How to Bleed a Boiler: Oil and Gas Boiler Bleeding Guide

By David Tait Reviewed by Cormac Breen

Editorial Manager

Bleeding an oil boiler consists of draining heating oil from the boiler to remove trapped air from the system. Bleeding a gas boiler requires bleeding individual radiators to remove air trapped inside the internal heating system. Bleeding your boiler is important in order to keep your boiler running efficienctly. If you have any doubt or are not sure how to bleed your boiler, always consult a trained boiler professional.

How Do I Bleed an Oil Boiler?

Oil boilers work slightly differently to other types of boilers as you have to bleed the oil boiler itself as opposed to the radiators as is the case with gas oil.

Before starting to bleed your oil boiler, make sure you have everything you need:

• A pipe wrench or an Allan key.
• A small container to catch the oil.
• Rubber cloves and rags to clean up any spillages.

In order to bleed your boiler, follow these steps:

1. Remove the Boiler Cover
   Begin by carefully taking off the cover located at the bottom of the boiler. This provides access to the internal components necessary for the bleeding process.
2. Remove the Bleed Nut
   Identify the bleed nut, which is usually found on the burner's pump. Use an Allen key or a spanner to loosen and remove this nut. Ensure you have the correct tool for your specific boiler model.
3. Run the Boiler and Bleed
   Switch on the boiler and allow it to run. Watch for the fuel to start flowing out of the bleed nut's opening. This indicates that air is being expelled from the system. Continue this process until there is a consistent flow of fuel without any air bubbles, indicating that the air has been cleared.
4. Replace the Nut and Test
   Once the fuel flows steadily without air, turn off the boiler and carefully replace the bleed nut, ensuring it is securely tightened.
5. Restart
   Restart your boiler to check if it functions properly. The boiler should now operate smoothly without air blockages.

If in doubt about which bung you should remove, and you can’t find the instructions, then consider asking the oil delivery person to help you. They will normally do so for free.

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When Do I Need to Bleed an Oil Boiler?

There are two main reasons why you would need to bleed your oil boiler:

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

Running Out of Heating Oil

Running out of heating oil can indeed require bleeding your boiler. Here's why:

1. Air in the Fuel Line:
   When the oil level in your tank drops below the fuel line, air can be drawn into the system. This air can disrupt the flow of oil to the burner, preventing your boiler from igniting or running properly.
2. Boiler Lockout:
   Some boilers have a safety feature that prevents the burner from igniting if it fails to start multiple times. This can happen when air is trapped in the fuel line.

Boiler Pressure Too High

If the pressure in the boiler is too high or fluctuates a lot, it can be an indication of air trapped in the system. It might not require a full boiler bleed and one of the following might help to reduce the pressure in the heating system:

1. Bleed your radiators:
   Bleeding the radiators may help reduce overall pressure in the system.
2. Pressure release valve:
   Some boilers have a pressure release valve that can be used to release excess pressure in the system.
3. Check the expansion vassel:
   It helps the boiler absorb pressure fluctuations in the system and if it's faulty or not working it can contribute to higher system pressure.

How Do I Bleed a Gas Boiler?

For gas boilers, including condensing and combi boilers, which are connected to your radiators, the solution is to bleed your radiators to remove air from the closed circuit heating system. Here are the quick, easy steps to bleed your gas boiler:

1. Turn Off Your Boiler
   Ensure your boiler is off and has cooled down for at least an hour before starting. This step is crucial to avoid the risk of scalding from hot water or steam.
2. Insert a Radiator Key
   Locate the valve on your radiator and carefully insert a radiator key. Position a basin or cloth underneath to catch any drips that may occur during the process.
3. Turn the Key Slowly
   Gently turn the key clockwise, being cautious to do it slowly. This prevents the pressurized water from ejecting rapidly and creating a mess or causing injury.
4. Close Valve After Seeing Water
   Listen for the hissing sound of air escaping. As soon as water starts to dribble out, promptly close the valve. This indicates that the air has been successfully bled from the radiator.

What Common Boiler Noises Are There When Bleeding a Boiler?

Here are some common boiler noises you may come across and how to quickly fix these issues yourself:

• Gurgling or Whistling Noises
  These may be caused by air in the system or a frozen condensate pipe. If you suspect air in the system, then you will need to bleed your boiler.
• What If I Have a Frozen Pipe?
  If you suspect a frozen pipe, verify if your pipe is outside and easily accessible. If so, you can thaw the pipe yourself with a hot water bottle or heat wraps. Were this the cause of the issue, definitely consider lagging your external piping to prevent it from occurring again.
• Banging Noises
  Often caused by something called 'kettling'. Kettling is a build-up of pressure in the system, normally caused by an accumulation of limescale deposits restricting water to the heat exchanger. Kettling can cause serious damage to your boiler, so if you think this may be the issue, then call an RGI professional straight away to scrub out your heat exchanger.

How Do I Check the Boiler Pressure When Bleeding?

Knowing how to check your boiler pressure is the first step when bleeding your boiler. Here are the steps in knowing what to look for when checking the boiler pressure:

1. Boiler Pressure Check
   Verify whether the boiler pressure is either too high or too low. Ideal pressure levels are typically around 1 to 1.5 bars, but consult the manufacturer’s guide for specific guidelines.
2. Handling High Pressure
   In cases of high boiler pressure, check the pressure valve for proper closure, particularly after repressurisation.
3. Pressure Reduction Procedure
   To lower high pressure, bleed the radiators. If this method is ineffective, professional assistance is recommended.
4. Addressing Low Pressure
   For low boiler pressure, carefully open the pressure valve to adjust the pressure between 1 and 1.5 bars.
5. Professional Consultation for Persistent Low Pressure
   If pressure adjustment does not rectify low pressure issues, seeking the help of a qualified engineer is advised.

Why Should I Bleed My Boiler?

Boilers work by heating water contained within the internal heating system, which is then spread to the radiators around your house and in turn heating it.

Any heating system with air trapped inside works less efficiently, meaning you’ll end up paying more on your energy bill for less heat. If you notice any of the following, it may be time to bleed your boiler:

1. A type of 'gurgling' sound coming from your radiators.
2. A metallic, banging side coming from the boiler.
3. Your radiators are hot at the bottom but cold at the top.

To bleed your radiators, you will need a radiator key, which can be bought at any hardware store. If you don't have one, you can also use a screwdriver, but this may damage the radiator valve.

• 🔧 Advantages of Bleeding Your Boiler
• Improved Efficiency - Your boiler runs smoothly with reduced energy consumption.
• Lower Energy Costs - A more efficient heating systems means less wasted energy and wasted money.
• Longer System Life - Bleeding your boiler extends its lifespan and avoids replacement.
• Reduces Noise - Any knocking sounds from your boiler will be eliminated
• Even Heat Distribution - No more cold patches in your radiators

What Other Boiler Bleeding Maintenance Is Needed?

Aside from bleeding your radiators, repressurising your system, or thawing a condensate pipe, do not attempt any DIY gas boiler repairs.

Never open up your boiler - this is incredibly dangerous. It will also void any warranty, and may also void your home insurance cover.

It's good practice to keep an eye on your boiler and take note of any strange noises or actions it may be making, but for anything beyond that we recommended contacting an RGII registered gas installer to carry out any service or repair work on the boiler.

To find a Registered Gas Installer in your area, you can use the RGII offical website to search for an RGI in your county.

Should I Get a Boiler Service?

Having your boiler serviced regularly can save up to 15% on fuel costs and keep your boiler running in top condition.

Some providers even offer boiler service:

For more information about Bord Gais and Energia's boiler service, you can find more information through the following links:

1. Bord Gais Boiler Service
2. Energia Boiler Service

Boiler cover will generally cover parts replacement (up to a certain amount), emergencies, callout service, and maintenance. When an RGI is called out to your house, they will also check for proper ventilation and whether the flue has degraded.