History: Vented central heating system. Dec 03. Central Heating flushed with Sentinel liquid and each radiator removed and flushed outside (lots of sludge etc). Refitted with TRVs. Sentinel inhibitor added. Early 04. CH flushed again (just clean water came out I think) and boiler replaced. Sentinel inhibitor added.
Now that I'm coming up for 1 year since inhibitor was added I wanted to check whether the inhibitor is at the correct concentration. I tried bleeding the system the other day and nothing was required - water came out immediately and it was perfectly clear. From this I suspect that everything is okay - although I shall drain some water off into a jam-jar and leave a nail in it and see if it rusts.
Is there anything else I can do to check whether I need to top up the inhibitor? I've not been able to find any test stuff to actually check the concentration which I've heard people talk of before.
Thanks
David