Intermittent pressure drop on combi boiler

Discussion in 'Plumbers' Talk' started by raac, Jun 1, 2022.

  1. raac

    raac New Member

    Suggestions for approaching this problem, please:

    House renovation has a new central heating install and new Vaillant Combi boiler: five rads. On two occasions now, however, there have been seemingly sudden pressure drops in the central heating system down to 0.6 bar. These have both happened recently, during warmer months, when the central heating was off so the only indication that it happened was when hot water stopped being produced. Events were 6 weeks apart.

    There are no visible leaks anywhere on walls or ceilings. The floorboards on the ground floor can easily be lifted: visual inspection on joints is good. The main pipes going vertically through the building don't have any evidence of leak at the ground floor. The joints (including elbows under the floors) of three of the rads could easily be inspected and all looked good.

    Any ideas how to proceed?
     
  2. raac

    raac New Member

    Also relevant: PRV and condense are both piped away using a suitable condense pump. So
     
  3. Mike83

    Mike83 Screwfix Select

    Could the drain point be leaking?
    When the pressure drops to 0.6bar are you topping it back up.
    The pressure sensor can give an inaccurate/ erratic reading if it starts to get blocked. Doubt it’s that though.
    You could also try closing the isolation valves then check if the pressure holds or still drops. This is a good time for the test especially when no heating is required.
     
  4. raac

    raac New Member

    The system drain point? I don't think so but there might possibly be and I will double check. Technically there is no drain point yet as we will add one when the final rad goes in next to the front door. However, we do have a slightly dodgy temporary one that I should inspect more closely. Last time I looked at it, though, I saw no obvious sign of leakage.

    Yes, I top back up to 0.6 bar. Last time it ran for 6 weeks after I topped up to 1.1 bar.

    So you are suggesting I pressurise then isolate the heating system for a few weeks and see if during normal operation with the valves closed the problem comes back? Or are you saying that after, say, 8 weeks I reopen the valves and see I get a sudden pressure drop?
     
  5. Mike83

    Mike83 Screwfix Select

    Pressurise to 1.5-2 bar. This slightly higher pressure won’t cause any issues.
    Then close the flow and return isolation valves. Isolate for say 6 weeks and check pressure and compare with what it was set at.
    Then open the valves and see if it drops.
    If it’s a new system though the installer should be doing all this to find the leak.
     
  6. raac

    raac New Member

    He didn't suggest that. I'll give it a shot, though. Easy enough and it makes sense.
     

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