It is shown that during non-stationary heat flow in a metal the condition o
f charge neutrality inside the metal is violated by transient space charges
which accompany the temperature relaxation. As expected for an effect of t
hermoelectricity, the magnitude of the space charges is small. It depends o
n a new parameter Q(1) which is related to, but different from the absolute
thermopower. Both the case of an infinite system and that of a parallel pl
ate with free boundaries are treated.