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Boiler pressure issue

Discussion in 'Plumbing and Central Heating' started by GrumpyBadger, 2 Nov 2020.

  1. GrumpyBadger

    GrumpyBadger

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    Hi,

    I've got a Main combi eco 25. When CH is running a lot, the pressure reaches 3 bar and the PRV seems to kick in as I can see a slow dribble from the outside pipe. I think what happens next is when CH is off for a while, the temp drops and the pressure also drops low enough so that the boiler won't start and gives the low-pressure error code.

    We had a loft conversion over a year ago and I've also temporarily removed a couple of radiators for decorating and then re-hung them (and bled). I've wondered if this tinkering has got air trapped in the boiler..

    From what I've read, it could be an issue with the expansion vessel. Could it also be an issue caused by air in the boiler? I've bled all rads with no joy.

    The manual (link below) mentions an automatic air vent on the heat exchanger (at the top of the unit) and another one on the pump. We need to top up the loop at least once a day at the moment. When I do so, I loosen the screw on the top air vent, as that's accessible, and sometimes get a small escape of air but never very much. I don't get how the automatic vents are supposed to work - should you need to loosen the screw occasionally as that doesn't seem very 'automatic' :)

    Any ideas on things to try? or ways to get air out of the system? I've seen mention of an anti-aeration process in manuals of other models but the eco 25 doesn't seem to have that function.

    Any tips appreciated.

    Will

    Manual - https://baxi-public.partsarena.com/Documents/DOC_004447/DOC_004447.htm
     
    Last edited: 2 Nov 2020
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  3. footprints

    footprints

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    Sounds like classic pressure vessel failure, unless you "vented air" from a tyre type valve fitted to a red tank when refilling which would have removed the air cushion from the vessel.
     
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  4. MeldrewsMate

    MeldrewsMate

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    Often the built-in expansion vessel (built into the boiler casing) is of inadequate capacity. Adding extra volume to the system, eg extra rads in the loft, can exacerbate this.
    In your case I suspect, as above, that your built-in expansion vessel has lost much of its 'air side' charge. It may simply be a case of recharging it using a car tyre pump, or bicycle pump with pressure gauge. Equally it may not be that simple, but worth trying this as a first response.

    1. Drain the system until the pressure gauge shows 0psi; this should only be a couple of pints or so. Do not attempt to empty all water.
    2. Attach air pump to the pressure vessel (expansion vessel) air connection - it's a car type Schraeder valve, often accessible at the top-rear of the boiler.
    3. Pump up to 2 bar.
    4. Re-drain water to 0 psi as in 1 above.
    5. Repeat 2 and 3 and 4 until the air pressure is at 2 bar and the water pressure at or near zero.
    6. Reduce air pressure to the initial design pressure. This will depend on the location of your boiler relative to the highest point of the heating. If you don't know it then gauge how high the tallest radiator on the top (loft?) floor is above the boiler. Set initial pressure (in psi) to this height (in metres) x 1.2; with a minimum air pressure of 10 psi.
    7. Fill the water system to initial air pressure x 1.2 with the system water cold.
    8. Check that the external discharge pipe is not discharging.
    9 Bleed rads, and reset water pressure as in 7 above.

    When doing the above air gurgling into the heating system/boiler is not a good sign, neither is water continuing to discharge from the discharge pipe. If either of these conditions occur refer back with clear symptoms.

    9. Important: Note the cold and hot system pressures for future reference. A pressure swing of more than 1.5 bar indicates an undersized expansion volume.

    PS Auto air vents are just that - leave the dust cap slightly loose.
     
  5. GrumpyBadger

    GrumpyBadger

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    Thanks for the replies. @MeldrewsMate, if I slightly loosen the screw on the top valve, water starts coming out.
     
  6. MeldrewsMate

    MeldrewsMate

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    I suggest you close it now, then explain where or what this valve is. A photo would be good (of the valve, I've seen leaking water before!)
     
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  8. MeldrewsMate

    MeldrewsMate

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    Your original post made no mention of water coming out of the top air vent, just air sometimes.
     
  9. GrumpyBadger

    GrumpyBadger

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    The auto air vent at the top. Manual says it's the heat exchanger one.


    I hadn't mentioned it as I assumed water coming out was normal, like when you bleed a rad.
     
  10. ianmcd

    ianmcd

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    Get a proper heating engineer to pressurise your expansion vessel properly, and stop listening to idiots that havent a clue what they are talking about
     
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  11. footprints

    footprints

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    That is an air bleed if you have water coming out it is correct, the schrader valve will be on the expantion vessel it's self you have not disturbed it if the one you show is the only one you have touched. If you can get a gas safe engineer out that is your best bet. He can tell if repressurising will work or the vessel is faulty.
     
  12. GrumpyBadger

    GrumpyBadger

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    Well it's overdue a service so I'll call someone out. What's a fair price for a service these days?
     
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