Here is the simplest axiomatization:
The file plumbing2.pl contains a full axiomatization.pressurised(hws) <- on(t4) & pressurised(p2). pressurised(p4) <- pressurised(hws). pressurised(p5) <- pressurised(hws). flow(shower) <- on(t5) & pressurised(p5). wet(sink) <- on(t6) & pressurised(p4).