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9th Aug 2019

How do you bleed an oil boiler?

In many ways, oil boilers are very similar to gas boilers. There are however important differences, including the fact that unlike gas appliances, oil boilers can run out of fuel. If this happens, air may get trapped in the fuel line, stopping the appliance from working.

If you have an oil boiler and you want to know what to do if you accidentally run out of fuel, keep reading. Here, we explain what happens if your oil tank runs dry, and the steps you can take to bleed your boiler and get it working again. We also offer advice on oil boiler pressure, and how you can adjust it.

What happens if you run out of oil?

No one wants to run out of heating oil. Failing to top up your tank with kerosene or gas oil in time could mean you’re left without heating and hot water. This can be inconvenient at any time of the year, but it’s especially problematic during the autumn and winter when temperatures tumble. And it’s not just your comfort that’s at stake. Running out of heating oil risks causing problems with your heating system.

The good news is, oil boilers have mechanisms built into them to automatically switch the burner off if there is no oil to combust, so a lack of fuel won’t cause any safety issues. However, letting your tank run down until it’s dry can mean that the residue and dirt that often collects at the bottom of these containers is drawn into the fuel line, potentially creating problems for your heating system. If this happens to your boiler, you might need to get an OFTEC registered engineer out to clean your fuel lines or replace your filter. This is obviously an expense and a hassle best avoided if possible.

At the very least, you’ll need to reset your boiler to get your heating system back up and running once you’ve topped up your tank. You might also find that air has become trapped in the fuel line, meaning your boiler doesn’t work when you hit the reset button. If this happens, 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.

How do you 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 than this. Firstly, it’s important to ensure you don’t rush this process. If you have an old metal tank, wait for a couple of hours after you’ve received your fuel delivery before you attempt to fire your boiler back up. This is because when your tank is topped up, sludge can be stirred up from the bottom, and it’s best to let this settle before you restart your appliance. Especially if you’re itching to get your heating system and hot water back online, this can be difficult – but it’s not worth hurrying the process if this risks damaging your system.

Once you’re confident that the fuel is OK to use, you can try to switch your boiler back on using the reset button. If you’re lucky, your appliance will simply kick into action straight away and start heating your home normally. If air is trapped in the supply line though, your boiler may be unresponsive. Don’t keep pressing the reset button as this isn’t good for your appliance.

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 – including a pipe wrench or Allen key and a small container. To keep mess to a minimum, it’s also a good idea to wear rubber gloves and have some rags handy to mop up any spillages.

Once you’re ready to get to work, make sure your boiler is turned off. Next, take the front cover off your appliance and find the bleed screw on the fuel pump. This will be near to where the oil line comes in. You should then place a small container underneath this bleed screw ready to catch the oil as you drain the line. At this point, you can start to loosen the bleed screw using your wrench or Allen key. Do this gradually, and only loosen it part way. If you remove it completely, oil will eventually come spurting out at high pressure, leaving you with a nasty spillage to deal with.

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. You should hear a hissing sound as air starts to escape. This will be followed by a sputtering of oil mixed with air, and after this, you should see a steady flow of oil. As soon as this happens, tighten the screw and make sure it’s fastened securely. Your boiler should now be functioning as normal, so you can simply replace the casing and let it do its thing.

Note that 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 removed.

How can I ensure I don’t run out of fuel in the first place

As the old adage goes, prevention is better than cure. To save yourself the hassle of having to either bleed your fuel line yourself or get an engineer out to do this for you, there are a number of steps you can take that should ensure you never run out of heating oil.

For example, get into the habit of checking your fuel tank on a regular basis. You can do this by physically going to your tank to look at it, but it’s much easier to have an electronic tank gauge fitted that allows you to monitor your oil usage remotely. This setup also allows for you to arrange with your supplier to automatically deliver fuel to you once your reserves get below a certain level.

oil gauge showing nearly empty on a tank

Take steps to protect your oil against theft too. A secure tank lock is a must, and if possible make sure your tank isn’t visible from the road.

What pressure should my oil boiler be?

Apart from running out of oil, there are other things that can prevent your boiler from operating properly. One of these is incorrect pressure. The optimum pressure for these appliances differs from model to model, so if you’re not sure if your boiler is operating at the proper pressure, the best thing to do is to consult your manual. Typically, manufacturers recommend between one and three bars, often pointing to around 1.3 bar as ideal.

If your boiler pressure is too high or low, it may lockout. So, if you spot a problem, it’s important to take action. It’s possible to adjust your boiler pressure yourself, but if you lack experience and confidence doing this, you might want to contact an engineer to fix the problem instead.

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. If you decide to do this yourself, you’ll need to locate the drain cock or a radiator bleed valve. Loosen this slightly to allow water to escape slowly. Do this until the pressure reaches the recommended level. If you’ve used a radiator bleed valve to do this, 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 to get it back up. 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.

Whether it’s 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, it’s always better to get an OFTEC registered heating engineer to look at your appliance.

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