i'll have to agree with SBR plumbing. There seems to be sludge build up. You can add sentinel X300 sludge buster to system . Run it for the duration indicated on the instructions, no more. As it can eat through some old pipework if system is extremely old.. Flush all evidence out from system then add sentinel x100 inhibitor, when satisfied. As SBR said, you may need the system balanced too.
lee S. Wales
January 2007