def calc_DH_return(t_0, t_1):
    """
    This function calculates the return temperature of the district heating network according to the minimum observed
    in all buildings connected to the grid.

    :param t_0: last minimum temperature
    :param t_1: current minimum temperature
    :return: ``tmax``, new minimum temperature
    """
    tmin = min(t_0, t_1)
    return tmin