there is a chance you have bacterial or fungal growth in the F&E. Scoop some out and see if it is slimy. If you get a bottle of clean water, and add a few spoonfuls of water from the F&E to it (leave the top off) does anything seem to grow in it? Ordinary sediment will fall to the bottom after a few days, but fungus will float on the top in a skin until it dies and sinks. Bacterial growth makes the water slimy and like a thin jelly. X100 does not seem to help with these but Fernox AF-10 (?) does (it is quite expensive). These growths make gas when they get into the got radiators and die. Alternatively, if you scoop some into a jug, and stir in a couple of teaspoosnful of bleach, does the goo disappear overnight? (bleach kills it)
If you have brown mud in the bottom of the F&E, it is usually iron oxide corrosion sediment. Very fine, will swirl up if you stir it into the water, and takes days to settle. It is faintly attracted to a powerful magnet (not as strongly as black iron oxide). I prefer to scoop it out and sponge the tank clean before draining, refilling or adding chemicals.
BTW did you see them using a powerflushing machine? It is quite big and noisy so you would have noticed.
I am a householder not a pro.
If you have brown mud in the bottom of the F&E, it is usually iron oxide corrosion sediment. Very fine, will swirl up if you stir it into the water, and takes days to settle. It is faintly attracted to a powerful magnet (not as strongly as black iron oxide). I prefer to scoop it out and sponge the tank clean before draining, refilling or adding chemicals.
BTW did you see them using a powerflushing machine? It is quite big and noisy so you would have noticed.
I am a householder not a pro.