How to Bleed an Oil Boiler After Running Out of Fuel
A Practical Guide from the Engineers at Hinckley Plumbing and Heating
Running out of heating oil is one of the most common reasons an oil boiler suddenly stops firing. Once the tank is refilled, many homeowners expect the heating to start working again immediately. Instead, the boiler often locks out and refuses to ignite.
In most cases, the issue is simply air trapped in the oil supply line.
When air enters the pipework between the oil tank and the burner, the fuel pump cannot draw oil properly. The burner attempts ignition, detects no fuel reaching the nozzle, and the boiler shuts down for safety. This is where bleeding the oil boiler becomes necessary.
Bleeding removes trapped air from the fuel line and restores a steady flow of heating oil to the burner, allowing the boiler to ignite normally again.
At Hinckley Plumbing and Heating, this is one of the most frequent call-outs we attend after a customer has topped up their oil tank. The process itself isn’t complicated, but it does require patience and care to avoid spilling oil or damaging burner components.
Why Oil Boilers Need Bleeding After a Run-Out
Oil-fired heating systems rely on a continuous, pressurised fuel supply between the oil tank and the burner pump.
When the tank runs empty:
- Air enters the oil line
- The burner pump draws air instead of fuel
- The ignition sequence fails
- The burner enters lockout mode
Even after the tank has been refilled, that trapped air remains in the pipework. Until it is removed, the burner simply cannot deliver fuel to the combustion chamber.
Bleeding the oil boiler removes this airlock in the fuel line, allowing the pump to pull clean oil through the system again.
What You’ll Need Before Starting
Before attempting to bleed the burner pump, gather a few basic items to keep the process controlled and tidy.
You should have:
- Rubber gloves to protect your hands
- Old rags or a small container to catch any oil drips
- A screwdriver, small spanner, or Allen key depending on the bleed screw fitting
- Good lighting so you can clearly see the oil pump and bleed point
Different oil boiler manufacturers use slightly different burner assemblies. Systems from Worcester Bosch, Grant, Firebird, and Warmflow often use burner units manufactured by companies such as Riello, Danfoss, or Bentone, and the bleed screw position may vary slightly between models.
Having another person nearby can also help, as one person can operate the reset button while the other monitors the bleed screw.
Step-by-Step: How to Bleed an Oil Boiler
If your boiler stopped working after running out of oil, the following method will usually restore fuel flow.
1. Confirm There Is Oil in the Tank
Before doing anything else, check the oil tank gauge.
Bleeding will not solve the problem if the tank is still empty. A small amount of oil remaining in the tank can also draw air into the pipework, so make sure there is sufficient fuel above the pickup level.
2. Turn the Boiler Off
Switch the boiler off at the programmer, fused spur, or isolator switch before removing the front casing.
This ensures the burner is not active while you access the pump and bleed screw.
3. Locate the Burner and Oil Pump
Remove the boiler’s front panel to access the burner assembly.
The oil pump is typically mounted on the side of the burner, where the oil supply pipe enters. On many Worcester Bosch and Grant Vortex oil boilers, the pump becomes visible once the front casing is removed.
Look for a small screw on the pump body — this is the bleed screw.
4. Position a Rag or Container Beneath the Bleed Screw
Even a small amount of oil escaping from the pump can leave stains or smells around the boiler.
Place a rag or small container directly beneath the bleed point to catch any oil that emerges.
5. Loosen the Bleed Screw Slightly
Using the appropriate tool, carefully loosen the bleed screw by a quarter to half a turn.
Do not remove the screw completely.
The fuel line operates under pressure, and fully removing the screw may cause oil to spray out quickly.
6. Press the Burner Reset Button
Press the red reset button on the burner unit to initiate a firing cycle.
This activates the oil pump and begins pushing air and fuel through the pipework.
7. Watch and Listen for Air Leaving the Line
Initially you will likely hear a hissing sound as air escapes.
Shortly after, a spluttering mixture of air and oil will begin emerging from the bleed point. This stage is normal and simply indicates the airlock is clearing.
8. Tighten the Screw When Oil Flows Smoothly
Once the oil begins flowing cleanly and consistently with no bubbles, tighten the bleed screw again.
The goal is to stop the flow once the fuel stream becomes steady.
Tighten firmly, but avoid excessive force, as overtightening can damage the threads on the pump housing.
9. Replace the Boiler Casing and Restart
Refit the boiler panel and allow the burner to complete its ignition sequence.
On some systems — particularly condensing oil boilers from Worcester Bosch or Firebird — the burner may take one or two ignition cycles to fully prime. This is normal after bleeding.
Important Safety Points to Remember
Although bleeding an oil boiler is relatively straightforward, a few safety precautions are essential.
Avoid repeated rapid resets.
Pressing the reset button multiple times in quick succession can flood the combustion chamber with unburned oil. If the burner eventually ignites, that oil could ignite suddenly.
Allow five to ten minutes between reset attempts if the burner does not fire up immediately.
Clean any spilled oil straight away.
Even small amounts of oil left around the boiler casing or floor can present a fire risk and make future leaks harder to detect.
When Bleeding Doesn’t Solve the Problem
If the boiler still refuses to fire after two or three careful attempts, the issue may lie elsewhere within the burner system.
Common underlying faults include:
- A blocked burner nozzle
- A faulty solenoid valve
- Worn ignition electrodes
- A failing oil pump
- Extended airlocks in long oil supply pipe runs
Homes where the oil tank sits a significant distance from the boiler — typically more than 10–15 metres — can sometimes be more difficult to prime manually.
When to Call a Professional Oil Boiler Engineer
While bleeding after a fuel run-out is sometimes manageable for homeowners, it’s best to contact a professional engineer if:
- The boiler locked out for reasons other than running out of oil
- The fuel line between tank and boiler is particularly long
- You’re unsure about working around the burner assembly or fuel system
- The boiler still fails to ignite after bleeding
Persistent ignition issues often require proper burner diagnostics and component inspection, plus we can also advise on a replacement oil boiler if required.
Preventing Airlocks with Regular Servicing
Regular oil boiler servicing helps prevent many of the issues that lead to lockouts and airlocks. During a service, engineers check and clean the burner, filters, and fuel components to ensure oil flows correctly and the boiler ignites reliably. Annual servicing also helps keep the system running safely and efficiently
Oil Boiler Not Starting in Hinckley?
If your oil boiler still won’t fire after bleeding the fuel line, it’s worth having the system inspected properly.
At Hinckley Plumbing and Heating, our APHC-accredited engineers regularly diagnose burner lockouts, airlocks, and fuel supply problems across the Hinckley area. In many cases, we can restore heating quickly by resolving the underlying fault and ensuring the burner is operating safely and efficiently.
Getting the system checked early not only restores heating faster, it also helps prevent further damage to the burner or fuel components as winter demand increases.