Sealed System - drawing air in

Discussion in 'Plumbers' Talk' started by redtomato, Jan 30, 2012.

  1. redtomato

    redtomato New Member

    Hi all,

    I have a sealed heating system. My boiler is an Vaillant Ecotec 630 and there are 12 radiators.

    Every two days the topmost radiator becomes cold to touch (at the top) but the system pressure gauge shows no loss of pressure.  When I vent this radiator the system pressure drops by about 0.1 to 0.2 bar. I have checked all the pipework and fittings and cannot find any evidence of a leak.

    Today I spoke to a leak detection company who advised that it would be very difficult to find a leak of this size and suggested that it is more likely that the system is drawing air in rather than leaking. They advised to check all radiator valves and fittings again.

    My worry is that although this is a small leak, I am adding fresh water to the system on a regular basis which will eventually dillute the inhibitor and cause oxidation, rust and long term problems.

    I have previously added fernox leak sealer but this hasn't fixed the problem.

    Is there anything else to look at or check?

    Many thanks.
  2. Blimey... Just how much air comes out when you bleed it? How long does it go 'ffsssssssssssssssssssssss' for?

    I'm sure you're thinking 'how can a pressurised, sealed system draw in air?' I can't think of any way either. So, I'm thinking you either have some serious corrosion going on inside your rads - which gives off a gas in the process - or else your expansion vessel diaphragm is punctured - in which case it'll soon be out of air anyway and that part of your problem will stop... :)

    I'm not sure what to suggest until the pros come charging in and start arguing amongst themselves. Do you know where the exaonsion vessel is inside your boiler? There should be an air valve on the top which looks like a car tyre valve. It would be interesting to see if some water spurts out off there along with air if the valve thingy is pressed in. Mind you, if you release too much pressure then it'll need charging up again - with a bike or car pump.
  3. G Brown

    G Brown New Member

    Almost certainly the expansion vessel needs recharging or replacing.
  4. redtomato

    redtomato New Member

    Thanks for the replies.  When I bleed the radiator is goes "fsssssssssss" for about 10 seconds sometimes a bit more sometimes a bit less.

    I drained the system two weeks ago and checked the pressure in the expansion vessel, it was about 0.9 bar. This is internal to boiler.  No water was ejected when I pressed the valve on this. I closed off the drain cock and repressured the vessel with a car tyre pump and then filled the system again.

    Another thing I have tried it to tie a plastic bag around the vent pipe for the expansion vessel. I left this in place for a couple of days but there was no water in this. I think if the expansion vessel was faulty then water would have discharged through the vent pipe and into the bag.

    Any other ideas?
  5. Onetap

    Onetap New Member

      Where do they think the water is going to then? It has a finite capacity, lets say 50 litres. If it is full and you add one or two litres a week, every week, does it get bigger? Will it hold 100 litres after a year?

    There is a leak somewhere. You may be getting hydrogen generated in the system as well (search archives) but there has to be water getting out. Leaking heat exchanger? Leak under floors? PRV lifting?
  6. Mememe 2

    Mememe 2 New Member

    An air leak on the suction side of the pump will suck air in and not let water out till the pump is switched off even then it will be very little maybe just damp, it will show on your finger if you run it around the fittings.Allow the system to go cold and check for any slight damp patches around the rad valves etc.
    My guess pending no leaks, rusting rads, could be a hydrogen build up due to corrosion.
  7. Hmmmmm, this is a pressurised system. I just don't think a pump sucking away even to the best of its ability is going to draw in air to a system which is pressurised to around 1.5bar...
  8. redtomato

    redtomato New Member

    Thinking about it now, if air was being drawn in and water wasn't escaping then surely the pressure would be increasing? Similarly, if it was a hydrogen corrosion problem, then the build up of gasand no loss of water, would increase the pressure in the circuit too?

    My problem is that the pressure is not increasing at all. The pressure remains the same until I vent the radiator at the highest point in the system at which point it falls. Therefore I am inclined to agree with onetap that there is a leak and I think it is under the tiled living room/kitchen :(
  9. twotwo22

    twotwo22 Member

    You cannot draw air into a sealed system with an expansion vessel pre-loaded to 1 Bar and installed in the correction position on the inlet side of the pump unless the system has points higher than 30 feet above the attachment point of the expansion vessel.
    I would check if it is air that you are bleeding by trying to ignite it.
    Hydrogen burns hot with a clear flame, be careful.
    If you have a water leak, which you may have or course, then this would just mean that that pressure in the system decreases and needs topping up with more water, it would not mean you need to bleed a radiator, unless the leak is so bad that you lose significant pressure.
    If you can bleed 'air' from an upstairs radiator then this would indicate that there is still significant pressure in the system at that point.

  10. And the reason the pressure doesn't appear to increase even if air was being drawn in (which it ain't) or gas is being produced internally (which it might) is because your expansion vessel is designed for just the purpose of keeping the pressure relatively constant.
  11. twotwo22

    twotwo22 Member

    Time to test that 'air' and report back.
    I did this on my one radiator that always needs bleeding and expected the blast of 'air' to blow out the flame of the gas cigarette lighter.
    Contrary to my expectations, I burnt my finger on the invisible flame emitting from the quickly melting plastic bleed vent.
    My system is well dosed with inhibitor so should have no corrosion and the water is in hand.

  12. What caused the corrosion, then 2x4?
  13. twotwo22

    twotwo22 Member

    You mean on my system ? I don't know.
    I am planning to replace an old bathroom radiator then run for a few weeks with a flusher in there, and then clear it out out and re-dose.
    Just haven't had time since being made redundant

  14. redtomato

    redtomato New Member

    This evening I bled the top most radiator, not much air in it. This time, as suggested, I tried introducing a lit lighter to the gas being expelled. It blew out the flame. I tried again but reduced the force of the expelled air by only slightly opening the bled valve but the flame would not ignite. My conclusion is that if it did contain hydrogen then it was not in a sufficient enough quantity to ignite the flame, therefore it is unlikely that the air being introduced to the system is from corrosion. Worringly, it appears to be from a leak. :(
  15. ... and that leak is very likely to be in your expansion vessel.

    Ok, I'm coming at this from a lay-man's point of view, 'cos I ain't a plumber. But it strikes me that there is simply no way that air can be introduced into a pressurised system unless it's from a greater pressure zone... And the only place that occurs in in the e-v.
  16. redtomato

    redtomato New Member

    As I didn't experience any water from the EV value when pressed, is there another way of testing it? I have previously tied a plastic bag to the EV vent pipe and left this for a couple of days but it was empty.
  17. I'm assuming air from the greater-pressure side of the diaphragm is passing through to the system side, not water escaping into the air side. So, not necessary to find evidence of water coming out t'valve. You got a pump pressure gauge? Take daily readings of the exp vessel (but don't let any air escape past when taking readings...)
  18. TooManyTools

    TooManyTools New Member

    I realise this is an old thread but it's the only one I found when having a similar issue. I can confirm that it is possible for the pump to suck in air without leaking. I only have 1 upstairs radiator connected early on in the circuit. I started getting a sound like gravel coming through the rad valve whenever the heating switched on. It abated after the system had warmed up, presumably expansion under heat sealing the leak.

    On mine, after bleeding the pump with the centre screw I noticed a slight weep from the plastic surround. Did a bodge fix with some parcel tape to see if it it reduced the problem. It did. I have now sealed around the edge of the plastic surround with flu sealant (due to its heat resistance). Problem solved. Hopefully this may help others in the future. I imagine it's a pretty rare problem.

    IMG_3776 (1).jpg
    Devil's Advocate likes this.
  19. Debra Gibbs

    Debra Gibbs New Member

    My mum had a combi boiler fitted in October last year, as yet she has not had a full week of it working. She has old pipe work and rads. Firstly the boiler kept cutting out as a lot of air in System, after a lot of complaints they eventually reflushed rads and although air not as bad in rads air goes into boiler and cuts out. We released air out of Boiler, then it would start after repressurising . So company fitted auto release valves on pipe work before the boiler, so now the pressure drops every day, well, through the night when it cools down, and will not work in the morning untill it's been pressurised. The company does not seem capable of working out what the problem is. Please Help help HELP
  20. Devil's Advocate

    Devil's Advocate Well-Known Member

    Debra, too many issues here, and the installer is already on the case - they need to carry on and sort it.

    Really this is as simple as getting them back to fix it until they do - and it shouldn't be at any extra cost to you (unless a part of your original system is faulty that they had no control over).

    You have a combi system, so it is sealed and pressurised. Air cannot get 'in' to that sealed system, so it's either air that they have yet to fully remove, or else it's hydrogen produced as a by-product of corrosion (I'd have thought this unlikely as they will have added chemical inhibitor).

    If your installer ultimately gives up and says there's nothing they can do, you then get someone else to come in and fix it - and then sue the first plumber for the cost (depending on the cause - is it something the first plumber should have ID'd and fixed as a matter of course).

    (Do you have 'Legal Protection' on your home insurance?)

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