When my C H overflow pipe drips it leaves a blackish/grey mark on the path,so i assume that it is not clean water dripping out.Can anyone explain what causes this ?
from what I remember of the last visit from a boiler-man, he said the water, although supposedly in a sealed system isn't pure due to it going round the pipes etc
(should have paid more attention to his explanation, sorry, but he did mention something about acids accumulating)
if it is a condenser style boiler then I think there usually is a drip from condensation on the outside wall... the sysyem also has additives for circulation anti rusting etc... so I was told anyway when something or another was added at some cost !!