I keep getting mayonaise floating on the water in my unpressurised system header tank. I clean it out but within a couple of weeks, it's coming back. I've put a temporary trap under the vent to see if any water is flowing back into the header but nothing ever does. Any ideas why this is happening please?
That is mould/fungus. You will get warm water flowing back through the feed/outlet as the system warms and water expands.
It's a bacterial growth,you see it in attic that are warm, & tanks are open,time for Fernox AF10 biocide or similar.. You got any lids on those tanks,not sure how you dose system correctly,years since I dabbled with AF10, I've seen it done where they suspend float float valves, empty both tanks add AF10, then let tanks refill, can be added to Feed and Expansion tanks,details in links. . https://fernox.com/product/af10-biocide-500ml/ https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=2ahUKEwi68unYjPLdAhVbF8AKHbOpAd0QFjABegQICBAC&url=https://fernox.com/wp-content/uploads/sites/3/2018/01/af10_v.001.pdf&usg=AOvVaw1_AnUtTPRNKzE6gvNi9Oxs
The tank has a lid but it's not airtight where a hole has been cut in it for the expansion pipe to drop down into the tank. Should it be fully airtight? I was wondering if the expansion pipe should really enter through a boss on the side of the tank (after dropping down from above) to keep the tank airtight.