Help - fed up with bleeding radiators

I need to ensure I've got all the air out of my closed pressurised system in order to resolve/eliminate another issue. Pressure is around 0.8bar when I start, down to about 0.5bar when I've finished and I then repressurise.
The bleed screws are those little square ones at the back (not side) of each radiator so when they squirt out they hit the wallpaper.
The property is a bungalow so all radiators at ground level but the pipes go up and over via the attic.
Anyhow most of the radiators instantly let out a silent stream of water.
A couple sputter, water comes out but with a sputtering sound and/or a whistling sound and this is where I'm confused. It seems no matter how much I let out (I'm talking cup fulls not gallons) they still seem to sputter. They sputtered yesterday, today and I expect they will tomorrow.
Is this air in the system or is it just the shape of the screw?
Am I correct in assuming that air cannot be introduced into the system unless there is serious corrosion or the like going on?
How much oxygen am I reintroducing repressurising the system?
--
AnthonyL

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On 07/11/17 10:29, AnthonyL wrote:

I'm debugging a similar (and new) system - my pipe runs are tortuous out of necessity (upstairs is a dormer, so pipes go:
Up from boiler
Down to north side rads.
Down to interlink pipe to south side.
Up to distribution pipework
Down to rads.
Needless to say, the south side has two cold rads on the ends of the distribution leg.
Trouble was expected - so after seeing how naive pipework behaves (better than expected to be honest) I'm getting the plumbers back to insert bottle type auto vents in several key locations once I can determined where specifically the air seemed to be getting trapped.
A thermal camera would be dead useful here - are those hireable?
Otherwise, we'll do it the old fashioned way by feeling the pipes for the lukewarm (neither hot flow nor cold return) bits.
So probably the same for you - add some bleeders in the high bits, especially the returns.

They would generally - the air locks are in the pipe above.

Probably air - you'll find there are one or two rads that seem to catch odd bubbles of air that the pump manages to shove down a pipe.

Correct.

It won't matter if you add inhibitor.
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Tim Watts submitted this idea :

Not quite so convenient, but you can do it with an IR thermometer and from a distance.

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wrote:

I think my problems started after adding inhibitor :( (following a PRV replacement).
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AnthonyL

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On 07/11/2017 10:29, AnthonyL wrote:

You don't own a towel?

You can fit an automatic air vent up there.

Is the radiator hot right up to the top?

Very little. Just the oxygen dissolved in the water.

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On 07/11/17 10:42, GB wrote:

Oh I forgot - small towell, rag, hanky, kitchen roll - all of these should result in near zero spillage.
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If there is air in a rad, the very top won't get as hot as the rest.
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Dave Plowman (News) explained :

More obvious, if the heating has quite recently fired up.
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On 07/11/2017 10:29, AnthonyL wrote:

Usually you find that any air in a system will mostly gravitate to one or perhaps two radiators that are the highest in the system. The tops will be noticeably cooler when the rest of it is hot if air is there.
If there is a lot of air in a poorly designed system after refilling then you will hear it bubbling up in any vertical pipe runs and a change in pump noise when there is air going through it.

If it sputters it is probably from gasses in the system.

Only if you bleed it with the pump not running. If you try to bleed with the CH pump running all bets are off. It can sometimes suck air in.

Worst case not a great deal. And perhaps none at all if the repressuring is done by a piston with the air in a bladder which is how the one I have to sort out from time to time seems to be engineered.
It might be worth adding a corrosion inhibitor if it is a new installation (elderly systems can react badly to any kind of change).
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Regards,
Martin Brown
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On Tue, 7 Nov 2017 11:29:53 +0000, Martin Brown

I'm not sensing much difference. My old house had a top up tank, an upper (bathroom) radiator often needed bleeding and it was obviously cold on top and air (and just air) would come out until water came out.

I turn the whole boiler (CH and HW) to Off before starting.

It's an elderly system (like me). Never had any problems with my coal fired back boiler CH system in 30yrs apart from the occasional bleed of the bathrooom radiator mentioned and that was a two storey house compared to this bungalow.
--
AnthonyL

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On 07-Nov-17 10:29 AM, AnthonyL wrote:

Fit automatic bleed valves and stop worrying about it :-)
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Colin Bignell
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On 07/11/2017 12:36, Nightjar wrote:

+1. Fitting one of these in the loft next to the water tank completely eliminated any problems in my mother's bungalow.
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On Tue, 7 Nov 2017 14:47:00 +0000, newshound

I'll enquire about them - do they work on a closed pressured system?
And how come everything has been fine for 2 years before I had some boiler work?
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AnthonyL

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I'd have thought the up and down nature of the plumbing is an air trap waiting to happen permanently myself. Brian
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AnthonyL wrote:

I think* the pressure should be 1.5 bar, mine is. If the pressure drops to 0.8bar I'd say that you have a leak somewhere, we did.
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On Tue, 7 Nov 2017 17:56:01 -0000, "Mr Pounder Esquire"

I've dropped the pressure to that level. AIUI it is within spec. I can increase it if it has any bearing on my problem, it isn't leaking (since the PRV was replaced).
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On 07/11/2017 10:29, AnthonyL wrote:

One of the bleed keys with attached receptacle might solve that:
http://amzn.eu/dGXFk4i

Some have the outlet positioned such that you can't actually bleed all the air out - there will be a little pocket of air at the top.

No... some will come with the fresh water - so each time you add more a little air will also be added.
You can also get air drawn in if you have a very slow leak somewhere.

A little but not much. That is one of the things the corrosion inhibitor is for - to mop up any excess O2.
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Cheers,

John.
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On Tue, 7 Nov 2017 18:12:08 +0000, John Rumm

Doesn't work on little square bleed valves that are on the back of the panel
https://www.diynot.com/diy/media/untitled.85254/full
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AnthonyL

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