The installation of a properly designed heat exchanger allows an increase in the temperature of the water supplied to Oviedo of up to 18 C when the initial temperature is lower than this. The value of 18 C has been chosen as the maximum temperature because it is lower than the temperature reached during the three months of the summer and is lower than 20 C which is considered to be the limit under which the harmful bacteria Legionella do not appear [36].