http://www.telegraph.co.uk/property...e-Is-my-central-heating-system-corroding.html
Jeff Howell gives it to you straight
Q.
I always keep an anti-corrosion liquid such as Fernox in my central-heating system. Last year I had to drain the system for some alterations to the pipework, so I took the opportunity to clean out the system with a de-scaler. It was then flushed thoroughly and re-filled with anti-corrosion treatment. I have noticed now, whenever I need to bleed a radiator to remove any air, that the water which comes out is black. Does this indicate a problem and what might be the cause please? CO, by email
A.
The "system" water in a central heating circuit is locked in, and can circulate around this closed network of boiler, radiators, pipes and hot-water cylinder for years without being disturbed. There will always be a small amount of corrosion taking place inside the system, due to the combination of different metals – copper, steel, aluminium, zinc and possibly lead – which react with each other in the presence of water to produce oxides and other residues. The fact that you have to regularly bleed gas out of the system might indicate a small leak on the return side of the pipework which is allowing air to enter, and this will increase the possibility of oxidation. On the other hand, the gas might be hydrogen sulfide (smells like rotten eggs, and ignites when a flame is applied), which would indicate a more serious corrosion problem.
Serious corrosion will result in a build-up of sludge over the years, which will eventually cause blockages. In your case, this seems unlikely, however, as your system has been drained down and refilled using a corrosion inhibitor. So any slight discolouration of the system water is unlikely to indicate a serious problem. Regardless of the water colour, the most important point is to check that there is always a sufficient concentration of inhibitor in the system water. Fernox and Sentinel both supply kits to test for this (available from plumbers' merchants). The method is to collect a little water from a radiator bleed valve, add tablets, and note the colour.
Or you could try the old trick of bleeding radiator water into a glass jar and adding a steel nail and a 1p coin. If the nail turns rusty after a month, then that will be happening to your radiators too, so you will need to add more inhibitor until the system stabilises.