Air in sealed heating system

Discussion in 'Plumbers' Talk' started by Chilla, May 5, 2016.

  1. Don't worry about the eV if it was to small for the system you would have water coming from the pressure relief valve if that's not the case then you have a small leak somewhere on the system
  2. kiaora

    kiaora Guest

    I installed lead pipes and coal fired back boilers, parkray 88s I think?
    But now it has changed, you can now become a plumber / heating engineer in a few months.

    It just take another 10 or 20 years before you start to understand what's really going on.

    How can a pump cavitate,

    Simply explain and you are on the road to being a plumber.

    Sermon over
    KIAB likes this.
  3. KIAB

    KIAB Super Member

    And Baxi Bermuda 551 & 552 fitted in the 80's, now 30 years plus later I'm ripping them out now.
  4. Chilla, can you post a sketch of your layout - particularly that of the replumbing on the upper floor? Showing how the pipes run?

    Can you confirm that when you bleed that upper rad, air does come out quite successfully & forcefully - you can feel it and it goes psssssssst? And, you stop bleeding only when a bit of water comes out too?

    Been thinking about what you said before about having the system pressure higher than normal because of the high rads - I can see now that that would make sense; when filling a system, it's the pressure of the incoming mains water that is used to drive the water upwards against gravity to the highest rads. For instance, if your water pressure was only 1 bar at ground level, then in theory that would only push water up to a height of ~10 metres - I'm guessing your house is taller than that?

    In which case what you said was true - you'd need to pressurise your system to a higher level just to fill these upstairs rads. (And, by implication, I was wrong - but we won't dwell on that bit...:oops: )

    Ooookkkaaaayyy, with a normal 'vented' system which has a storage tank in t'loft, and so generally works under a lower system pressure, it is not uncommon - when it's poorly designed - for air to be drawn in by the circulating water (I mean this shouldn't happen, but certain circumstances can cause it - partial blockages and poor design, etc.)

    Right, this shouldn't ever happen with a pressurised system as it's sealed, but if there was a leak there at a very high point, then in theory it could happen.

    Your boiler is on the ground level? It's showing 1.8bar pressure? I'd blindly assumed that the pressure in all your system would therefore be at 1.8bar. But, since hydro-whatsit pressure drops orf at around 1 bar per 10m height, your building - how tall? Around 10m? - might have well under 1 bar pressure in its system on the upper floor.

    And, once the pump starts circulating - which cause a higher pressure before it and a lower press before (ie - the 'return' will be at a lower pressure) - if you have a part of your system which could allow air to get drawn in on that return loop, then maybe - maybe - it's possible...

    That's all theory.
  5. CraigMcK

    CraigMcK Screwfix Select

    Unfortunatly along with decreasing skills comes google to give you the "answers"
  6. 'Cavitation' is the turmoil caused by empty pockets of irrelevant thread postings... :p
  7. kiaora

    kiaora Guest

    you may have missed my point?

    what happens if you get a low pressure on the feed to the pump?

  8. If it's a negative head pump it won't matter if it's a positive head pump it won't work
  9. kiaora

    kiaora Guest

    Maybe a bit more research required ?

    Low pressure of water changes the boiling point,
    Low pressure at inlet of pump, will result in the water boiling! Producing air!
    It's not boiling as in.. Water boiled at 100 c at sea level.

    It's boiling at a lower pressure........

  10. How does boiling water produce air? By expelling the air that's dissolved in the water?
  11. kiaora

    kiaora Guest

  12. KIAB

    KIAB Super Member

    A* :p
  13. But the air in the water will ultimately be expelled anyways, won't it? Regardless of whether it's boiling of chust at 80oC?

    In any case, surely it's not what's giving Chilla his gas? After a couple of bleeds, there would be no more air left in the water.
  14. Crowsfoot

    Crowsfoot Screwfix Select

    Apologies for jumping in (I may have missed something)
    I would totally disconnect the filling loop from the system.
    The expansion vessel size could be too small.
    +10% of the systems capacity is the formula used for sizing a EV.
  15. Chilla

    Chilla New Member

    Hi, Was away for the weekend, thanks for all the replies.

    DA: 1st bit is easy: Yes, the upper rads bleed quickly, so I don' think the air is coming from up there. A drawing?! eek, not sure. It is an old house and I am sure there are many 'interesting' bits of plumbing under the floor boards but I could try...

    Kiora: So I think you are saying that with a drop in pressure the water should/could boil (hence air), and that this would happen before the pump (i.e. on the low pressure side). [​IMG]

    So if the low pressure side was at 0.69 atm( or bar) water could boild at 90degC. But since my system is >1.5bar (above atmospheric) already this is a drop of 2.19 bar by the pump, I am not sure if this is possible. Would it be a good test to just drop the temperature of the rads? (currently on 2 out of 5).

    But also if the pump *could* drop it's intake pressure to below atmospheric, then this could be where the air is coming from? Any one know if this is a possibility?
  16. Chilla

    Chilla New Member

    Thanks for suggestion, this is possible, as 4 of the radiators I swapped were flat panel before I replaced them with cast iron ones (higher volume for same output). But how does this introduce air?
  17. Chilla

    Chilla New Member

    Remembering my physics here. But boiling water produces water vapour (not air). But water can disolve air, and does release it when at higher temperatures.

    But *here is a new theory* Probably a load of rubbish though... :) It is dissolved air, degassing! Cold water holds/dissolves more air (and I did the refit in the winter), as I did the refill in the winter the new water had lots of air in it which is getting released when the boiler heats up the water. It then gets stuck in the badly positioned radiator, causing gurgling. I tend not to use all the rads in the house (just the ones I need), so there is plenty of water that could still have high air content.

    I think I can test this by running the system with little/no heating. Then heating up the whole system really hot (hotter than I normally run). This should degas, I then vent, and drop back down to normal temperature. Do you think this idea makes sense or is this a waste of time/energy...?
  18. kiaora

    kiaora Guest

    The excellent graph you show, may confuse some, it appears to show that at the top of Everest the pressure is say 0.9 bar, and the pressure is lower as you
    I know that's not the case, but it looks a bit like that.

    I think I'll leave it there!

  19. Hi Chilla.

    If the upper rads bleed quickly and easily, then they have enough pressure in them. And if the boiler says 1.8bar, then even the upstairs rads will be at the best part of one bar - so certainly not the issue I was thinking about.

    (I was chust wondering if - due to your house's height - the top rads could be at near zero bar when system is off, and then possibly the flow of circulated water allowed one side of the circuit there to drop below zero and draw in air? But, in reality, it's pretty clear the pressure is plenty high enough, and also there would actually have to be a faulty joint or summat that would allow air in - and not water out! So, silly theory... :oops: )

    Ooookkkaaayyy, dissolved air is gonna come out at some point or other - it doesn't need anything funny done like starting off cool and then heating. A certain amount of dissolved air will be expelled when the system water is heated up - full stop. But, obviously, that amount will fall off dramatically with time. So your rads should require less and less bleeding as time goes on.

    And, really, it should be a pretty small bit of air anyway.

    You are still having to re-bleed on a regular basis? Man, I'm stumped... :(
  20. Ryluer

    Ryluer Well-Known Member

    It can suck in air if the piping is not sized correctly. Round the gland and flange seals.

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