How to Bleed an Oil Boiler After Running Out of Oil

To bleed an oil boiler, you loosen the bleed screw on the burner’s oil pump, press the reset button so the pump runs, and let air and oil escape until a steady stream of bubble-free oil comes out, then tighten the screw. It is usually needed after the tank has run dry, because air gets into the line and the pump can’t push oil through it.

It sounds simple, and often it is. But it means taking the casing off and working right next to the burner, so read the whole guide first and be honest with yourself about whether you are comfortable doing it. There is no shame in calling an engineer, especially on a cold night.

How do I know my oil boiler needs bleeding?

The classic sign is a boiler that locks out after the tank has been refilled. The burner runs, tries to light, gets no oil at the nozzle and shuts down. You may hear the motor run for a while with no flame. Bleeding is also often needed after new oil pipework, a filter change or a tank replacement.

If your tank didn’t run low and nothing has been disturbed, air in the line is less likely. In that case the cause is probably elsewhere, and our guide to an oil boiler not firing up is a better starting point.

Should I bleed it myself or call an engineer?

Call an engineer if any of these apply:

• The boiler locked out for a reason other than running out of oil.
• You smell oil, see a leak or notice wet pipework.
• The tank is a long way from the boiler, as long runs can be hard to prime by hand.
• The boiler is still under warranty and you are unsure whether DIY work affects it.
• You have pressed reset several times already.
• You are not comfortable removing the casing or working around the burner.

Otherwise, if the tank simply ran dry and you have refilled it, bleeding the pump yourself is something many oil users learn to do.

What do I need before I start?

Item	Why
Disposable or rubber gloves	Kerosene irritates skin and the smell lingers
Old rags and a small container	To catch oil from the bleed screw
Screwdriver, small spanner or Allen key	Bleed screws vary by pump make
Torch	Pumps sit low down inside the casing
A second person (optional)	One presses reset while the other watches the screw

How do I bleed an oil boiler, step by step?

1. Check there is enough oil in the tank. Make sure the level is comfortably above the outlet, not just a few centimetres in the bottom.
2. Wait a while after the delivery if you can. Giving the oil time to settle lets sediment sink before you draw it through.
3. Switch the boiler off at the programmer and the isolator before removing the casing.
4. Find the oil pump. It is mounted on the burner where the oil pipe connects. Look for a small screw or vent on the pump body. On many Riello burners it is clearly marked, but check your manual if you aren’t sure.
5. Put a rag or container underneath the bleed point.
6. Loosen the bleed screw a quarter to half a turn. Don’t remove it completely.
7. Switch the power back on and press the burner reset once so the pump runs.
8. Watch the bleed point. Air will hiss out first, then a spluttering mix of air and oil.
9. Tighten the screw once the oil runs steady and bubble-free. Firm is enough; overtightening can damage the threads.
10. Wipe up every drop of oil, refit the casing and let the burner run through its start-up.

The burner may need one or two cycles to fully prime. If it locks out, wait at least five to ten minutes before trying again so any oil in the combustion chamber can clear.

Why is waiting between resets so important?

Every failed attempt to light can leave a little unburnt oil in the combustion chamber. Reset again and again in quick succession and that oil can ignite all at once with a bang when the burner finally catches. Give it time between attempts, and stop altogether after two or three.

I’ve bled it and it still won’t fire. What’s wrong?

Something else is stopping it. The common culprits after a run-out are:

• A filter clogged with sediment pulled up from the bottom of the tank.
• A blocked or worn nozzle.
• Worn electrodes or a dirty photocell.
• A failing oil pump or sticking solenoid valve.
• A long supply pipe that needs priming from the tank end with proper equipment.
• An air leak at a fitting, which lets air back in after every bleed.

Any of these need an OFTEC registered engineer. The brand guides for Grant, Worcester Bosch and Firebird oil boilers go into the model-specific faults.

How do I stop it happening again?

• Don’t let the tank run low. Reorder at around a quarter full, sooner in winter when deliveries take longer.
• Use a remote tank gauge if your tank is out of sight at the bottom of the garden.
• Get the boiler and tank checked every year. A proper oil boiler service includes a new nozzle, a filter check and a look over the tank and supply pipe.
• Keep the tank lid and gauge cover sound so rainwater can’t get in.

Is running out of oil bad for the boiler?

Not usually. The burner locks out rather than doing damage. The real risk is the sludge and water that collect at the bottom of the tank, which get drawn into the filter and pump when the level drops right down. If you run out often, or your tank is getting on, a new filter at the very least is a good idea, and an old tank may be due for replacement. We check the tank and oil line on every oil boiler replacement.

Bleeding an oil boiler: quick answers

How long does it take to bleed an oil boiler?

Usually 10 to 20 minutes for a straightforward bleed at the pump. Long pipe runs or repeated lockouts can take longer.

Can I damage my boiler by bleeding it?

The main risks are overtightening the bleed screw, spilling oil and resetting too many times. Take it slowly and stop if it doesn’t fire after two or three careful attempts.

Do I need to bleed it after every delivery?

No. Only if the tank ran dry or very low before the delivery. A normal top-up shouldn’t let air into the line.

Is oil spilt inside the casing a fire risk?

Yes. Wipe up every drop before restarting, and dispose of oily rags safely outside.