How often to change Central Heating inhibitors.

[rustic]

Well the central heating went Kaput on Sunday, conventional pumped system, old fashioned cast iron boiler, and header tank. The boiler fired up and was able to heat the hot water, but not the central heating on its own, so the three port valve was suspected, but the fault wasn't like it was when it failed 10 years ago when it was a cheap and chearful motor exchange, the lever on the side felt odd, didn't return...
It could be the gearbox, or the valve body, rather than mess about I was 100% certain the valve was faulty, and a valve 20 years old must fail sometime, so part drain down time. Wow the header tank was full of gunge, so I connected a hose and syphoned off the gunge, and flushed the tank, before draining down.
So I tied up the ball cock, emptied the header tank.
The valve is upstairs, so I thought I would take another 2 gallons out of the heating system, from a drain downstairs. I took a sample and put some steel wool in there to check how good the inhibitors were... Now wait and see for a week or so lol...
I changed the valve, no water spillage, not a drop, filled up, all ok valve works, we are warm again...

Now... thanks for staying with me on this one....
How long do you leave chemicals in the system, before flushing out and refilling?
The internet has different ideas.
They say test the chemicals.... How ? Well this is where the steel wool idea came from...
I fully flushed it 7 years ago added inhibitors etc... I think it needs flushing again, but then web sites say... adding fresh oxygen rich water encourages corrosion......

Any ideas... it is not a cheap and nasty combi boiler with a thin aluminium heat exchanger with an 8 year life lol...
It is a cast iron Potterton boiler, with copper pipes, steel rads, copper immersion tank and header tank.
Many thanks.
Rustic