X400 needs to be left in for a minimum of four weeks and is better left for five but should be drained and flushed clean by six weeks.
Only if its well clean should you add the inhibitor as further cleaning may be required if the system is still dirty or circulation faults have not been resolved.
Quote from Sentinel's web site about X400; my bold.
Where systems are badly fouled, a longer period of circulation (e.g., up to 4 weeks) is recommended, but the product will not clear a completely blocked pipe. There is no limit to the time for which the product can safely be left in the system.
X400 needs to be left in for a minimum of four weeks and is better left for five but should be drained and flushed clean by six weeks.
Only if its well clean should you add the inhibitor as further cleaning may be required if the system is still dirty or circulation faults have not been resolved.
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