Having TRVs fitted next week and the plumber tells me he’ll have to drain the system. While he is fitting the valves and the cold water tank in the loft is empty, is there anything I can do to steralise it? We have a big problem with biofilm and if anyone knows if there is something that I can do to minimise the black slime blocking the taps, shower heads and I suppose pipes then I’d be grateful for the information.