I recently installed a thermal store in the garage, to replace our gravity-fed hot water. The good news is that we now have hot water at mains pressure, and it was a fairly easy splice into the existing pipes. The only problem is that soon afterwards, and continuing for a few months, there have been small sticky black bits in the hot water for the bath. They are easily removed with white spirit, but I am wondering what might be the cause. There is a flexible pipe supplying the tap, but I have replaced it and cut open the old one, and it had not deteriorated. The installation has resulted in water now flowing in the opposite direction to what it did before; and my assumption is that this, combined with the vastly increased flow, is now shifting the black deposits that I know have built up in the (40 year-old) pipes. Is this a reasonable guess? Has anyone else done similar work and had the same thing happen?