Why is My Diesel Boiler’s Burner Blocked?


The burner is, without a doubt, one of the most (if not the most) important components of your boiler. This part of your boiler provides you with domestic hot water and helps you to live comfortably in your home. When there is a blockage in your diesel boiler’s burner, it will usually stop working completely. This can be incredibly frustrating and it always seems to happen at the most inconvenient times.

It is usually better to prevent this problem from happening in the first place and to have it maintained or repaired before the cold season starts. With this in mind, we are going to discuss in detail some of the causes of your diesel boiler burner not starting, and how you can resolve these problems. In some cases, the burner (or some of the burner’s parts) will be damaged and you will need to replace the damaged components. At the end of this article, we have included a complete list of control units and photocells for all the main boiler brands that are available for purchase at Suner.

Diesel supply failure.

One of the most common reasons your diesel burner’s boiler is not starting is because there is a diesel flow failure. This is evident when the fuel does not pass through to the burner nozzle.

There can be many factors causing this: problems with the pump hose or solenoid valve (the part that causes pressurized fuel to flow to the nozzle), diesel pump failure, or failure of other parts such as the coupling (this is the plastic component connecting the motor and the pump), or even a key that should be open but is closed.

In such cases, the first thing you should do is to make sure that the pump is actually taking in diesel. This can be done by means of a very effective test which consists of loosening the return hose and placing it in a bucket. You then need to observe it to see if any diesel comes out when the pump is operating. If it isn’t, the problem is simple: the tap is closed, and simply need to open it for the fuel to flow again.

Poorly regulated or poorly installed burners.

One of the burner’s main functions is to create the perfect balance of air and fuel so that the burner can function optimally. If it fails to do this, the unit will not operate properly. To check whether your burner is well or poorly regulated, you need a device or a meter that can verify the chemical and physical composition of the gases. Boiler technicians normally have such equipment.


However, you probably don’t have these instruments at home, and if you are not planning on calling a technician in who has the necessary tools to check your burner, you could always try the following visual test, which is easy and practical. Look at the color of the flame:

  • If there is not enough air, the flame will be dark in color. This is not a good scenario because your boiler will generate smoke that can block pipes and the burner.
  • If there is too much air, the flame tends to be white or bluish-white in color. This is not good, either, because the boiler is not using fuel efficiently, and this does not comply with the anti-pollution regulations (Royal Decree 1042/2017 of December 22), as well as making it difficult to ignite your boiler.

If your boiler is one of the brands where you can’t see the flame directly, you can test it by paying attention to how the smoke flowing out of the chimney behaves:

  • If the smoke exiting the chimney is dark, you will need to increase the amount of air in the burner.
  • On the other hand, if the smoke is white, you will need to decrease the amount of air in the burner until the smoke is no longer white.

When your boiler is operating as normal, the smoke coming out the chimney will be colorless. The smoke will once again be this color once your technician has regulated the burner. In both cases, after assessing the situation, you would still need to call in a technician to have the burner regulated. Unless you are well-versed in regulating your burner in a safe way, it’s always best to seek professional help.

When the airspeed is faster than the flame spread

Something else that can cause problems with diesel burners is a unit failure when the fuel tank has just been refilled. This does not only happen when the filters are clogged and dirty, and there is the same chance of this happening if you have clean filters. The burner may be injecting diesel fuel correctly, but it could fail to ignite.

Obviously, in some cases, the problem is that the boiler has been refilled with poor quality diesel. If you are able to rule this out, then the problem is likely that the speed at which the air travels through the burner nozzle is greater than the flame propagation speed. This is what actually causes the burner to stop working. The solution here is to calibrate the air outlet speed and the flame spread until you have the correct balance of air and flame.

Problems with the diesel burner’s protocell or photoresistance

As mentioned above, when there is a blockage in a diesel boiler’s burner, the boiler will stop working. Since burners are made up of several parts, any one of these components can fail. We will now discuss the photocell. Whenever there is a blockage in your burner, you should assume that the photocell has potentially failed.

Photocells are vital for the burner to be able to switch on and off properly. It is also very sensitive to light. Its main function is to detect fire, and, depending on whether or not it detects a fire, or how much of a fire there is, it will cause the burner to be turned on or off. In other words, the photocell can detect the level of light or flame that is present in your boiler, and depending on how much fire there is, the electronic board will decide how the burner operates. Obviously, if this process fails, the burner will not work. The problem may be with the photocell itself, or its connection with the control unit. If the photocell is faulty, dirty, misaligned, or has been incorrectly connected, the burner may not work.


There are normally 2 kinds of photocell breakdowns that affect how your burner operates:

  • When the photocell detects that there is fire, but there is actually no fire. In this case, the boiler will be locked during the first few seconds of operation.
  • When the photocell does not detect any fire, even if there is one. This could be because it is dirty or has faulty connections. In this case, even if a flame is generated, the photocell cannot detect it, and the burner will be blocked since there has been no flame detected (we’ll discuss this in more detail in the next point).

Some photocell models get dirty faster than others due to misalignment with the burner. If the photocell is poorly connected or is misaligned, it must be positioned correctly, and this should be done by a technician. Malfunctions in photocells are often simply because they are dirty. In this case, you can clean it with some kitchen paper or a dry cloth. However, bear in mind that it might also be broken and need replacing.

Steady flame or blockage (red light) due to no flame detection

When the red light on the boiler comes on, it means that the equipment has been locked. This typically happens when it is first ignited or after a long period of inactivity (after the summer, for instance). Your first step would be to make sure that the pilot light is working – if not, the problem may simply be because the boiler is unplugged.


Apart from this, there can be various causes of the blockage, from lack of gas or poor combustion to excessive temperature (possibly due to the smoke outlet being obstructed or in poor condition). It could also be that the room thermostat is faulty, there is not enough water in the mechanism, or even a problem with the electrical supply or bad wiring.

Whenever your home has a demand for space heating or domestic hot water, the boiler should turn on automatically. However, if the boiler has not been ignited after a period of 10 seconds, the boiler will activated the misfiring lock. This is a safety measure that it uses.

To prevent this kind of blockage from occuring, the boiler must be switched on and then off: rotate the main selector and put it temporarily in the RESET position (or press the reset button, depending on your boiler). It should be noted, however, that if there are repeated episodes of blockages, you would need to contact a technician. Regular annual maintenance is also very important.


Boiler transformer failure

This kind of fault happens when you try to start your boiler and it trips. In these cases, the problem could be with the transformer. Paradoxically, it is far more rare for an old boiler transformer (30 years or older) to fail or burn out than a newer transformer. Older models were made to last longer.

The problem could also be indirect. There could be a breakdown in another component that is causing the transformer to run continuously. In these cases, the transformer will not be able to release all the heat resulting from its operation. This results in a short circuit and can cause the unit to burn out or break down, and it will always trip. When the transformer fails, the burner will fail as well. The solution might be to reinstall it correctly, as it may have been incorrectly installed. However, sometimes the part will have to be replaced for a new one.

Something that often causes confusion is a very dirty boiler (due to lack of maintenance), resulting in it not being able to start, or switching off as soon as it starts. It then becomes difficult to determine if the blockage is caused by dirt in the boiler or failure of the transformer. A great trick to determining what is really happening is to listen carefully and see if you can hear the sound that the spark makes when the boiler starts. If you can hear the sound of the spark, the problem is probably because of dirt because the transformer is still working. However, if you don’t hear the spark making any sound, the problem is probably being caused by transformer failure.

Failure of the flame control unit

You might have experienced a situation where, after pressing your boiler burner’s on button, it works fine until it reaches the set temperature, where it turns off. When turned on again, the burner button goes off. In these cases, it might be that the electrodes have been damaged, and they need to be replaced with new ones.


The problem could also be with the switchboard. The control unit’s (also known as the control box) function is to automatically control all the devices that cause the burner to ignite (the gas valve and the ignition electrodes), as well as to detect the flame (the ionization electrode). Its main function is to switch the burner on or off, according to your home’s heat requirements.

The control unit is obviously very important when it comes to the burner’s operation, as well as its precision. If the control unit is faulty, it will prevent the burner from operation correctly. To resolve faults with the control unit, you would need to call in a technician, and you may also need a new part.

ECU spare parts by brand

The following are all the spare parts for your boiler’s control unit. Simply select your brand, click on the link and visit the Suner store directly. The buying process is very easy.

Photocell spare parts by brand

Here is a list of spare parts for your boiler’s photocell. All you need to do is select your brand, click on the link and visit the Suner store directly. You will notice that the process is very simple.

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