Condensing boiler with zero pressure

Discussion in 'Plumbers' Talk' started by Sartois, Apr 1, 2017.

  1. Sartois

    Sartois New Member

    Hi all,

    My first post here, so my apologies if I'm asking stupid questions...basically I'm interested in buying a house, but the survey raised an issue that the boiler (a Worcester Greenstar 24i) had zero pressure.

    Now, the owner of the house is actually a retired plumber and installed the system himself - it used to have a cold water tank at the top of the house, but now has a large pressurized cylinder (in addition to the condensing boiler), approximately four feet high in the same room as the boiler.

    Firstly, I'm not exactly sure what this cylinder does but I'm assuming it holds hot water? Apparently it also has an immersion heater, but from what the guy said (over the phone) that doesn't appear to be its main function.

    Secondly, he said that water does tend to 'leak' out of the system (for lack of a better layperson word) and it has to be topped up. I'm fine with that, having had to do that myself on my boiler, but I wouldn't expect the boiler to be at zero pressure at any point?

    He's completed a full Landlord/Home Owner Gas Safety Record, with serial number, etc. but I'm still a bit confused by the setup of the system and why the Greenstar would be at zero. Does it sound ok to you professionals? :)



    p.s. Some additional information from the completed record: the flue type is RS, the operating pressure was 18Mb, and the combustion analyser reading was 9.8.
  2. Hi Sartois.

    You have a 'system' boiler on an unvented system. That tank is a mains-pressurised hot water cylinder which gets heated up by the boiler and then delivers this hot water to your hot taps under mains pressure - so you should hopefully have a really good flow from your taps?

    The PS info doesn't help - that's all to do with the boiler's gas setup and testing.

    So, I'm guessing you are referring to the system pressure as shown on the gauge on the front of the boiler? If the system leaks somewhere, then that pressure will fall. Left long enough, it'll hit 'zero'. So that's no great surprise.

    The issue is - where is it leaking from? If it's from a 15mm copper pipe which points outside the house, then it's chust a leaking PRV in the boiler - a relatively cheap and easy fix.

    However, if there's no sign of a leak, then you could potentially have a more serious issue such as a leaking pipe under yer floor.

    Any idea how quickly it loses its pressure after being topped up?
  3. Sartois

    Sartois New Member

    Hi Devil's Advocate,

    Thanks for your reply, it's much appreciated, and thanks for the clarification re. the tank. He said he fitted it 13 years ago or so, and brought it across from Norway - were those tanks new tech at that point?

    Yes, you're right - the gauge on the front of the boiler was showing as zero. And indeed, if it is zero, this does infer a leak, with subsequent questions about where it's leaking.

    I guess this is the issue which is confusing me...would it be viable to have a system where the boiler pressure would be zero when it wasn't on, and when turned on it came up to pressure? If not, then I'm concerned as to how a certificate has been issued on the property.


  4. Sartois

    Sartois New Member

    Sorry, I forgot to address your last question - he said he kept the pressure between 1.2 - 1.4 bars, but I'm not sure how often he had to top it up.
  5. I guess he topped up the system to get it to a working pressure so's he could turn on the boiler and check the flue gases etc for his tests.

    If you look outside the house, usually very close to where the boiler is sited, you should find a 15mm copper pipe terminating there, pointing downwards or back against the wall. See if that is dripping after the system has been topped up.

    It is astonishing that the guy - a plumber - hasn't sorted this issue himself! Ask him where he believes the leak to be.

    Also, as a landlord, I wonder if he is allowed to carry out the gas safety check? I dunno, but worth finding out.

    Anyhoo, if he doesn't produce decent answers, cost in repairs - I dunno, £500 (to cover worst case?)

    But it could be a lot less than this, but you don't know!

    Yes, the rest of the world had these systems long before they were popular here.

    Another point - you need a special qualification to fit these pressurised systems - does he have this?!

    Bottom line - these systems can be dangerous, and this guy sounds like a bit of a cowboy. I would strongly suggest an independent evaluation of the whole caboodle, ideally before you but. Then you can legitimately price in a suitable reduction.

    Bottom bottom line tho' is that, for most houses, a faulty CH system isn't reason enough to walk away - you just have to make sure you have the funds to fix it.
  6. kiaora

    kiaora Well-Known Member

    To check the system is working, it's just a matter of topping up the system pressure to around 1.5 bar,

    Then the boiler can't be turned on and tested ,

    There will be a filling loop somewhere to top up the pressure,

    As DA says, often the water is lost via the pressure relief valve, either its faulty, or the expansion vessel is faulty

    Not a big problem...

  7. That is about right. How quickly it looses pressure is important - it should maintain that for a year at least. Even then, it suggests a tiny leak (my system hasn't needed topping up ever. But then, it was fitted by a genius...)
  8. Sartois

    Sartois New Member

    Ok - it does sound like something is awry in the system then. I'll get someone to have a look at it and preferably be there at the time, so they can talk me through the system. :)

    Thanks again,

  9. Sartois

    Sartois New Member

    Hi all - sorry for the thread resurrection, but it's about the same system...

    Basically I bought the house and had the system tested out by the plumber I've used many times in the past. As before, it's a Worcester Greenstar 24i boiler and has a mains-pressurised hot water cylinder. This is capped out at 1.3 Bar though, which neither my plumber or I could quite fathom why the original guy would have capped it at that. In addition, the original owner (a plumber) created a system where there's two thermostats (upstairs and downstairs) and you can control each separately.

    Now, the issue which I originally posted about - the zero pressure - still exists. The odd thing is that when the boiler turns on, the pressure slowly cranks itself up to 1.3 or thereabouts, and the heating comes on. For reference, if I add water to the system, it tops out around 1.3 but when the boiler is off, it'll return to 0.

    Any thoughts? :)


  10. What do you mean by 'capped out'? What prevents it from going higher than 1.3bar? Is there a pressure release valve that opens at this point? If so, that's low.

    If a sealed system boiler like yours has zero pressure when cold which rises when heated, then that suggests an expansion vessel that's either faulty or needs recharging with air.

    Ok, a thought... You might have a 'heat only' boiler which is essentially the same as a 'system' boiler except it's used on a 'vented' system and not a 'sealed' type. BUT, many (most? all?) heat-only boilers can be adapted for a sealed system (like yours) but it will then require an expansion vessel to be fitted on the system somewhere - wherever there's easy access to a pipe.

    With an exp vessel fitted (or with an internal one as with a 'system' boiler), the pressure should be at around 1 bar (perhaps a bit more), but this should remain pretty steady when the system heats up, and not rise significantly higher. Basically, the pressure should be fairly 'stable' (and that's what the exp vessel does - it takes in the expanding water when hot, and releases it back to the sealed system as it cools = 'stable' pressure).

    So I wonder if the installing plumber was a complete git and used a 'heat-only' boiler on a sealed system, but DIDN'T add an exp vessel?

    That's just a guess. But it adds up.
  11. Sartois

    Sartois New Member

    Hi DA!

    Nice to hear from you again! :) Yes, sorry, capped out meant the pressure gauge doesn't go more than 1.3 Bar.

    Hhmmm, I just about followed your argument, heh - is there any way I could establish whether there's an expansion vessel or not?


  12. DIYDave.

    DIYDave. Well-Known Member

    If the system has been topped up on a regular basis to maintain pressure, then any inhibitor flowing around the system may be at a very low concentration - not good for a healthy boiler and pipework. Concentration can be checked with a kit

    Also regular water top ups adds fresh oxygenated water to the system, along with a low level of inhibitor, not a good mix and can cause rads to prematurely rust (amongst other issues)

    Sounds like your on the case so hopefully will be sorted :)
    Deleted member 33931 likes this.
  13. Sartois

    Sartois New Member

    This boiler is really screwing around with thought my bad sleep was due to work stress, etc. but actually it's because the boiler starts up at multiple times during the night. It's done it at 03.00 or thereabouts before, but last night it started up at 04.30 (heating up all the downstairs radiators, plus the towel rail upstairs). I went downstairs and reset the entire system, whereupon half an hour later it was doing it again...

    Going to get a Worcester specialist to come and have a look at it, but I'm considering getting a quote to rip out the entire lot (including the pressurised tank) and get a new boiler in. I reckon it's about 14 years old anyway.

    Sleep deprivation FTW! ;)

    Edit: The boiler is actually a Greenstar 12i, and the pressurised tank an OSO 20D / 20I.
    Last edited: Nov 27, 2017
  14. It's worth getting a specialist in right enough - it could all be quite simple. Possibly.

    I mean, there's a chance that the pressure is actually ok, and it's the gauge that's reading low! (the zero to 1.3bar max reading is actually 1.5 to 3 bar...)

    I don't know about that boiler, but most current models will simply not fire up if the pressure reads too low (is the pressure gauge analogue or digital, and is there one on the boiler casing itself?)

    Ok, the boiler coming on at random times is weird, but that's probably not the actual boiler's fault!

    Keep us posted - it's a mystery system :)
  15. KIAB

    KIAB Well-Known Member

    Could be 'frost setting' kicking in, 14 years old, it's had a good life.

    Look at Vaillant for new boilers, had various models over the last 25 years or so, nrver had one break down, current Exo Tec 824 boiler was poorly last year, a new heat exchanger soon problem, first new part in 9 years.:)
    Deleted member 33931 likes this.

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