Auto air bleeding of central heating systems?

I have a pressurised c/h system. I can hear bubbles trickling around the pipes but no air ever gets trapped in any radiators for me to bleed out.
There is a bleed point upstairs at what is probably the highest point of the pipes (not including radatiors). This sometimes has a tiny amount of air in it
I wondered if I could replace the conventional bleed valve with one of these (part number 7498)
http://www.bes.ltd.uk/products/114.asp
Or does this not do what I think it does?
Cheers
Nick.
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NickW wrote:

These are Ok if you are in a soft water area. They consist of an adjusting screw on the top pressing down on a stack of fibre washers which close off a hole leading into the pipework. Assume the washers are dry and the screw is set to allow as small gap to release the air. The air comes out driven by the head of water in the system. When the air has been expelled, the water then swells the fibre washers and closes the gap and seals the pipe. The valve warms up, dries the fibre, water weeps out seals the hole etc. Thus there is a low but finite evaporation or water from the valve. If used with hardwater, the fibre washers become impregnated with calcium carbonate which being porous means the valve never seals fully and becomes useless.
HTH
Bob
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Bob Minchin wrote:

This might be a hard water solution? http://www.denc.co.uk /
Richard
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On 21 Oct 2003 11:54:51 -0700, snipped-for-privacy@bt.com (NickW) wrote:

If you mean their part 7498, they are not very good, as Bob says.
Bottle air vents like their 7703 or 7704 are better and work with a float valve inside the brass chamber which closes off an air hole when the chamber fills with water.
Honeywell make a better version which you can get from heating or plumbing merchants like Plumbcenter.
http://content.honeywell.com/uk/homes/Catalogue/Non%20Electric/5.3%20EA122.pdf
I fitted an air separator from Reliance Water Controls http://www.rwc.co.uk/rwc/hoprofs.htm ASEP 192 102
This is even better than the Honeywell unit and has a stainless steel mesh which helps with separating the air. There is a much better quality mechanism as well.
Having said all of this, if you have an ongoing issue of air in the system, then the cause should be checked. It may be hydrogen from corrosion, for example......
.andy
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