• Boilers Why is My boiler leaking?
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  • Help Why is My boiler leaking?

    Boiler Leaking? But Why



    We get many calls from customers asking why is my boiler leaking.

    Boiler leaking, It’s never done this before, 

    it now stopped working and showing a fault code and the pressure 

    has dropped to zero, what has happened?


    Many of today’s boilers now work under high pressure and should the pressure 

    drop due to the boiler leaking, they will go into safety mode and shutdown.

    This is to protect themselves from overheating and damage.

    So why is my boiler leaking?

    First we need to establish when this problem started. 

    If the boiler has been installed for a while and there were no problems before, 

    then the chances of a leak on the pipework are very slim.

    When we attend a job where the customer states my boiler is leaking please help, 

    we carry out a few checks before checking over the boiler itself.


    First we ask if anything has changed before the problem started, has a radiator been removed for decorating and then fitted back. 

    We then check all of the radiators and valves within the property for signs of leakage. 

    If no signs of leakage are present we then move on to the boiler. 

    Now, we need to know when the boiler was last serviced, this is very important. 

    Most of the time we find that the boiler has not been serviced for a while, or it may have been serviced but not correctly.

    The first thing we do is isolate the boiler from the rest of the system and connect a hose to the boiler drain point. 

    Once that is all set up, we now check the expansion vessel. 

    The vessel can be located within the boiler or external on a pipe close to the boiler.

    The expansion vessel is a very important working part of the boiler. 

    If it is not working properly then the boiler will shutdown and 

    show a fault code and the pressure will most of the time be at zero.

    Most of the time when we attend a call re leaking boiler, the customer tells us I think my boiler is leaking. 

    90% of the time we know it’s the expansion vessel causing 

    the problem and not a leaking boiler.

    So what does the expansion vessel do?

    When you turn your boiler on to heat the hot water or your radiators, the water within the boiler and system starts to expand, this is a normal process. 

    The expansion of the water has to go somewhere, this is 

    where the expansion vessel starts to work. 

    The expansion vessel is like a sponge, 

    it takes up the expansion of the water.

    When the expansion vessel is fitted to the boiler or the system, 

    it is pumped up to a certain pressure, overtime this pressure will drop out. 

    On most combination boilers the vessel is fitted within and set at the factory.


    The expansion vessel should be set according to the whole system.

    Should the expansion vessel loose its pressure then the water expansion 

    has nowhere to go other than to the safety relief valve, where the pressure will drop down to zero.

    When an annual boiler service is carried out the 

    expansion vessel seems to get overlooked for some reason, but it needs to be check without fail. 

    So when we are asked to help my boiler is leaking, 

    we know most of the time that it’s not the boiler but the expansion vessel.

    So now when you have your boiler serviced ask the 

    service engineer if they have checked the expansion vessel and reset it, if needed