Mike Reed
pfm Member
This is to make sure that the hot water in the radiators doesn’t burn you when you start to bleed the radiators.
So it's advice linked to safety, not efficacy.
My hot water doesn't get hot enough to burn me at the bleeder valve Anyway, I wrap cloth round to absorb the outflow. Ellenor cracked this query in post 778 for gravity-fed (as mine is) systems
here is a chance it can draw air in through the bleed valve.
Yes that was true on very old systems where the pump was on the return pipe back to the boiler.
I rather think that my pump (over 20 yrs old; amazing!) is on a return pipe, and the boiler/pump is certainly old. Thought pump life was about 10 years, as has been mine in previous houses. A new kitchen was built over mine 15 years ago, unfortunately, despite my best efforts at the time with the pair of kitchen fitter Neanderthals. Not the easiest access !