  Fluctuations of energy and heat are investigated during the relaxation
following the instantaneous temperature quench of an extended system. Results
are obtained analytically for the Gaussian model and for the large $N$ model
quenched below the critical temperature $T_C$. The main finding is that
fluctuations exceeding a critical threshold do condense. Though driven by a
mechanism similar to that of Bose-Einstein condensation, this phenomenon is an
out-of-equilibrium feature produced by the breaking of energy equipartition
occurring in the transient regime. The dynamical nature of the transition is
illustrated by phase diagrams extending in the time direction.
