Corrosion and clogging are hidden hazards in various engineered systems dealing with water as storage or heat transfer medium. The related efficiency and serviceability losses may entail expensive remedial measures and costs up to the range of millions (e). Especially modern low-exergy installations are affected. They may suffer from oxygen ingress through capillary tubes, higher microbial activity due to moderate temperatures and increased vulnerability of complex components to particulate corrosion products. This field study presents in-situ investigations of chemical and microbial water quality and corrosion in pipes of different non-residential buildings. It identifies processes and recommends measures to prevent corrosion. Causes of corrosion and clogging were found to be i.a. related to operational errors.