In the cooling state no water is being drawn from the EWH
and the water inside the pipe is stagnant. The temperature of
the outlet pipe and the water are equivalent and can be assumed
to remain in good agreement throughout the duration of this
state as the thermal conductivity of copper is high (385W
m
◦
C)
and the pipe wall is thin, therefore the temperature drop
across the pipe wall is negligible [15]. Since the temperature
of the system is higher than the ambient temperature of its
surroundings, heat is dissipated to the environment through
the pipe surface during this state as a result of the temperature
difference. The temperature of the system will continue to
decrease until it reaches the ambient temperature of the air
surrounding the pipe, or another usage event occurs.
Figure 3 shows the thermal equivalent circuit for the outlet
pipe system as sensed by the temperature sensor. R and C are
the total thermal resistance and capacitance, respectively, for
the outlet pipe system during a specific state. The temperature
of the pipe at a given time (t) can therefore be modelled using
an RC transient circuit analysis and is determined using the
following equation: