12 months ago ,i have a plumber who doesnt stick a wet finger in ghe air , doesn't like using plastic plpe and can design systems for very large houses and they work properly,his knowledge is immense and i bow to his knowledge ,and they way he makes his plumbing look artisticMust have had money to burn eh, that or you needed it that high due to the poor efficiency of the boiler or super sludgy radiators.
When was the last time you power flushed your wet system?