An oil boiler that has run out of fuel, had a long period without use, or recently had the tank refilled sometimes locks out and refuses to fire. In many cases this is simply an air lock in the fuel line that you can clear yourself in ten minutes without calling an engineer.
Bleeding a boiler involves working with fuel oil. Ensure the boiler is switched off before you start. Work in a ventilated space, keep a rag handy to catch drips, and never use a naked flame near the boiler. If you are in any doubt, call a registered OFTEC engineer.
Unlike gas boilers which receive a continuous supply from the mains, oil boilers draw fuel from a storage tank. If the tank runs dry, air enters the fuel line. Even if the tank is refilled immediately, that air pocket sits in the line and prevents fuel reaching the burner. The boiler fires, fails to ignite, and locks out: usually showing a lockout light or fault code.
The same thing can happen if the boiler has been unused for a long period, if the fuel filter has been changed without care, or occasionally after a very large delivery that disturbs sediment in the tank and temporarily blocks the filter.
The bleed screw is on the fuel pump, which is part of the boiler burner assembly. On most UK oil boilers (Worcester Bosch, Grant, Warmflow, Firebird, and others) the pump is a grey or black cylindrical component attached to the burner. The bleed screw is a small brass screw, usually with a slot for a flat-head screwdriver, on the side or front of the pump body. It may be covered by a small rubber or plastic cap.
Make sure the boiler is switched off at the programmer or thermostat and at the main isolation switch. Do not attempt to bleed a boiler that is running or attempting to fire.
Hold the container or rag directly below the bleed screw to catch any fuel that escapes.
Turn the bleed screw anticlockwise by about a quarter to half a turn using the flat-head screwdriver. You do not need to remove it fully. You should hear a faint hiss as air escapes. Keep the container below the screw.
Initially you may see bubbles or a mixture of air and fuel. Wait until a steady stream of clean fuel flows without any bubbling. This confirms the air has been purged from the line.
Once fuel flows steadily, tighten the bleed screw clockwise. Do not overtighten. A snug fit is sufficient.
Press the reset button on the boiler (usually a red button on the burner). Switch the boiler back on and call for heat at the thermostat. The boiler should fire within a few seconds. If it locks out again, repeat the process once more: sometimes two bleeds are needed if there was a significant amount of air in the line.
If the boiler locks out repeatedly after bleeding, the problem may not be an air lock. It could be a faulty nozzle, a blocked fuel filter, a failing pump, or a fault with the ignition electrodes. At this point call a registered OFTEC engineer rather than continuing to reset.
Some older or simpler installations have a gravity-fed fuel line rather than a pump-fed system. In these cases there is no pump bleed screw. Instead, air in the line can sometimes be cleared by opening the manual bleed valve on the filter housing (if fitted) or by checking that all isolation valves between the tank and boiler are fully open. If none of these resolve the issue, an OFTEC engineer will need to prime the system properly.
Call a registered OFTEC engineer if:
OFTEC-registered engineers can be found at oftec.org. Always use a registered engineer for boiler work: unregistered work can invalidate your home insurance and, more importantly, is a safety risk.
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