  We consider an interacting pion gas in the regime where thermal but not
chemical equilibrium has been reached. Approximate particle number conservation
is implemented by a nonvanishing pion chemical potential $\mu_\pi$ within a
diagrammatic thermal field theory approach, valid in principle for any bosonic
field theory in this regime. The resulting Feynman rules are then applied
within the context of Chiral Perturbation Theory to discuss thermodynamical
quantities of interest for the pion gas such as the free energy, the quark
condensate and thermal self-energy. In particular, we derive the $\mu_\pi\neq
0$ generalization of Luscher and Gell-Mann-Oakes-Renner type relations. We pay
special attention to the comparison with the conventional kinetic theory
approach in the dilute regime, which allows for a check of consistency of our
approach. Several phenomenological applications are discussed, concerning
chiral symmetry restoration, freeze-out conditions and Bose-Einstein pion
condensation.
