I used Fernox DS-3 annually as a relatively idiot-proof descaler in my steam launch boiler (3-drum Blackstaff type water tube), together with tannin feed treatment.

I am not too sure exactly how effective this was, but I am sure it did no harm.

When the boiler (built 1981) expired the other year we cut a slot in the steam drum to check thew all thickness and a rather large amount of scale fell out.

Perhaps I should have left the fernox in longer!

Peter,

I am guessing the scale that fell out of your boiler when you cut it open was made up from iron deposits. Fernox DS-3 is a limescale remover and does not dissolve iron - for obvious reasons!

The way to stop of this type of scale forming is to blow down regularly or wash your boiler out regularly if it has the luxury of mud doors.

Don.

Thanks - I wondered about that.

Looks like regular blow downs did not shift this stuff, I now suspect it was largely iron oxide scale accumulating in the steam drum, as the water durms have wash out plugs.