Feed Tank has black water

[oldhenry]

I have an old Ideal 'E' type CF boiler system.80,000 BTU Pumped primary and gravity HW. The feed tank does not have the expansion pipe over it as the domestic water supply tank has both expansion pipes- I presume this was done for the fitters' convenience. It is 37 year old - like the house. The boiler is kettling a bit so I wanted to add Sentinel X200 to the system Draining some water off and examining the Feed tank it is pretty grotty. Brown scum and black water. How as it get there? It must be through the feed but how it does that I do not know. The system has always had inhibitor in ,replaced whenever the system was drained for any reason such as moving radiators when decorating.
Any clues to how/why water would get back into the Feed tank? The heating/boiler heats up fine and over the years I have lived here (19) I have taken radiators off and flushed them as I decorate. They have never been sludge ridden nor have cold spots. Sorry about the length of this epistle.

 

[Belboz]

Is your F&E tank in the loft and is it covered?

Some older systems just have an open tank which means that any old detritus can fall into it e.g. dust, roof debris etc.etc.

Over time this will cause the tank and the water in it to become dirty.

Got any pics??

 

[oldhenry]

Thanks
I made a lid a long time ago out of wood and covered by insulation so nothing can fall into the tank. The roof is felted and generally not too dirty. I cleaned the tank so will keep an eye on it and check it again in a couple of weeks. But the water was black as from dissolved iron - from radiators I suppose- but the question is how it got there with no expansion pipe over that tank?