Central heating biocide additive

8 Feb 2009
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United Kingdom
I'm wondering whether a biocide is needed in my CH expansion cistern. It's a conventional boiler and Y-plan setup with 6 rads. I have recently replaced and relocated the cistern.

The old cistern always had a yucky layer of 'soggy cardboard' of the surface. I would syphon it off, drain and flush the system, add F1 to the cistern. The yuck always came back.

The new cistern is a Polytank with the snap on lid. What I've done is drain the system, add 500ml of F3 cleaner, refill, run it for a week, then drain and refill (3 times) to flush this out. Finally I added 250ml of F1 inhibitor, via a radiator this time, and refilled the system.

Now after running it for a week there's a thin layer of scum on the surface. I will monitor this but I'm wondering if it gets worse should I add a biocide?

I'm also wondering if it could actually be a bi-product of the F1 inhibitor rather than biological growth?

And if I do add a biocide, which one? Fernox F7 seems ridiculously expensive and there's something way cheaper called E700 Biocide Concentrate.
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