Hi all, I am about to do my annual system check, fit another TRV, flush out and re-charge with inhibitor. For the second year running, the expansion tank has a 'sludgy skin' on top of the water. I don't know what it is or what causes it.
Can anyone help please.
Thanks from the latest newbie.

Could the crud just be the deposits taken out of the water by the inhibitor. Maybe you could check with the makers of this - it might well be a fairly common question