Advice for replacing boiler

Discussion in 'Plumbers' Talk' started by Mike2021, Sep 30, 2021.

  1. Mike2021

    Mike2021 New Member


    Short of getting someone in (at this stage), figured I'd see if anyone here can help so I at least know what to look for.
    I'm in a flat with an old gloworm micron 40ff, which according to online resources is a "conventional boiler", I also have an unvented cylinder.
    I'm a bit confused here because it's my understanding that an unvented cylinder gets it higher pressure from the cold supply coming in, but I'm also reading that conventional boilers need a cold water storage tank to feed them.
    So, is my unvented water supply at the pressure of the cold water tank feeding into the boiler? Surely the boiler would need to keep up with that pressure too which doesn't sound like something to expect from any type of boiler? am I totally misunderstanding how an unvented cylinder works?

    I'd like to put a nest thermostat in so trying to figure out what I should replace the boiler with to make sure everything will all work together.
  2. pppmacca43

    pppmacca43 Super Member

    You are correct the unvented cylinder will be at mains pressure. The boiler is a completely seperate system and can either be tank fed or a sealed system that is filled up with mains water to a certain pressure.
    Mike2021 likes this.
  3. Mike2021

    Mike2021 New Member

    So mains water is coming in to the tank to give it pressure but the boiler is also putting hot water in? How does the mains water coming in not just fill it with cold water seeing as that will be at a higher pressure?

    Sorry if these are silly questions
  4. terrymac

    terrymac Screwfix Select

    The cylinder takes in mains pressure cold water ,and that water is heated by the boiler by sending central heating water through a coil that's inside the cylinder. as hot water from the boiler passed through the coil ,it heats the cold water around it ( bit like an electric kettles heating element heats the water in the kettle) the electricity doesn't mix with the water ,and your boilers water doesn't mix with your domestic hot water.
    The boiler and central heating / radiators water is supplied from the loft tank ( which in turn fills from the cold mains when necessary ,by means of a ball cock ,rather like a toilet cistern fills itself when necessary). But can also be a sealed system ,with no loft tank.
    Hope this helps !
    Last edited: Sep 30, 2021
  5. Mike2021

    Mike2021 New Member

    Ah thank you, yes that clears it up now!
    Given that setup, do you know whether I should be going for another (but more modern) conventional boiler to replace this old non condensing one?
  6. exbg

    exbg Active Member

    "Conventional" can refer to it being tank fed, or non condensing (Or both) . While I am not particularly conversant with your model, I am fairly sure that it is not condensing, so in that context is "conventional". it IS suitable for a sealed system OR a tank fed system. I am guessing that you have no individual loft? In which case it is more likely that the heating system is "sealed".

    I have not seen this all the way through, but may help:
    Mike2021 likes this.
  7. terrymac

    terrymac Screwfix Select

    Just replace like for like ,the updated ,more efficient version of what you have now.
    Mike2021 likes this.
  8. Mike2021

    Mike2021 New Member

    Thanks everyone, that's cleared up my confusion!

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