CH on or off, or does it depend on the design of the rad??

shouldn't make any difference but off if it bothers you

On, as the heat will expand the air present and help ensure you get it all out. The higher pressure of the system running also helps.

the can of worms is open now

its not voodoo

just bleed them

You're just getting shut of air trapped in a rad, it doesnt matter unless you have had a leak elsewhere and are trying to push the bubbles around to the bleed point.

I'm not "scared" of doing it, on or off makes no odds to me – I just wondered if there was a proper way of doing it. I'd not thought of the expansion with heat, that's a good point. Most of the rads have worked fine with the CH on, but there's one rad downstairs that spits out water when the CH is on, but bleeds nicely when it's off, guess it's something to do with flowing water being more turbulent and interfering with the bleed port but settling when off – hence wondering if one way was more correct than the other.

The instructions for my combi say you should bleed the rads with the boiler off.

If you're bleeding it properly you'll get water coming out at the end anyway, obviously, so I'd not worry and do it however is best for you. I do mine cold because it doesnt hurt as much (pressurised system).

http://britishgas.home-maintenance.info/heating/radiator-bleeding/

Use an old towel, ie water comes out dirty and under pressure, so use towel to collect and protect wallpaper, bedding etc

Also, unscrewing the bleed valve too far could upset you ……

sorry

if it's a combi system (s opposed to open with a haeder tank in loft etc) you need to know which valve on boiler etc refills system. (My gfs is like that and when I bled it her boiler wouldn't restart – pressure I think).

RTFM I'd say

off (or you just get water out – not air), but they should be warm

Cheers for the input. All done now, house lovely and toasty!

Off is best just in case the system is under negative pressure, although unlikely these days. Bleed them all, then run the system for a bit, then re-bleed, and don't forget the one on the cylinder / any high level pipework etc.