On Monday, April 16, 2012 5:00:15 PM UTC+1, Fred wrote:
Hi,

Please can anyone tell me how often the CH should be drained down and
the inhibitor replaced? I've got two years in my head but I might be
thinking of car antifreeze (the blue rather than red version)!

You don't drain it, just top it up. You need to drain car anti-freeze because the ethylene glycol degrades with age and turn into an acidic compound which starts rapid corrosion.

Domestic CH inhibitors are usually sodium molybdate, so you can't overdose it SFAIK.

If you're considering changing to a sealed system, I'd suggest you shut off the fill valve and monior the water level. An open vented system can top up a leaking systems for years without being noticed; it causes corrosion problems. The same leak on a sealed system will require regular topping up.

You need to check your boiler is suitable, the manual will tell you. It needs a manual- reset overheat thermostat; not all have them.