Mechanical properties and morphological studies of polypropylene thermopipes manufactured in Jordan and usually used in heating system networks, have been investigated. Samples were chosen to represent new and different working periods (1,3.5 and 6y). The flow resistance of water inside the pipes showed a slight decrease in the first year followed by an increase after the second year of use. A decrease in resistance was observed with increasing working temperature and Reynold＇s number. The stress-strain tests exhibit property changes towards brittleness and glassy behaviour, observed after 6y working service. Evidence of deformation was seen by scanning electron microscopy (SEM). Deterioration and cracks on the internal surfaces of aged thermopipes were also noticed.