Legacy single speed fans are converted to variable speed fans configured to operate at reduced speeds. A conversion controller operates in conjunction with the existing controller. The conversion controller is configured with a 24 volt digital input module and a 0-10 volt analog output module. The input module is electrically connected to the 24v AC output signal wires from the existing controller. The input modules are also configured with a fan quantity selection switch to input the actual quantity of 24v AC signals for fans. The conversion controller registers the quantity of 24v AC contact signal inputs received from the existing controller, compares it to the fan quantity selected on the fan quantity selection switch, and sends a signal to the variable speed fans. The output modules are electrically connected to variable speed fan signal inputs.