 30 psi relief valve is installed and not leaking; if pressure is too high, bring it down to no more than 30 psi.
7. Check that any third-party automatic fill valve is working correctly.
8. Remove the pressure sensor and inspect for debris; clean gently with a soft bristle brush, rinse with clean water and reinstall.
9. Repeat the pressure check, fill and deaeration steps; if the sensor reading is still inaccurate, replace the pressure sensor.