Two years ago i cleaned my heating system with fernox3 into the header tank like it said on the bottle, i did the same thing this year but after the last flush and refill there is a blue tint to the the hot water. Can anyone tell me why this? Also i bought a bottle of Fernox1 Inhibitor and it says put in expansion tank, why one in header and other in expansion?
Header and expansion are different things. The header tank is not only a storage vessel for keeping a head of water in a circuit (it can also be referred to as a cistern) but it also doubles as an expansion tank to cope with the expanding water.
Maybe you are confusing the terminology with an expansion vessel which is basically a cylinder containing a diaphragm and has the sole purpose of maintaining an even pressure in a system regardless of water state.
I have no suggestions regarding the coloured water other than to examine the hot water header tank to ensure no debris or other matter has fallen in and contaminated the water