We demonstrate a new type of non-Hermitian phase transition in open systems far from thermal equilibrium, which takes place in coupled systems interacting with reservoirs at different temperatures. The frequency of the maximum in the spectrum of energy flow through the system plays the role of the order parameter, and is determined by an analog of the -potential. The phase transition is exhibited in the frequency splitting of the spectrum at a critical point, the value of which is determined by the relaxation rates and the coupling strengths. Near the critical point, fluctuations of the order parameter diverge according to a power law. We show that the critical exponent depends only on the ratio of reservoir temperatures. This dependence indicates the non-equilibrium nature of the phase transition at the critical point. This new non-Hermitian phase transition can take place in systems without exceptional points.
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