Abstract
Premise plumbing systems are critical control points for drinking water safety, particularly regarding opportunistic pathogens such as Legionella pneumophila. While hot-water systems are typically maintained above 60 °C in Norway to suppress Legionella, cold-water systems are expected to remain below 20–25 °C. However, recent field observations have shown unexpectedly high cold-water temperatures upstream of hot-water heaters during low-demand periods, indicating “thermal contamination” driven by heat transfer within connected plumbing elements.
This work presents ongoing research combining laboratory measurements, hydraulic–temperature modelling, and design mitigation concepts to better understand and reduce thermal intrusion into cold-water systems. Laboratory experiments using a premise plumbing pilot rig demonstrated that convective heat transfer between connected hot- and cold-water pipes can produce substantial cold-water warming under specific pipe configurations, particularly when hot water flows below stagnant cold water. In contrast, conductive heat transfer alone resulted in significantly lower upstream temperature increases, although warming above 25 °C was still observed under certain conditions.
Building on these results, a calibrated temperature model was developed that accounts for both radial heat exchange with the surroundings and axial/convective and conductive heat transfer between connected elements. The model successfully reproduced observed temperature fluctuations in a real case study from a Norwegian nursing home, where cold-water temperatures exceeded 55 °C overnight due to minimal net water demand. These findings highlight that thermal contamination may create unintended temperature conditions favorable for Legionella growth in locations assumed to remain cold.
This work is situated at the intersection of drinking water safety and building energy performance. The EU Drinking Water Directive emphasizes Legionella control in domestic distribution systems, particularly in priority premises such as healthcare facilities, while recent energy efficiency policies are driving efforts to reduce thermal losses and optimize hot-water operation in buildings. Design measures such as heat traps—vertical pipe configurations that limit buoyancy-driven convective heat migration—are being evaluated as a practical mitigation strategy.
The study combines model developments, experimental validation, and early design insights to support risk-based management of temperature in premise plumbing systems, contributing to safer and more energy-efficient building water installations.