Mould in header tank

[gash2024]

Last year i flushed out the heating system with fernox F3, flushed all the radiators, refilled the system and added fernox F1.
I noticed at the start of this year whilst in the loft there was a small amount of white scum in the tank that looked like dried paint, i removed it and i thought that was the end of it but on inspecting the tank at the weekend it has got more of what is obviously mould and very little of it is white and instead is black.
Why am i getting mould as i have added fernox and what can i do about it.

 

[RigidRaider]

Did you keep half a pint of Fernox back to treat the water in the tank?

 

[gash2024]

I kept a bit back but only about a cup full, i put it in when the system was free of air but i have since removed one radiator to decorate.

 

[RigidRaider]

Then maybe the concentration of the fungicides in the tank has fallen below the effective level. Clean out and refill with a fresh dosing of Fernox?

 

[gash2024]

Its just strange that it is there because the system was very dirty before but never had any mould.
I've just drained a little of the water out at the boiler and its got black sediment coming out so iam going to give it another flush and refill, it does'nt surprise me its got dirty again because the previous owner hadn't cleaned it for about 10 years!.
So if i keep about half the bottle back then that will be enough to treat the tank?.

 

[RigidRaider]

Easily enough.

Leave the rest near the tank and don't forget to add some next time you drain a rad.

 

[JohnD]

if you can do basic plumbing and can afford £100, you might consider fitting a Magnaclean on a 22mm vertical Return pipe.

It will collect all the remaining black sediment that is travelling around in the water (flushing never gets it all out) and will prevent it being able to accumulate into new blockages. I have not used the Fernox cleaner, but with sentinel X400, it loosens existing sediment and the Magnaclean traps it from the circulating water for weeks afterwards.

I guarantee you will be amazed and delighted to sed how much it traps, and it continues working permanently.

BTW if you have a fungus or bacteria problem, clean out the F&E throroughlky, and wipe the tank and lid with bleach to kill any spores, and wipe off. If you still get probelms, you can order Fernoc AF-10 from your plumbers merchant. It is a universal biocide but quite expensive. A little bottle is about £20 but will be twice what you need to treat an average house. You tip it in the F&E after you have filled the system, and stir it in, so its concentration is highest in the F&E. I have known people add a drop of bleach instead, which certainly works, but I don't know if that has any unwanted effects.