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Discuss Getting last of air out of sealed system in the Central Heating Forum area at

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  1. hba

    hba New Member

    Our old sealed CH system would always get a lot of air in after it was drained down, resulting the boiler overheating and cutting out. I learned in time that the boiler inlet and outlet pipes, above the boiler and a couple of inches below the ceiling, needed to be loosened to purge the air as part of bleeding the system after draining down, and one plumber installed bleed valves in them.

    After the latest draindown, the boiler isn't overheating but some air remains in the system, enough for the circulation pump to make whooshing sounds from time to time. Not loud but a bit irritating and none of the air ever gets to a bleed valve where I could deal with it.

    The photo of the top of the boiler shows that the back bleed valve rises a little above the pipe. The front one is teed off horizontally or a little below horizontal, so the air doesn't rise into it. OK for purging large amounts of air, useless I fear for that last little bit.

    So my question is, how to get this last air out? I'm running the hot water and switching to central heating for a few minutes each day, hoping to move the air into the rads where I can get at it. But each time I switch back to hot water, after a minute or two the air whooshes through the pump again. It may be a little better, but this has lasted several weeks and two plumber's visits.

    My first thought is that I cut a hole in the ceiling to the loft space above and get a plumber to turn the horzontal tee vertical to a pipe into the loft, with a bleed valve, auto or manual, on the end.

    Do people think that's likely to be the source of the problem? If so, can a plumber do what I ask, in terms of being practical and within regs?

    Or do you have any clever tricks for this sort of problem?

    Many thanks in advance.
  2. Best

    Best Trusted Plumber

    Firstly the bleed points could have been installed better. The brass elbow closest to camera could have been a brass tee with a bleed point directly into the vertical part.
    Air is usually just in small bubbles and will keep moving through system pipework. Some will catch in a radiator - like where the water spreads and slows, such as a towel rail (ladder type).
    There is a few devices designed to catch the air. A Flamco air eliminator is a clever auto air bleed, but probably too bulky above your boiler.
    If you can get an auto vent on the flow pipe above your boiler (might need hole in ceiling though) - then it will automatically take some, but not all air out. Manual vents, like you have a present, are fine, but won’t bleed 24/7 obviously.
    If your system had have been piped carefully when installed, then air would have easily moved into radiator(s) after a few days.
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    Last edited: Jun 4, 2018
  3. hba

    hba New Member

    Thanks Best, glad to have a professional confirm I'm thinking along the right lines. The last plumber can put a vent on a vertical pipe through the ceiling but suggests a tee above the circulation pump might work and be easier/cheaper. But he'll need to see it again first.
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