Boiler pressure issue

Discussion in 'Plumbers' Talk' started by Manuel Rathmann, Apr 7, 2021.

  1. Manuel Rathmann

    Manuel Rathmann New Member


    I have a 2 year old Viessmann Vitodens 050-W gas condensing boiler and a problem with keeping the pressure in the central heating system. When I pressurize the system to 1 bar and isolate the boiler the pressure holds so I assume there isn't a problem with the boiler itself.

    When I pressurize the boiler to 1 bar and let it run normally with the heating on it takes around 1 hour to drop from 1 bar to 0. At the same time I lose around 170 ml in condensation and it takes me 30 sec. to fill the system up from 0 to 1 bar again.

    Here is where I could do with some help from people who know far more than myself:
    1) Am I on the right track thinking this clearly has nothing to do with the boiler itself and that I have a leak somewhere in the central heating system?
    2) Given it only takes an hour to drop 1 bar and I need 30 sec for a refill am I correct in thinking the leak is fairly substantial? I am asking because if this is the case then it's fairly safe to assume it is somewhere under the floorboards on the ground floor - otherwise I would have seen water marks/damage somewhere

    Thanks for your help,
  2. kiaora

    kiaora Screwfix Select

    It sounds like you do have a leak on the system, but...

    Quite often the leak can be the prv letting by, due to failed expansion vessel.

    so when the boiler is on and the water expanding, with a failed expansion vessel, the pressure relief valve will open
    Try looking at the safety pipe outside,

    good luck
  3. Manuel Rathmann

    Manuel Rathmann New Member

    Hi thanks for the response. I forgot to mention I did check the condensation/PRV outlet and I lost about 170ml in an hour. Meanwhile I've re-pressurized the cold system to 1 bar, bled all radiators and with the boiler isolated I lost 0.5 bar in 1 minute! Suspect I have a massive leak somewhere under the floor. Of course everything is either tiled or covered in laminate :(
  4. quasar9

    quasar9 Active Member

    The condensate and PRV pipe are different, the first usually plastic, the second copper ! You will get quite a lot of condensate but PRV should be dry. Tie a plastic bag to PRV pipe to check. Also, does the pressure go up when system is running ? Does it creep upwards of 3bar, as PRV open just above this point. If all good, it’s the pipe work
  5. pppmacca43

    pppmacca43 Active Member

    The condensate is a plastic pipe that is connected to a drain or a waste pipe. The prv will be a 15mm copper pipe that most of the time goes out of the wall where the boiler is sited to outside and normally terminates just as it passes through the external brickwork.
  6. pppmacca43

    pppmacca43 Active Member

    I suspect your PRV is letting by which could be caused by a faulty expansion vessel and would need a plumber to check, but to me it doesn’t sound like u have a leak on the pipe work. The reason is u said u lost 0.5 bar in a minute with the boiler isolated, so the chances are the water is being lost from the boiler not the system and that’s why it went down so quickly when the boiler was isolated.
  7. pppmacca43

    pppmacca43 Active Member

    If u take a photo of where u collected the 170mm of water and upload on here someone can advise for sure if it’s your prv
  8. Manuel Rathmann

    Manuel Rathmann New Member

    Sorry just realized I confused myself - the cold water test with the 0.5 bar drop in 1 min was indeed with the boiler connected/un-isolated but off...

    The condensation and PRV outlet come together in the same pipe as my boiler is in the garage without an outside wall nearby. The pipe then goes into a pump which drains the water into a hole in the ground...

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