Baxi 800: Heating intermittently

1. 1. Verify gas supply to the property and boiler.

   Why: No gas supply will prevent the boiler from firing, leading to intermittent or no heating.

   How: Check if other gas appliances in the property (e.g., hob) are working. Ensure the main gas meter valve is fully open.

2. 2. Check boiler pressure.

   Why: Low system pressure is a very common cause of intermittent heating or boiler lockout, often without a specific fault code being displayed immediately.

   How: Locate the pressure gauge on the boiler. The pressure should be between 1 and 2 bar when the system is cold, ideally around 1.5 bar. If low, repressurise the system using the filling loop as per manufacturer's instructions.

3. 3. Inspect the condensate pipe for blockages.

   Why: A blocked condensate pipe, especially in cold weather, can cause the boiler to shut down intermittently as waste water cannot drain.

   How: Locate the condensate pipe (usually a white plastic pipe exiting the boiler). Check for any visible blockages or signs of freezing, particularly on external sections. Thaw with warm (not boiling) water if frozen.

4. 4. Check the room thermostat and programmer settings.

   Why: Incorrect settings or a faulty thermostat can lead to the boiler not receiving the demand for heat, resulting in intermittent operation.

   How: Ensure the thermostat is set to a temperature higher than the current room temperature and that the programmer is set to a 'heating on' period. Check thermostat batteries if applicable.

5. 5. Bleed radiators for trapped air.

   Why: Air trapped in the heating system can prevent hot water circulation, leading to cold spots on radiators and intermittent heating.

   How: Turn off the heating and allow radiators to cool. Use a radiator key to open the bleed valve on each radiator, starting from the lowest point in the system, until water appears. Close the valve and recheck system pressure.

6. 6. Verify central heating pump operation.

   Why: A faulty or seized central heating pump will prevent hot water circulation, causing intermittent or no heating to the radiators.

   How: With the boiler firing for heating, carefully check if the pump is running (listen for operation, feel for vibration). If accessible and safe, remove the pump head to check for a seized shaft. (Requires electrical isolation).

7. 7. Examine the diverter valve operation (for combi boilers).

   Why: A sticking or faulty diverter valve can prevent the boiler from directing hot water to the heating system, especially if domestic hot water is working correctly.

   How: Observe if the boiler provides hot water to the taps consistently. If hot water is fine but heating is intermittent, the diverter valve may be sticking. This often requires professional intervention for diagnosis and replacement.

Commonly failed parts

• Central Heating Pump
• Diverter Valve Cartridge
• NTC Thermistor (Flow/Return)

Sources

The AI cross-checked the following manufacturer and trade sources while producing this checklist.

• justanswer.co.uk
• checkatrade.com
• homecureplumbers.co.uk
• boxt.co.uk
• fixandrepairengineers.co.uk
• iheat.co.uk
• justanswer.co.uk
• youtube.com
• justanswer.co.uk
• 0800homefix.com