Boiler Service

In many ways, oil boilers are very similar to gas boilers. However, there are some key differences: unlike gas appliances, oil boilers can run out of fuel. If your boiler runs out of fuel, air may get trapped in the fuel line and stop the appliance from working.

We’ll explain the best ways to restart your boiler after running out of oil. If your oil tank runs dry, follow these steps to bleed your boiler and restart your heating. We’ll also offer advice on oil boiler pressure, and how to bleed your boiler to adjust the pressure inside.

Oil boiler bleeding instructions for trapped air

If you discover that you need to bleed air from the system and you want to do this yourself, you’ll need some basic equipment.

What equipment do you need for bleeding trapped air from an oil boiler?

  • A pipe wrench or an Allen key
  • A small container
  • Rubber gloves and rags to mop up any spillages and to keep mess to a minimum

Step-by-step guide to bleeding trapped air from your oil boiler

  1. Make sure your boiler is turned off.
  2. Take the front cover off your appliance and find the bleeder valve/screw on the fuel pump. It will be located near to where the oil line comes in.
  3. Place a small container underneath this bleed screw ready to catch the oil as you drain the line.
  4. Start to loosen the bleed screw using your wrench or Allen key. Loosen the screw gradually, and only loosen it part way. If you remove the screw completely, oil will come spurting out at high pressure, leaving you with a nasty spillage to deal with.
  5. Once the screw is sufficiently loose, push the reset button on your boiler. Keep your wrench or Allen key on the screw ready to tighten it back up. 
  6. Listen out for a hissing sound as air starts to escape. The hissing will be followed by a sputtering of oil mixed with air. After the initial air has escaped, you should see a steady flow of oil.
  7. As soon as you get a steady flow, tighten the screw and make sure it’s fastened securely.

Your boiler should now function as normal. Simply replace the casing and let it do its thing.

Note: When bleeding the system, you may find your boiler reset cycle finishes before all of the air is drained from your fuel line. If this happens, you’ll need to press the reset button again to continue the process until the trapped air is completely clear.

How can I adjust the pressure on my oil boiler?

If your boiler pressure is too high, you’ll need to bleed water from the system.

Oil boiler bleeding instructions for reducing pressure

  1. Locate the drain cock or a radiator bleed valve.
  2. Loosen the valve slightly to allow water to escape slowly. Do this until the pressure reaches the recommended level. It’s much easier if you have someone helping you as one person can release water while the other monitors the pressure reading on the boiler.

If the pressure is too low, you’ll need to add more water to the system. You can do this by opening the valve on the external filling loop, which is usually located below the boiler casing. Your instruction manual should explain the process.

When will I need to bleed my oil boiler?

There are two key scenarios when you might need to bleed your oil boiler:

  1. If you run out of heating oil and air becomes trapped in the fuel line.
  2. If your oil boiler pressure is too high or the boiler pressure keeps dropping unexpectedly.

We go into detail below on how to deal with both situations.

1. What happens if you run out of heating oil?

No one wants to run out of heating oil. Failing to top up your tank with kerosene or gas oil could leave you without heating and hot water.

An empty tank can be inconvenient at any time of the year, but it’s especially problematic when temperatures tumble in winter. Beyond discomfort, running out of heating oil risks causing problems with your heating system.

The good news is, oil furnaces have mechanisms built into them to switch off automatically if there is no oil to combust. So, a lack of fuel won’t cause any safety issues. However, if your tank does run dry, you’ll need to:

  • Refill your tank with heating fuel oil
  • Bleed any excess air from your system
  • Reset your boiler to get your heating system back up and running 

You could also encounter some problems that can affect your boiler and require action:

Residue & dirt can get drawn into the fuel line

Letting your fuel tank run dry can lead to residue in the fuel line. The dirt that often collects at the bottom of your oil container will be drawn into the fuel line. 

If your boiler becomes clogged with residue, you will need an OFTEC registered engineer to clean your fuel lines or replace your filter. Avoid this added expense by keeping your tank topped up.

Air can become trapped in the fuel line

If you run out of oil, you might also find that air has become trapped in the fuel line. This means that your boiler won’t work when you hit the reset button. If air does get trapped in your boiler , you’ll need to bleed your fuel supply system to get rid of the air. You may be able to do this yourself, but if you don’t feel confident that you can get this right, it’s best to call an expert.

Top Tip: Avoid running out of heating oil by keeping track of how much oil you’re using. Read our handy guide to measuring your oil usage.

How to restart a boiler after running out of oil

Restarting your appliance after you’ve run out of fuel could be as simple as pressing a button, but it may be more complicated.

Don’t rush the restart process

If you have an old metal tank, wait for a couple of hours between your fuel delivery and attempting to fire up your boiler. Allow time for any tank sludge to settle before you restart your appliance. Hurrying the process risks damaging your system. We always recommend reading your oil boiler manual and tank instructions if you’re ever unsure. 

Once you’re confident that the fuel has settled in your tank, try to switch your boiler back on using the reset button. Hopefully, your appliance will simply kick into action straight away and start heating your home normally.

If air is trapped in the supply line, your boiler may be unresponsive. Don’t keep pressing the reset button as this isn’t good for your appliance.

2. What if the pressure in my oil boiler is too high?

Apart from running out of oil, there are other things that can prevent your boiler from operating properly. One of these is incorrect pressure.

What pressure should my oil boiler be?

The optimum pressure for oil boilers differs from model to model. If you’re not sure whether your boiler is operating at the proper pressure, consult your manual. Typically, manufacturers recommend between one and three bars, often pointing to around 1.3 bars as ideal. The priority is to steer clear of the red zone on your boiler pressure gauge!

If your boiler pressure is too high or low, it may lockout. Checking your boiler regularly can help to prevent issues. Adjust your boiler pressure yourself, or contact an engineer to fix the problem instead.

Speak to our boiler team for help with adjusting your oil boiler pressure.

Whether you’re bleeding air from your fuel line or repressuring your oil boiler, you should only attempt these fixes if you’re confident that you know what you’re doing. If you’re not, ask an OFTEC registered heating engineer to look at your appliance.

If you need help with your oil boiler, please contact our experts.

Alternatively, get a quote for your heating oil delivery today.