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  • The Boiler Pressure


    We get called out a lot these days with issues with boiler pressure,

    cause many central heating systems are now pressurised and should the pressure drop then the boiler stops working. 

    I have been to many jobs where the client tells me they must have a leak somewhere because the boiler pressure keeps dropping. 

    When I ask why they say they must have a leak, most tell me that they have another plumber round to check the 

    boiler pressure and they are told that they must have a leak under the floor somewhere. 

    Then I am called in to try and resolve the issue. 

    Asking the client a few questions on why the first plumber could not resolve the problem,

    they normally tell me that the plumber was not Gas Safe Registeredor were too busy.

    With the boiler pressure problem, it’s normally a simple fix which takes about half an hour to resolve. 

    What normally happens is the client wakes up in the morning and find the boiler is not working dure to lock out,

    they also find the boiler pressure on zero. 

    They then refill the heating system to 1 bar and push the reset button then the boiler fires up,

    but as the boiler heats up the 

    pressure starts to rise to around 3 to 3.5 bar, if left running all day then the boiler locks out, 

    as the system cools down then the pressure drops to zero.

    So what causes this to happen?

    The cause normally 9 out of 10 times is the boilers expansion vessel.

    Inside the expansion vessel is a big thick piece of rubber, 

    this vessel is pressurised and you normally set it up the size of the heating system,

    should the vessel loose pressure then the pressure 

    relief valve cut in and lets the pressure out, it can be only a very small amount of 

    water that you probably would not even notice it. 

    Once the expansion vessel is pressurised and reset then all is back to normal.

    So why does the vessel lose pressure

    Over a 12 month periodthe vessel will lose pressure this is normal, it normally takes 2-3 years for the vessel to lose all

    its pressure, this is when the boiler problems start. 

    The vessel works by taking up the expansion from the hot water within the boiler, 

    so it is always working, it’s a little like a balloon. 

    As the boiler heats up then the expansion of the hot water has to go somewhere, this is where the expansion vessel does its work. 

    As the boiler cools down then the vessel is not under pressure.

    My boiler has just been serviced why was this not checked is the next question

    What I have found over the years is that not many boiler installers and service engineers understand the expansion vessel,

    they know that if the boiler is pressurised then it will need an expansion vessel either internal or external.

    I have been to many boilers where the vessel is full of water

    The vessel should not have any water in it, 

    once it enters the vessel then it starts to rust inside, once this happens then its a new vessel. 

    The vessel should always be checked on the service. This boiler pressure problem keeps us busy here at Duval Heating.

    The right size vessel for the heating system

    I have seen many combi boilers with over 20 radiators fitted to the system, with no extra vessel fitted.

    The average combi can cope with 10-12 normal size radiators with the internal expansion, 

    if you go above 12 then you will need an extra vessel fitted to the system, that’s not to say it won’t work it will,

    but you will have a problem later with boiler pressure dropping. 

    Also with the correct size vessel, you will find that all radiators heat up at the same time, as long as the system has been set up correctly.

    If you are having problems with your boiler pressure and would like us to contact you,

    then enter your details below and we will arrange a time to visit your boiler.