Central Heating expansion vessel faulty.

Discussion in 'Plumbers' Talk' started by SurreyCumbrian, Jan 19, 2023.

  1. SurreyCumbrian

    SurreyCumbrian New Member

    I had my plumber round recently to do an annual service on my boiler. He checked the grey central heating expansion vessel pressure (photo), and said it was too low and so raised it. I think he said he set it to 3 bar. My system is non-vented /sealed. I have a megaflo for hot water. Ours is a big 25+ radiator system, also with a separate NuHeat wet UFH.

    Since the plumber left the c/h pressure now fluctuates a lot more than it ever did and I am losing pressure to almost zero in about 2 days. I am reasonably confident that there are no leaks on the system and I have bled the rads at various points in the house.

    I phoned the company back and I spoke to the engineer who made the change. He said it sounds like the expansion vessel has developed a fault/leak. It’s only about 6 years old - as prior to this the system was open vented with a header tank. Wished to get an opinion here. Is the pressure correct? Are the symptoms likely of the expansion vessel now having an issue (leaking bladder/ diaphragm?)? I think if he hadn’t have fiddled with it, things would have been ok. I had though observed a very slight pressure drop over time previously (2 weeks to reduce from 1.5 bar to below 0.75 bar say.) Which is partly why he checked it.

    Grateful for some experienced heads advice here. Thank you.


    Attached Files:

  2. Mike83

    Mike83 Screwfix Select

    Vessel should be at 1 bar not 3.
    If it’s set at 3 it simply won’t work.
    It would cause the pressure loss.
  3. SurreyCumbrian

    SurreyCumbrian New Member

    Many thanks - I will check the pressure myself and discuss with the engineer. Maybe he did set it lower. He also checked the megaflo (hot water) expansion tank and adjusted this pressure. What should this one be at?

    For the central heating expansion tank - if its above 1 bar, will it cause an issue?
  4. quasar9

    quasar9 Screwfix Select

    An empty EV (water drained from system) should be charged with air between 0.75 and 1 bar. When filled with water the system pressure should be around 1.5 bar. When operating and hot this will rise but any thing approaching 3 is bad. After this point the pressure relief valve can open dumping the excess pressure. Most only go up a max of .25 bar.
  5. SurreyCumbrian

    SurreyCumbrian New Member

    Have set the pressure of the vessel at approx 0.8bar. It was at 2.1 bar, so the plumber set it too high. The system pressure is set at 1.5bar and much more stable now. I bled some of the rads but no air in the system. Will monitor going forward.

    Attached Files:

  6. SurreyCumbrian

    SurreyCumbrian New Member

    Still getting a slight drop in 48 hours. Have bled some more air out and retopped up to 1.5bar.

    is it common for these expansion vessels to develop a leak? It’s only 5-6 years old.

    Can’t see any obvious water leaks anywhere on my system so fairly sure it’s pressure tight.
  7. quasar9

    quasar9 Screwfix Select

    EV - they last a lot longer than 5/6 years.

    your problem may be that before you reduced the excess pressure in the EV, the pressure relief valve in the boiler was lifting, binging the pressure rapidly to zero. These PRV rarely seat back completely once open and may be weeping.

    tie a plastic bag to the pipe outside the house where the PRV dumps the water and see if it collects any over a day or two.

    if it does, replacement of the PRV is the only answer.
  8. SurreyCumbrian

    SurreyCumbrian New Member

    Thanks… I can see the PRV drain/tundish and will check there. All of the boiler drains into the master pipe that takes the kitchen /utility outlet to the foul drain
  9. SurreyCumbrian

    SurreyCumbrian New Member

    Had a look at the tundish. It’s dry. So PRV seems fine. Will continue to monitor.
  10. diymostthings

    diymostthings Well-Known Member

    If the internal membrane is leaking, water will come out of the Schrader valve when the centre pin is depressed. If the membrane is intact, air will come out. It is important to note that when setting the air pressure in the expansion vessel, the 1 bar refers to the COLD condition.

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