Applicable Systems
Baxi Heating Boiler
Symptoms
The boiler water pressure has dropped below 0.8 bar (12 psi). The pressure switch opens and the boiler stops firing to prevent damage from low water conditions.
Possible Causes
- System water lost through a leak in the piping, radiator valve, or heat exchanger
- Expansion vessel charge lost, causing the pressure relief valve to vent water
- Pressure relief valve stuck open or passing, bleeding system pressure
- Automatic filling valve failed closed, not replenishing lost water
Troubleshooting Steps
- Step 1: Check the system pressure gauge and compare to the required 1.0-1.5 bar cold fill pressure
Repressurize using the filling loop if pressure is below 1.0 bar - Step 2: Inspect all visible piping, radiator valves, and the boiler flange for water leaks
Repair any visible leaks before repressurizing - Step 3: Check the pressure relief valve discharge pipe for water flow
If water is flowing, the valve is passing and needs replacement - Step 4: Test the expansion vessel by pressing the Schrader valve
If water comes out, the diaphragm is ruptured; if air only, check the precharge pressure
How to Reset
Reset: Re-pressurize the system to 1.2-1.5 bar using the filling loop. The pressure switch closes and the boiler restarts automatically.
Common Replacement Parts
- Pressure relief valve: 3 bar (43 psi) set point (Match to boiler model)
- Expansion vessel: 8L or 12L, 1.0 bar precharge (Match to system water volume)
- Filling loop hose: Flexible, 15mm compression (Disconnect after filling per regulations)
Frequently Asked Questions
- Q: What is the correct boiler pressure?
Cold: 1.0-1.5 bar. Hot: up to 2.5 bar. If pressure exceeds 3.0 bar, the relief valve opens and discharges water.
- Q: How do I repressurize the system?
Connect the filling loop between cold water supply and system return. Open both valves until the gauge reads 1.2-1.5 bar, then close and disconnect.
- Q: How often should the expansion vessel be checked?
Annually. Check the nitrogen precharge (should be 1.0 bar) and verify the diaphragm is not leaking by checking the Schrader valve for water.
Source: OEM Installation Manual
Category: Home Boiler