in the above example with similar logic employed and similar results achieved it should be understood that while the logic discussed in connection with fig5 and 6 related to a particular system the logic may be applied to the other systems or subsystems disclosed herein for example the logic of fig5 may be easily adapted for use with the auxiliary system if so desired similarly the logic discussed in connection with fig6 may equally be applied to operation of the main fan or possibly the control of the main ventilation inlet while the invention may be susceptible to various modifications and alternative forms specific embodiments have been shown by way of example in the drawings and have been described in detail herein however it should be understood that the invention is not intended to be limited to the particular forms disclosed rather the invention is to cover all modifications equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims for example it is contemplated that while the embodiments and techniques described above have been shown to be combined into a single system they may operate as individual systems or as subsystems for example what has been described as the auxiliary system i e fig3 need not be connected to the same control unit as the systems described in fig1 and 2 as noted above multiple controllers may be employed to operate the system in a similar manner it is further contemplated that a single control unit may interact with individual components of the system on an independent basis for example fig4 illustrates a system with a single control unit 122.