Why is my boiler leaking water?


Below are some answers and solutions.

If your boiler is leaking water, this could be a sign of various problems: the pressure may be too high, the expansion tank could be cracked, there could be a faulty safety valve, the radiators need to be bled, or leaking parts…Homeowners sometimes ask questions, such as “why is my boiler leaking water from underneath?”, or “why is my boiler losing so much water and pressure?”. This article will answer your questions about why your boiler is losing water.

Reasons your boiler is losing water

When pressure rises inside your boiler and it loses water, the problem is normally due to a pressure increase, and not the safety valve. Even though your boiler is losing water through the safety valve, this does not necessarily mean that there is a problem with it as the valve is performing its function by getting rid of excess water to control the pressure. It is not advisable for boiler pressure to be above 3 bars. Some reasons for leaking water include:

  • Even though the safety valve has been designed to automatically expel water when needed, it sometimes does this incorrectly when it is faulty.
  • The filler tap may not be closed properly, or it may be damaged due to minerals present in the water.
  • The radiators may be congested, in which case they will make strange sounds.
  • The expansion vessel may not be the correct size with regards to the boiler water flow and your home’s water demands. Otherwise, it may have a hardened or perforated inner membrane.
  • It’s important to note that your boiler might be releasing water because the pressure is actually too low (less than 1 bar). This could mean that the boiler is losing water and pressure at the same time, which prevents the hot water from circulating properly through the heating and DHW circuit, resulting in a dripping boiler.
  • Water leaks do not only occur through the safety valve. Your boiler may also have a faulty, porous, or leaky part. If your boiler is losing water in jets and not drops, the problem could be quite serious and further investigation is needed.
  • If the boiler is leaking or spilling water but the pressure is not affected (the manometer remains in the normal pressure range of between 1 and 2 bars), this means that there is a problem outside of the heating circuit. In this case, the problem will be with your domestic hot water circuit, which is an open circuit.

How to fix a leaking boiler

How you fix the water leak will depend on which one of the above problems your boiler is experiencing.


As previously mentioned, boilers tend to release water through the safety valve, and this happens when the safety valve is either faulty or (and this is normally the main reason) for some reason, the pressure in the boiler is rising and the safety valve is performing its main function of relieving the pressure by releasing water. The water leak may also be because the boiler’s pressure is too low, or the pressure may not be the issue at all. Let’s take a look at what you should do in each scenario:

  • If the safety valve is faulty (for instance, it may have mineralized due to the continuous flow of hot water), it is best to replace it with a new part. Click here to browse all the safety valves available in the Suner store.
  • If your boiler is leaking water and it is a Saunier Duval, bear in mind that this make often experiences problems due to the safety valve, which tends to get damaged as a result of wear and tear. We recommend checking the safety valve very carefully, but also not ruling out other options. Click here to browse all the Saunier Duval brand safety valves for sale at Suner.
  • If the problem is with the filling cock, we recommend that you replace it with a new part. Take a look at the filling cocks for sale at Suner by clicking on this link.
  • If the boiler’s radiators are congested, you should bleed them. You can check if this is the problem by bleeding the radiator closest to the boiler. If the bleeding lowers the boiler’s pressure, and stops the water leaking out of the safety valve, it is a sign that you will need to bleed the other radiators as well. If you do not know how to bleed radiators, this article will give you a step-by-step guide on how to do so. It is advisable to bleed your radiators at least once a year, as this makes it easier for the hot water from the heating system to circulate without a problem. In addition, bleeding the radiators improves the boiler’s energy efficiency and fuel consumption, and the radiators are also likely to last longer.
  • If the rising pressure is due to the expansion vessel, you should carefully check it. This is a very important topic, which we will discuss later on in this article.
  • If the water leak is due to decreasing pressure, the solution is to increase the boiler’s pressure until it reaches the correct level. The water leak should then stop as soon as you increase the pressure.
  • If the safety valve is not the cause of the water leaking problem and there is another part that has been damaged or torn inside the boiler, you would need to find which part is faulty or leaking, and then replace that part with a new one. You can click here to browse the Suner store’s extensive catalogue of spare boiler parts. Another reason (apart from the safety valve) why the boiler may be leaking water is because of a leak between parts that are joined together. This can be because the joint has become too weak or has been broken. Consider tightening the joints or replacing the gasket with a new part.
  • If you have a water leak anywhere in your domestic hot water system, you should try and find the source of the leak and fix it, either by changing one of the pipes or by tightening or replacing a joint. Sometimes, these kinds of problems require the entire circuit to be replaced.

Another very important point to note is that sometimes parts break not only due to normal wear and tear, but also because the boiler has not been maintained regularly. As a general rule of thumb, you should have your boiler serviced once a year. If you do this, the spare parts will not break, or will take longer to do so.


When boilers leak water due to a broken expansion vessel.

One of the most common reasons for boilers to leak water due to increased pressure is that the expansion vessel is not working properly by absorbing the excess pressure. If the boiler is leaking water through the expansion tank, it is a good idea to check that it is working correctly. Otherwise, you should repair or replace it with a new part.

To check the condition of the expansion vessel, apply pressure to it and check what comes out of it. If air is released this means that the membrane is intact and not broken. However, if water comes out of the expansion vessel when you apply pressure, this means that the membrane has been torn and the water from the boiler is flowing into the air chamber, which should normally be completely separated from the water chamber. You can fill the expansion tank with more air, or you could try and fix it by following the steps in this article (we will also leave a link at the end). However, it is almost always better to replace it with a new part. Expansion vessel repairs are notoriously difficult and are often in vain, making it not worth the effort. Replacing this part is normally much easier.

If you need a good replacement part for your expansion tank, check out what the Suner store has available here. We stock only original products that have been made from the highest quality and most durable materials. We stock Beretta, Junkers, Ferroli, Roca, and Vaillant brand expansion vessels, among others. The Suner store is well-known for its simple purchasing process and fast and safe shipping.

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