Worcester Bosch Boiler Pressure Problems

Discussion in 'Plumbers' Talk' started by jsaipe, Dec 7, 2013.

  1. jsaipe

    jsaipe New Member

    I have a Worcester Bosch Greenstar 24i boiler with strange pressure fluctuations.

    For the last couple of days I noticed that the pressure was reading zero in the morning when cold. I also noticed a slow drip coming out of the drain valve thanks to it only being hand tight. So, I assumed that the pressure had dropped due to the water leak (albeit small). After tightening the drain valve I re-pressurised the system (when cold) back to 1.5.

    However, when the boiler is on, the pressure goes up to 3 bar and into the red. I've bled the nearest radiator to reduce the pressure back to 1.5 when the boiler is on (not sure if I should do that while the boiler is running?). And then it sits happily around 1.5-2 bar.

    However, when the boiler has been off for a while, the pressure is near enough back to zero after cooling right down which has forced me to re-pressurise to get the pressure back up to 1.

    I've been through this process a few times of re-pressurising when cold but then having to reduce the pressure when hot.

    Normally the boiler pressure is 1 and goes up to 1.5 when warm.

    I'm not especially technical, but from what I've read could it be my expansion vessel? Or could there be trapped air causing this? (FYI I've bled most of the radiators and none at the time had air in them).

    Any help would be appreciated.
  2. Devil's Advocate

    Devil's Advocate Well-Known Member

    Hi jsaipe.

    It's almost certainly what you suspect - a problem with the expansion vessel.

    It could also, so I've read, be due to a blocked pipe going to the E-V, but this is far less likely.

    If you are lucky, it's just a case of the E-V having lost air pressure (very common - it'll happen to most of us at some point), but it could also be due to the E-V having a ruptured diaphragm - so it'll need replacing (but not so common.)

    Another possible cause is the Safety Release Valve allowing system water to escape - which yours now clearly is (the dripping pipe.) That almost always does happen following a fault with the E-V, and it becomes an added issue. However, it's rarely the case that the SRV caused the problem - what usually happens is that the pressure fault triggers the SRV to open, and it often doesn't shut properly afterwards... :(

    Your system - with a 'combi' boiler - is 'sealed' and pressurised. When water is heated up, it expands. Your system, being sealed, has to have somewhere for this excess water to go temporarily, and be able to return that water when your system cools down again. This way, the system pressure should remain fairly constant at all times, at an ideal around-one-bar-ish.

    And the thingy that maintains this pressure is the E-V. Inside this tank (usually flattish, sometimes cylindrical - often coloured a dark matt red) is a rubber diaphragm with the system water on one side and air on the other. The air is pressurised usually via a normal 'car-tyre' Shrader valve.

    This E-V allows expanding water to flow in and this is pressed against the rubber diaphragm and air pressure - the result is the system pressure remains pretty constant. When your system cools, this water is pushed back into the system by the air pressure - ditto, pressure remains constant.

    When the E-V becomes faulty, the expanding water now has no-where to go. The result is that the system pressure shoots up as the water heats up. Once it reaches around 3 bar, the SRV opens and dumps this excess water out a discharge pipe and outside your house (via a 15mm copper pipe). The system pressure then typically remains high at over 2 bar while it's running. Once your system cools down, tho', there's now no extra water to keep it topped up as the water contracts - so the pressure falls to near zero. So what do we do? Yep - we top it back up and switch it on - HOT - EXPANDING WATER - INCREASING PRESSURE - OPENS VALVE - DUMP!

    A viscous circle. (sic)


    Ok, to test this vessel, locate the darned thing and the wee valve on top. It'll often be positioned to make access as awkward as possible, and some knuckle-shredding will ensue. Undo the safety cap and press the pin in the middle down (as if releasing the air from your neighbour's car tyre.) Does any water come out? Does it go PSSSSSS-FRRSSSTTTPSSTPPRRRP suggesting there may be some water there, or just PSSSSSSSSSSSSSSSSS with only air?

    If only air, that's good. If any water then your E-V is shot and needs replacing.

    You could then take a pressure reading from this valve, but it won't necessarily point to the problem as the water pressure side will be affecting the air reading (you won't know if the reading is driven by the air or the system 'water' pressure.)


    So, what you need to do is 'set up' the E-V air pressure.

    To 'set' the E-V up, the water side should be first drained to zero pressure. You can do this by draining water out the system drain cock (it should have one), or by 'bleeding' a radiator (it could take some time), or by manually opening the Safety Release Valve inside the boiler (which is not recommended as they often don't seal properly afterwards). Once the system's water pressure gauge is at complete zero (nothing comes out a rad bleed screw), you take a reading from the air valve on the E-V using a normal tyre-pressure gauge, and then pump it up (even a bicycle pump will do) to typically 1 bar (about 15psi).

    Finally, you top up the system 'water' pressure by opening the top-up tap which is on, or near, the boiler. Take 'er up to 1 bar - no need to go higher.

    And that's it! No cost! All is well!


    Except... Very, very likely your SRV will continue to drip, drip, drip. So your system pressure will slowly fall. So will need regular top-ups of water. Don't be tempted to over top-up - stop at, ooh, 1.2b max.

    Keep an eye on the safety discharge pipe outside - tie a bag around the end or something to monitor the discharge. If it does keep dripping, it'll almost certainly need replacing. Typically around - ooh - £80+ part and labour?



    (A temporary 'fix' for the whole E-V problem can be made by turning one of your rads into an expansion vessel! Some plumbers on here have suggested you could drain off half a rad's water content - you'll need to close both rad valves and slacken the nut between a valve body and the radiator - with a pan underneath. Open a bleed valve to allow air in. Stop when you reckon you've drained a fair amount, and re-tighten and reopen valves. The air inside will now compress to accommodate expanding water. This is a temporary fix only.)
    Last edited: Dec 7, 2013
  3. jsaipe

    jsaipe New Member

    Wow what an epic response - war and peace for plumbers. Thanks so much!

    I'm not confident enough to tackle this on my own, but the info above will at least give me half a brain to discuss the problem with Worcester Bosch. Again many thanks! :)
  4. Devil's Advocate

    Devil's Advocate Well-Known Member

    You mean I wrote all that for now't... :p.

    Ok, tell me where you live and I'll come and do it for you.

    Akshully, I wrote out all that detail so as to make it available for anyone who asks that same question - and it's a common one.
  5. plane-it

    plane-it Member

    Wow...brilliant post there DA!
  6. Devil's Advocate

    Devil's Advocate Well-Known Member

    Gee thanks. But I'm not one to be held hostage to flattery. Not at all.

    What bits did you like best?
  7. joinerjohn1

    joinerjohn1 Screwfix Select

    I liked the full stop right at the end DA. That one full stop, is your greatest literary achievement to date. Worthy of the Man Booker prize. ;);););)
  8. Devil's Advocate

    Devil's Advocate Well-Known Member

    I thought so too. It's soooo well-rounded. I didn't just throw that dot in, you know. It took care, consideration, timing...
  9. unique

    unique New Member

    Thanks DA just thought I would let you know you didn't write all that for nought, I will be giving it a go just need to pick up a pressure gauge. BTW that is by far the best post I have seen anywhere we'll looking for some answers.

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