These models are designed to provide combustion air for appliances when direct connection to the burner is not possible (when the appliance is located in a confined space). According to NFPA 54 and NFPA 31, an engineered system such as a CAS-3, 4, 6 or 7 may be used to overcome the lack of air in a confined space. The CAS-4MV is for use with 750 mV power pile gas appliances. Each is designed for optimum safety and includes a proving device that won't allow the burner to engage without the Fan-in-a-Can® operating. Multiple units or additional controls are required for multiple appliance installations. Each is equipped with a two speed control for enhanced versatility.