Boiler pressure keeps increasing!

Discussion in 'Plumbers' Talk' started by diy_master, Apr 30, 2018.

  1. diy_master

    diy_master Member

    We have just added a new radiator and now our a combi boiler (Gloworm 30cx) pressure keeps increasing from 1.0 bar to 2.5 bar when the heating is on and there is a leak of dripping water coming from under the casing on the front right hand side when the pressure reaches over 2.5 bar.

    When cold the pressure drops down to 1.5 bar, so we have been bleeding the radiators everyday to get it back down to 1.0 bar again.

    Could the expansion vessel just need re-pressurising? Could the leaking water be coming from the auto-air vent?

    Thanks for any advice.
  2. dcox

    dcox Active Member

    If the pressure is only going up when the boiler is fired up I'd be looking at the expansion vessel first. Do you know how to check it?

    If you feel confident, take off the cover to help see where the leak is coming from. Could be lots of places including the auto air vent, the expansion vessel itself or, if the pressure is getting high, the PRV and its pipework.
    diy_master likes this.
  3. Mike83

    Mike83 Screwfix Select

    Vessel needs charged.
    The water may be coming from the outlet of the prv.
    diy_master likes this.
  4. Mike83

    Mike83 Screwfix Select

    If the boiler is wet inside then there is a leak.
    If it's only wet outside the case then it's due to the prv lifting and the pipe not being tight enough or the washer missing. 16E9827C-DE71-4902-A5F5-47E2D06EBBFB.jpeg
    diy_master likes this.
  5. diy_master

    diy_master Member

    Been looking online, it says to use a pump with pressure gauge to check the expansion vessel pressure is at 0.8 bar.
  6. diy_master

    diy_master Member

    The water is water dripping from under the thermostat casing.

  7. dcox

    dcox Active Member

    Yes, but you need to let some water out of your system first until your boiler gauge reads zero. Then undo the plastic cap on the expansion vessel and put the Schrader bike pump on and check the pressure. If water comes out of the vessel valve then it will need to be replaced.
    diy_master likes this.
  8. diy_master

    diy_master Member

    Even if just a little bit of water comes out, does it definitely need to be replaced?

    Could it be the expansion vessel can't handle the extra radiator? As I've been reading about people adding an extra external expansion vessel once they add a few extra radiators in their extension.
  9. dcox

    dcox Active Member

    If any water comes out it means the vessel has failed.

    I'd be very surprised if the standard vessel size wasn't up to coping with your system, unless you have a very large property. How many radiators do you have?
  10. rogerk101

    rogerk101 Screwfix Select

    It could be that your pressure is rising because your filling loop shut-offs are leaking.

    I've have so many filling loops fail on my oil fired Grant Vortex combi boiler that I eventually moved it and the expansion vessel out of the boiler housing completely. The 'hot side of the filling loop was way too hot to touch (being connected directly into the heat exchanger), but now it only ever gets to a max of 50 degrees, because that's the max temperature I let my CH water get to. I can't say how much longer they'll last yet, but I'm sure I'l get many years out of them, which is a whole lot better than having to change them every year. Grant really messed up on that design! Even the double check valves were failing because of overheating.
  11. diy_master

    diy_master Member

    1 small, 3 medium, 4 large. So 8 radiators in total , plus a new double radiator 60cm x 50cm.

    The manual states the expansion vessel pressure should be only 0.5 bar. It seems low, as I thought it should be 0.2 bars below the recommended boiler pressure of 1.0 bar, so 0.8 bar.

  12. diy_master

    diy_master Member

    The pressure only rises only when the heating is on. Both sides of the PRV filling loop are shut closed, so does both sides of the filling loop have to fail for the water to pass through?
  13. Sammi

    Sammi Member

    Back to basics, Master.

    You added a rad? It is very unlikely that this is the cause of your problems. Ie - unlikely that this rad has increased your system size to the extent the EV is no longer coping. So I wouldn't worry about that.

    When you filled this new rad, you used the filling loop/top-up tap to do this?

    Ok, can you explain in detail what happens with the pressure? It starts at a cold 1 bar and this shoots up to 2.5bar when the heating is on and then you notice a leak from under the casing? Ok, what then? You say you bleed a rad until the pressure falls from 2.5bar to around 1.5bar, and it stays there whilst the system is hot? And, when it's cooled down, it goes back to 1 bar? And you don't add any more water, but yet it shoots up to 2.5bar again next time?!

    Question - where is all this mysterious water coming from?

    You might have a failed EV, but I can't see how that is keeping this crazy cycle going UNLESS you top up the system again when cold each time? If you DON'T top up the system, then something is amiss.

    This can be two things I can think off; the first is that your filling loop/top-up tap (which is it your boiler has?) is letting by. But this would surely see the pressure rising constantly, even if the boiler is off - and yours doesn't do that?

    The other is a failed component inside which is allowing mains water in to your system. The only part I can think of that would do this is the secondary exchanger.

    Firs things first; are you up for checking the EV? As you say, you need a tyre pressure gauge and a foot or hand pump. Shut off the power and remove the front cover (often just 2 screws under the bottom front edge at each end). Locate the Shrader valve. Take a quick pressure reading as it is, just to see if there is any pressure in it. If any water comes out, it's shot.

    Ok, assuming no water comes out, to repressurise you need to bleed a rad until no more water comes out. You then need to leave that bleed screw open with someone ready to catch additional water - more will come out as you repressurise. Take a reading, fit the pump and get it up to 0.5 bar - because that's what it says in your book. Yes, that seems a bit low - usually it's about 0.75bar, but do as the book says.

    Once it's up to the correct pressure, replace the cap, close the bleed screw and top up your system to, say, 1 to 1.2 bar. That's usually enough.

    Make certain that the filling loop or tap is fully off - what type do you have? If it's a 'loop', then can you disconnect it at one end?

    See what happens.
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  14. diy_master

    diy_master Member

    Firstly thank you Sammi for the detailed reply, much appreciated.


    Not exactly, when its cold it drops down to 1.5 bar max and stays there, so we need to bleed the radiators everyday to get it back down to 1.0 bar again.

    Yes, thats right the pressure only rises only when the heating is on. Both sides of the PRV filling loop are shut closed, however doesn't both sides of the filling loop have to fail for the water to pass through?

    Will try it today.
  15. Sammi

    Sammi Member

    Yes, both sides of the filling loop would need to let by. Rare, but not unheard of.

    From what you describe, you are regularly releasing water from your system via a rad, but not having to top it back up. That can surely only mean one thing - water is getting IN to your system from a higher pressure source, and that's the mains.

    I hope it's a failed filling loop because that will be a cheap fix. Why don't you disconnect one end just to be able to eliminate it from your enquiries?
    diy_master likes this.
  16. diy_master

    diy_master Member

    Ok, will try that.
    Sammi likes this.
  17. diy_master

    diy_master Member

    I read about that here:

    "If the heating system contains a lot of water (lots of radiators in the central heating system) an extra expansion vessel will be needed from the start, or you will consistently get a big rise in pressure."
  18. Sammi

    Sammi Member

    But your system is a fairly standard size, and one extra rad won't have suddenly tipped it over the edge :)

    It certainly wouldn't have caused it to go woooooooooops like yours has. As rads are added to a system towards the max capacity of the internal EV, then you'd expect to see an increase in 'hot' system pressure arrive in stages, and not as a sudden 'that's too much'.

    There is also the mysterious issue of your boiler magically producing water.
  19. diy_master

    diy_master Member

    Ok, just managed to pull off the top cover and noticed the sump was full of water with the probe inside and there was water directly under it near the flexible hose. So emptied it out, it was a murky grey colour. Turned back on the heating and bled radiators back to 1.0 bar, so lets see how it goes... Not received pumped and pressure gauge yet, so hope it works?!

    upload_2018-5-3_13-56-53.png upload_2018-5-3_13-59-32.png
  20. I must be sad

    I must be sad New Member

    You have pulled off the condense trap? That is supposed to be filled with water my friend? It keeps the products of combustion staying on the much preferred route down the flue, put that trap back on (before anyone notices the I.D situation)

    Put the heating on make sure the offending rad gets hot to eliminate a rather embarrassing hot or cold being connected to the rad (if so don't tell the Mrs)

    Have you got an integral filling loop on the boiler? Or one that's cut into the pipe work? In either case remove the link

    Turn off the valves to that rad, Then put the heating on, if it works ok and pressure doesn't rise, you know exp vessel is ok and that one of the tails on the rad is heating and other mains if you have a changeover valve in hot water mode when dormant,

    If pressure rises still you may of created a pin hole in exp vessel by decompressing on drain down, if water comes out of the air valve side this is the case.

    You may also have **** stuck in the changeover valve, run the hot water see if rads get hot at same time

    Or pin hole in flat plate will give you a pressure rise

    I doubt 1 rad will push the limits of the vessel
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