We consider heat fluctuations and fluctuation theorems for systems driven by
multiple reservoirs. We establish a fundamental symmetry obeyed by the joint
probability distribution for the heat transfers and system coordinates. The
symmetry leads to a generalisation of the asymptotic fluctuation theorem for
large deviations at large times. As a result the presence of multiple
reservoirs influence the tails in the heat distribution. The symmetry,
moreover, allows for a simple derivation of a recent exact fluctuation theorem
valid at all times. Including a time dependent work protocol we also present a
derivation of the integral fluctuation theorem.Comment: 27 pages, 1 figure, new extended version, to appear in J. Stat. Mech,
(2014