Loud knocking noise - Electric Immersion unvented boiler

Discussion in 'Plumbers' Talk' started by Ziv, Sep 22, 2021.

  1. Ziv

    Ziv New Member

    I need help/advice about my boiler, please.

    it’s an electric immersion unvented boiler (big 180L tank) - Tribune make

    A month ago I started hearing a loud knocking noise at night, which I though was originating from neighbours, and only in recent days I realised that it’s my own boiler.

    The knocking lasts for 5 hours (3:30am to 08:30am, most likely starts when the water starts being heated up.although though the boiler should be programmed to heat water for only 2 hours, from around 3-5am.

    Having googled the reasons for this knocking sound, the possible causes could be faulty thermostat, or a buildup. One other website mentioned that this knocking noise could indicate that the boiler is about to explode which could be fatal to those in the flat.

    I bought the flat 4 years ago, the boiler was installed when the building was built, so about 15years ago.

    I very badly want the knocking sound to go away, and the boiler fixed, because I have not been able to sleep past 3am.

    My questions:
    1. The boiler is 15 years old. Even if it gets fixed, how long do such boilers usually “live”? Should I replace it?
    2. Is it definitely about to explode, how urgent this issue is?
    3. I found that most companies seek Combi boilers, which I know nothing about. They seem economic but I’m unsure whether it might heat enough water for my needs. Should I replace the boiler with the exactly the same type (tank/immersion/unvented), or can any boiler be installed? Which boilers would be compatible? What should I be looking for? Which make/company would be advisable? What sort of cost am I looking at - installation and boiler price? Electricity usage? I use up all of the hot water on most days when I’m home, I like to have baths occasionally.

    I would really appreciate any sort of help/advise/recommendations.

  2. andy48

    andy48 Screwfix Select

    1. Firstly, it is a direct, unvented, hot water cylinder, not a boiler.
    2. They are meant to be serviced annually by an engineer with a "G3" qualification.
    3. Provided it was properly installed in the first place, it will have four "layers" of protection:
    3a. A basic thermostat which sets the water temperature. Once the preset temperature is reached, it turns off the electrical supply. The water cools over time, and the thermostat automatically reconnect the electricity.
    3b. An overheat thermostat which disconnects the heating element if the water temperature exceeds a set value, normally around 85 degrees. Manual intervention is required to reset this thermostat.
    3c. An expansion valve to relieve excess pressure of the water in the system.
    3d. A temperature and pressure relief valve which operates if the pressure becomes excessive or the temperature becomes excessive. This valve opens and lets excess water into a drain off pipe.
    4. There should be what is called a tun dish through which water from either expansion valve can flow. The tun dish has a small diameter pipe (usually 15 mm) going into the top, and a larger diameter pipe coming out of the bottom. Between the two pipes there is a gap through which you would be able to see water running.
    5. When the cylinder is making the noise you describe, locate and observe the tun dish. If there is water going through it then there is a problem requiring an engineer's visit:
    5a. If the water going through is cold, it is likely to be a problem with the expansion valve. This is normally part of a larger valve called the combination valve which also contains a filter and a pressure reducing mechanism.
    5b. If the water is hot, then it is likely to be coming from the temperature and pressure relief valve. In this case turn the system off at the mains switch, and do not use it until the problem is resolved.
    6. It should not be necessary to replace the entire cylinder. They should have a working life of at least 25 years, or at least they should if maintained properly.
    7. Do you have a gas supply to the property. If not, I'd suggest you will find it virtually impossible to find any form of water and space heating better than that which you already have.
    8. There are many reasons why your cylinder is making this noise. If you live in a hard water area it is possible that limescale has built up on the bottom of the cylinder to a level where it is touching the immersion heater (the thing which actually heats the water) set into the side. If so, the limescale doesn't conduct heat as well as water, leading to local overheating of the element and bubbling of the water when it finds the hot spots. The immersion can be removed (although often difficult to achieve) and the limescale removed.
    9. There is very little risk of an explosion, but please take note that:
    9a. Your system clearly needs a service and some repairs.
    9b. If it is leaking hot water, stop using it until it is fixed.
    Ziv likes this.
  3. Ziv

    Ziv New Member

    Thank you so much, Andy, for such a detailed reply!!

    I do, indeed, live in a hard water area.
    I did have an issue of the water running from a pipe into a tun dish last year and the year before. The plumber changed thermostat because thats what he guessed was faulty. There was no mention ever of servicing needed, or that lime scale buildup is potentially causing that water leak.
    I now only have the knocking noise and haven’t observed any water leaking this time.

    Before the plumber visit to fix the water leaking into the tun dish, I did speak to another plumber who was a G3 engineer I now remember - he did correctly identify, that I had an unvented cylinder, he said over the phone, however without a visit to inspect, he was convinced that I should change my whole boiler - I need to buy a brand new one which he could do for me, as my boiler would now be old, having been installed 13-14 years ago at the time.

    Andy, would you know any competent G3 engineers that I could contact to get my boiler serviced and fixed, please?

    I posted a job advert on another website, and I also called the plumber who came previously to fix the water leaking into the tun dish, however the plumber I used earlier isn’t sure why I’d hear the knocking sound from my boiler.

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