How to bleed a boiler in 4 easy steps


Bleeding a boiler can be useful for many different reasons. Amongst others, it helps to expel air from the system, increase the radiators’ performance, reduce the combustion temperature, and also simply helps to improve the boiler’s performance after noticing that it hasn’t been working as efficiently for unknown reasons. In most circumstances, bleeding the boiler is often the best solution. This article will explain 4 simple steps to bleeding a boiler and why this is beneficial.

Signs your boiler needs to be bled

When your boiler needs to be bled, it can make strange noises, especially when your heating is turned on. Boiler owners sometimes also humorously describe how they know their boiler needs to be bled. They often associate the strange noises their boiler makes with something out of a “horror movie”. There is the sound of knocking metal, liquid hiss-like sounds, vibrations in the pipes, echoes, and more…

If your boiler is showing signs that the air needs to be purged, a simple restart will not fix things. Resetting it may temporarily stop the noises, but this is not a permanent solution. Some boiler owners restart their boiler multiple times, causing it to crash, but the mysterious noises always resurface at some point, anyway.


These noises are actually not mysterious at all. They are simply due to the presence of air bubbles in the boiler, and are located mainly in the heating system and in the radiators. The only thing you need to be afraid of is your energy bill at the end of the month as these air bubbles slow down the water heating process, dissipate heat, prevent temperature exchange, and generally result in the boiler needing more fuel to work efficiently to reach the desired temperature. You can therefore safely say that when your boiler is making strange noises, this is a good sign that it needs to be bled.

Benefits of bleeding your boiler

This article will not focus on how to bleed radiators as we have already published an article about this topic (check out this link if you would like to read it). This article’s topic is about how to bleed a boiler, although, as you’ll see, also includes information on bleeding a radiator within a heating system.

The vast majority of websites out there confuse both of these bleeding procedures (i.e.boiler bleeding and radiator bleeding). However, they are not exactly the same. Radiators can be bled without having to bleed to the boiler if, for instance, you need to lower the system pressure. Boiler bleeding, on the other hand, is a more complete process, and can also include bleeding the radiator, so that the entire system is purged of air.

Performing a correct and well-timed boiler air purge is definitely the most common and effective method of improving your boiler’s performance if you have noticed a reduction in efficiency but your boiler is not showing specific signs of failure. Bleeding the boiler helps to prevent internal corrosion and malfunction of the various parts because air bubbles are eliminated (and these limit water circulation and dissipate heat from the water, causing an increase in fuel consumption and therefore also more polluting emissions).

The Institute for Energy Diversification and Saving (IEDS) has confirmed that heating accounts for 46% of all energy consumption in Spain’s residential sector, which implies a significant monthly energy bill. Each time you increase the temperature by one degree, it increases the fuel consumption and money spent by between 5% and 7%. It is therefore vital that your heating system functions in the most fuel-efficient way as this helps you to save money on excess energy costs.


Situations where bleeding a boiler is recommended

  • It is recommended that you bleed your boiler 1 to 2 times a year as part of its maintenance routine, even if there are no obvious problems. Bleeding can be done when your boiler is having its annual service, or at any other time convenient to you. With new boilers, bleeding can be carried out once a year, but older boilers need to be bled twice a year. The technician who normally services your boiler is the best person to recommend how often your boiler should be bled.
  • When summer has come to an end, your boiler will soon start working harder after a long period of inactivity. This normally happens at the beginning of autumn.
  • When installing a new boiler, it is always recommended that you bleed it. If the air is not purged, the gases will cause the temperature to get too high, which will result in the boiler starting to lock up.
  • When you have a blockage in your boiler due to excess temperature resulting from gas combustion. This is typically an unmistakable sign that your boiler needs to be bled.
  • When the heating circuit contains air bubbles, it will make piping noises and cause the ducts to vibrate, even if you have already bled the radiator. These air bubbles slow the heating process down, and therefore consume a greater amount of fuel and also emit more polluting gases.
  • When the boiler is not working properly and you are not sure what is causing this.
  • When the boiler and its circuits are working perfectly, but the radiators are not reaching the temperature you set.

Boiler bleeding methods

Automatic draining

Some boilers have an automatic drain in both the boiler itself and in the radiator, or in another part of the unit. This component allows the heating system to bleed automatically, without you having to do anything. An important thing to remember about automatic drains is that they can sometimes develop leaks, and these leaks can gradually become worse and end up flooding the room in which the boiler is located.


Manual bleeding

Not all bleeding is done automatically. Most boilers require manual bleeding and require some kind of action on the part of the user to expel air from the system. They normally have a cap or a hatch that must be manually opened. Manual bleeding can also be done via an automatic bleeder that has been designed for this purpose.


Step-by-step process to bleeding a boiler

1- Turn off the boiler completely

Your boiler must be completely shut down before you can start bleeding it. This is even more important if your boiler has a room thermostat. Bear in mind that if the boiler is not turned off, it can start working automatically at any time. If the boiler is turned off, it will be cold as the water will not be moving around. The air will separate from the water and will rise, making the bleeding process effective.

2- Locate the boiler bleed valve

Bleed valves come in many different designs. The design and location of the drain varies from brand to brand, but the bleeding procedure is always the same. If, for example, you are bleeding a Saunier Duval boiler, it is very similar to that of a Junkers, Vaillant, or Roca boiler. If needed, consult your boiler manual to find out what the bleed valve looks like and how it works. Then locate it inside your boiler.


Once you have located the bleed valve, you will notice that it has a specially designed plug. The plug’s design means that you do not have to completely remove the cap. Unscrewing it a little will allow some of the air to bleed out and be released. This is all that is needed for a boiler bleed.

After this, you should turn your boiler on and test it to see if it is no longer making any strange noises, vibrating or showing any other signs of needing a bleed. If so, the job is done. In some cases, bleeding the boiler alone is not enough to eliminate these signs, and purging the radiator and system will then be necessary.

3- Benefits to bleeding a radiator

Unless the radiator has an automatic purging system as part of the system and does not require manual bleeding, it is recommended that you bleed air from the radiators and pipes so that the entire system (and not only the boiler) can be purged of air. This can be done by using the bleed valve located on the radiator.

Bleeding radiators correctly involves starting with the radiator that is closest to the boiler, and moving in sequence to the one furthest away so that they are bled. If your home has more than one floor, it is best to begin with the upper floor, and then work your way down. A tell-tale sign that a radiator has been bled is the appearance of slightly cloudy water. When this happens, bleeding is complete.

(For more information, read our article on How to bleed radiators).


Should the boiler be on or off when bleeding the radiators? The answer is off. Your boiler must be completely switched off and cold. This means that it must have been off for quite some time before you start bleeding the radiators (approximately 1 hour before). This is not only for safety purposes, but also according to thermodynamic laws. The radiators should be bled when they are cold because the heating circuit needs to be stopped so that the pump doesn’t move the water around, and therefore also does not move the air that has accumulated in the system. Since there is no movement in the system, the air will rise to the top of the radiator and stay there, and this is what makes bleeding easier.

5- Refill the heating system

To complete the bleeding process, it is a good idea to fill the heating system with fresh water. This is a simple task, and involves opening the filling valve until the system fills up with water, and the boiler’s pressure normalizes. The normal pressure level is between 1 and 2 bars, but to find your boiler’s optimal pressure level, consult your user manual.

Once your boiler has reached its optimal pressure, remember to close the filler cock. If it is inadvertently left open, the pressure will rise too much, the expansion vessel will be under pressure, and the water will likely automatically be expelled through the safety valve in extreme circumstances.

Once your boiler is at the pressure (as recommended in your user manual) and you have closed the filling valve, the bleeding process will be complete and 100% ready for use.


Bleeding air from a diesel system

Bleeding an oil boiler is a different kind of job. Diesel boilers sometimes need air to be purged from the diesel circuit. Whenever a diesel burner starts tripping, a possible cause could be that air has entered the diesel circuit.

If the diesel tank becomes empty, a consequence of this could be that air is able to enter, or even that the boiler will not be able to restart when the tank is refilled. It is for this reason that it is not advisable to wait for the tank to get empty before refilling it.

If air has entered the unit, you will need to bleed it so that it can work properly again. This is done using the bleed valve, which is normally located on the side of the diesel pump. However, it is best to consult your user manual to determine exactly where the bleed valve is located and what it looks like.

Buy Suner bleed valves and other spare parts


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