Rhetorical question: how come, if you offer to help some – alternative – project to repair their heating or water system you always end up renewing the sewage infrastructure (too)?
(*sighs & rubs her hands clean…*)
in this case, it was a huge amount of paint residue causing the problem (silk screen workshop with a sink without a siphon … oh, and the pipes were going up & down, without being fixed except with about two cans construction foam mixed with glass wool inside the walls… *shakes head & tries not to ask for a ban on construction foam*)