A system, method, and computer program to adjust the effective width of centrifugal fan wheel(s) in a fan system during operation. The reason for the adjustment is to prevent stall when running the fan over a wider range of system operating points. This is of particular use in high turndown variable air volume systems commonly encountered in HVAC systems. An additional option function is to isolate a single fan, in a fan system, if that fan should fail, to prevent reverse flow through the failed fan.