Central heating header tank question

[Etaoin Shrdlu]

Our CH header tank doesn't normally cause concern, but last week, while draining the system to check the inside of my 3-way valve (I suspected the ball wasn't sealing properly), I discovered a brown crust on the water. My assumption is that something is growing in there. The tank isn't covered, although it has a jacket that is tied around the top. I assume that last time I was pouring something in, I didn't seal it well enough when I'd finished. I was just wondering what would be best to do with it. Can you get covers for these things? Cover it with cling film?

[Brian Gaff]

Are you sure its not just gunge from the system due to some kind of additive someone put in? Otherwise it sounds disgusting! Brian

[Etaoin Shrdlu]

Well, I put Fernox or Sentinel in there every year, so it could well be that. It's just that I'd never seen it like that until last week. I normally manage to keep the water quite clean. In fact, it is still quite clear underneath the crust. I'm just wondering if, in general, these tanks need to be very securely covered to stop new lifeforms evolving inside them :)

[Andrew Gabriel]

When it's like that, I ladle the crust out of the tank, rather than letting it wash into the system on a drain down. I believe it's a mixture of corrosion products and some bacteria which lives on them. Looks the same as the bacteria which eats things like the Titanic.

I only recall seeing it in uncovered tanks, but that maybe more a reflection of the age of the system, than with the tank cover itself.

[news]

My tank is covered (loosely - to stop stuff dropping in) and has contained Fernox (the old style high volume type, I forget the type name) since new. It too suffers from the brown "crust" you describe. The water underneath is clear and I do an annual "nail test" to ensure that I still have a reasonable degree of protection.

The system is 25 years old with no external sign of radiator corrosion and no cold spots on any of the radiators, so I don't worry about it too much.

[Etaoin Shrdlu]

I like the sound of that nail test, rather than just replacing it each year. Will no doubt save a lot of money.

[Etaoin Shrdlu]

Doesn't seem like it's anything to worry about, then. I suppose I could put more effort into making sure the cover is well sealed, but since there's an overflow pipe, there doesn't seem to be that much point.