On 4/25/2010 9:01 AM, Ian wrote:
Four years ago we had a new central heating boiler installed in the
airing cupboard upstairs. It is an icos HE24 condensing boiler, open
vent. Apart from the electronic controller failing a year ago,
everything has been fine. No problems of any kind.

A few weeks ago we had a radiator replaced. When draining down the
system, the plumber discovered an evil growth in the boiler
feed/expansion tank in the loft. The growth was a dark brown, gungy,
jellyfish-like mass about dinner plate size and about one inch thick. He
removed the tank from the loft, disposed of the gunge, and thoroughly
cleaned the tank before replacing it back in the loft and adding inhibitor.

Since then, there has been a strong, damp, "metallic" smell in the
boiler/airing cupboard, which appears whenever the boiler is firing.
When the boiler is switched off for a few hours, the smell goes away
There is no trace of the smell in the loft around the header tank.

There is no sign of any leaks around the new radiator or anywhere else.
As a double check, the plumber has just re-examined the header tank, and
reports that a new evil growth (a thick yellow skin on top of the water)
has appeared.

Is the water temperature too low perhaps, to kill off the evil bacteria?
(I don't know what the temperature setting is, but it hasn't been
altered since the original installation, and the new rad was a straight
replacement for an old one).

Could there be a problem if the new inhibitor was not the same type as
the old?

We're stumped. Any ideas on what to do next? I

Sounds like this stuff -

http://en.wikipedia.org/wiki/Iron_bacteria
http://www.hhs.state.ne.us/enh/IronSulfurBacteria.pdf

If it is, raising the temperature should help get rid of it.