Open vented system - air

[Nige F]

Well, it would stop air being drawn . I'm wracking my brain for a setup we did back in the 80's with the Glow Worm fuel saver boiler - seem to recall 22 coming out of the side of the F+E (25mm off the bottom) then after about 300mm a T rising about 55mm and then a 45 bend to take the vent up+ over the F+E. (Had a diagram of it somewhere .) After the T another 150mm or so then a bend downwards and to the flow and pump. 'twas a glorified combined feed/vent and was for low head installs - similar to what you have. You might find it in an old install manual ( on t'net)

 

[dormermike]

Interestingly I'm still getting air in the boiler bypass rad (and only that rad - no others) despite extending the expansion pipe to below the F&E water level... suggests a leak somewhere?

Most of my pipework is boxed in. zero signs of a leak though.. even when I bleed a lot of air (as in most of the rad seemed almost empty when I'd been away a week!) I never hear the feed tap running and it's right next to that rad.