In this Note, we consider the linear heat equation ut - Au= a(x)u in (0,T)xW,u=0 on (0,T)xaW, and u(0)=uº on W, where W C RN is a smooth bounded domain. We assume that a€L^loc(W) a >=0 and u>=. A simple condition on the potential a is necessary and suficient for the existence of positive weak solutions that are global in time and grow at most exponentially in time. We show that this condition, based on the existence of a Hardy type inequality with weight a(x), is "almost" necessary for the local existence in time of positive weak solutions. Applying these results to some "critical" potentials, we find new results on existence and on instantaneous and complete blow-up of solutions.