How does air get into a pressurised CH system?

Anyone care to explain how this happens?
About 18 months ago, I changed my CH to a pressurised system, and have since been tracking down leaks in the old pipework. The main problem being that the majority of these pipes are buried in the concrete floor. There is only one radiator that gets air in it. Why is this, and why is there still pressure in the system (0.5 bar)? Thanks, Jon
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Catch it in a test tube (or similar) and attempt to light it. If it goes pop, then it is hydrogen from the radiator corroding. Check your inhibitor level before you have no radiators.
In any case, systems seem to prefer having a specific radiator to collect air in. They are frequently upstairs. I think the air in lower radiators gets reabsorbed into the water and dumped in the lowest pressure radiator, but I could well be wrong.
Christian.
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Ah! That sounds likely.... We've yet to put any in due to having to drain it down every couple of months. However we've only had to top it up twice in the last 10 months due to the small drop in pressure that has happened over that time - during which we've found two self bleeding valves to be leaking (which we've isolated) and last night a small leak from a radiator feed pipe which we will need to drain down the system before we can fix it....... It's a bungalow with the pump in the attic - the radiator that gets the gas may well be the first one from the pump. Jon
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As other poster says, this may be hydrogen from corrosion.
However, strange though it might seem, air can leak into a pressurised system through any leaks, even if the system always remains pressurised. Air will leak in until the partial pressure of oxygen and nitrogen in the water is the same as that in the air itself. When the water is heated, the partial pressure (for a fixed amount of air) increases, and this forces the air out of the water, hence bubbles forming when you heat water. These will settle in some high point. When the water cools, the partial pressure will drop as this air cannot instantly be redissolved into the water (it would given a long enough time if the pump was left running with the system cold). So the effect is that any leaks will now start to let air in again, bringing the partial pressure of oxygen and nitrogen in the water back to atmospheric levels. You might think not much air could leak in through such small leaks, possibly too small to even observe a water leak, but remember air is very much less viscous than water, so it can get through such small leaks with relative ease.
When my system developed a small leak recently, the first hint that it had happened was that I could hear some bubbles being pumped in the pipework. The rate of pressure drop due to water loss was low -- probably would have taken several months to lose pressure to the point where it needed topping up. However, having fixed the leak (radiator blanking plug), the air has also stopped getting into the system and the sound of bubbles has gone.
--
Andrew Gabriel

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writes:

since
that
Now I know! Jon
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Ah, but how did you find said microleak :-?
--
fred

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I turned the heating off for a couple of days when I was away. With the pipework cold, and the house quite cold too, some water drops did form. On closer inspection, there were some small tell-tale traces of evapourated inhibitor, but I would probably have never noticed that.
--
Andrew Gabriel

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Thanks, useful tip. Perhaps I'll leave it a couple of months tho ;-).
--
fred

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since
replace all the lockshield radiator valves, with valves that have a drain incorporated in them. This will allow you in future to remove each individual radiator without the need to drain the whole system. see link for example.
http://www.plumbworld.co.uk/acb/showdetl.cfm?&DID &User_IDP98213&st92&st2Q926370&st3654859&Product_IDE37&CATID1
In my experience, I've had to drain the system losing the inhibitor on several occasions. At least this way your will get to keep most of your inhibitor. plus you should only need to bleed one rad of air.
regards mike
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Look closer... that valve does not allow you to drain the radiator individually. It just provides a drain point for the whole system, as it is on the outside of the radiator valve.
--
Tim Mitchell

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Might I also suggest to Tim that you invest in these rad valves as indeed they do work as I described. I bought my valves from Wickes, same as the ones I referenced to. Mine work fine. I can drain each individual radiator without the need to drain the whole system. regards mike
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Well... actually I have got some!! And I did not know that they worked like that, I thought they were just a system drain point. From looking at the outside of the valve I assumed that when the radiator valve was closed, the drain point was connected to the system and not to the radiator. So I must apologise for my misinformation.
--
Tim Mitchell

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no problem Tim, We are all here to learn and to share information. especially me !!!!!!!!! :o) regards mike
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They do allow you to drain a radiator. The valve washer pushes down from the top and blocks off the bottom (supply) pipe only. This leaves a clear channel between the right (radiator) and left (drain tap) pipes when the tap is closed, but blocks the drain tap from the supply.
Christian.
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http://www.plumbworld.co.uk/acb/showdetl.cfm?&DID &User_IDP98213&st92&st2Q926370&st3654859&Product_IDE37&CATID1
Good call! Jon
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