A very common issue that we encounter is rust in heating systems.
Generally, when a sample of heating water is inspected and it appears black that is iron ferrite. This occurs over time and provided it has not been left too long to the point which it becomes sludgy then it is not detrimental to the system. On average, a well installed leak free system will have this black iron ferrite cleaned out twice in the boiler’s life. Once when the boiler is installed and possibly again after five or six years of service.
If the sample appears red/brown that indicates iron oxide. You can only form iron oxide if oxygen is present. As the water in a heating system doesn’t change there is a limited amount of oxygen available to form rust and once that oxygen is used the water becomes inert. Ideal for heating systems.
When you have a little, insignificant, leak on a heating system the water lost is replaced via the header tank or by the owner re-pressurizing the system. The fresh water is oxygenated and creates rust on the system components.
The owner of course is oblivious to the damage being caused and continues to ignore that little leak, refilling the system, adding more oxygen.
Eventually, leaks begin to appear in multiple places, pipework and boiler components become blocked and the damage now is very significant and irreversible.