Posting a message to the forum will remove the above advertisement
  1. Checkout our Plumbers Insurance area - heavily discounted Public Liability Insurance and Van Insurance specifically designed for plumbers.

Discuss Do I need an additional expansion vessel in the Plumbing Forum area at

  1. Trevor C

    Trevor C New Member

    Before I call a plumber ...

    My boiler (Gloworm 18sxi) slowly loses pressure and I'm having to regularly top it up. There are no obvious leaks anywhere and no discharge from the PRV as far as I can tell.

    Just reading through the manual for the boiler, it says that it has an internal 8 lire expansion vessel and a maximum system content of 130 litres and there is a note saying "For larger systems use an additional expansion vessel". It provides a schematic on where to place the additional vessel.

    I have a 4 bedroom house with 15 radiators, five of which are doubles. I've read that a rule of thumb is 10 litres per single radiator, so that would make my system something closer to 200 litres.

    1. Do I need to get a plumber in to add an additional expansion vessel?
    2. Is this likely to have anything to do with the loss of pressure issue?

  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Normally 1l of expansion per rad so a 15l one is needed but that wouldn't cause your loss of pressure are there any valves under the boiler (on the 22mm pipes) ?
    • Like Like x 1
    • Agree Agree x 1
  3. Darren Jackson

    Darren Jackson Plumber GSR

    1, Yes you could do with an additional expansion vessel.

    2, No it would not normally cause the system to loose pressure.

    Is there a constant drip or running water coming from the condensate pipe? I've had a few glowworm SXi heat exchangers fail and one of the symptoms is loss of pressure.

    How often is a top up of pressure required?
    • Like Like x 1
    • Agree Agree x 1
  4. Trevor C

    Trevor C New Member

    This is what is under the boiler if it helps .

  5. Trevor C

    Trevor C New Member

    Unfortunately I cant tell if there is a constant drip or running water from the condensate pipe because it goes through the wall and is joined directly into a guttering downpipe.

    I'm having to top it up every 4-5 days at the moment.
  6. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Turn the boiler off

    Turn the red valves off left and right

    And leave it for a day

    If the pressure drops it's inside the boiler if it doesn't it's on your system
    • Like Like x 3
Similar Threads - need additional expansion Forum Date
Advice needed... Gas Safe Registered Plumbers Only Jun 11, 2018
price of petrol up n up? Do you need to put hourly rate up? Plumbers Cafe Jun 5, 2018