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-3 is designed for use with oil appliances. The CAS-4 is for use with gas appliances with 24 VAC gas valve controls. The CAS-4MV is for use with 750mV 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.