Thanks for all of that. I think I can see both sides of the argument. All I know is what I saw come out of the radiators! I am no plumber and I don't expect drinking water to be in there either, however, the contents suggest that there is a problem with the original setup.
I will approach...
I own a new house (less than 2 years old) with a condenser gas central heating system.
I have just had the radiators in the bathrooms replaced with heated towel rails. Upon draining the old radiators the plumber found the contents to be black and lumpy and very smelly.
He has advised a...