An HVAC system has at least one variable speed controller for an AC induction motor, especially of the Permanent Split Capacitor type. The comparator has its input end electrically connected to the current detection control unit and its output end electrically connected to the drive control unit, which is electrically connected to the second MOSFET for synchronous rectification. Use of calcium carbonate in tablets containing at least one biologically active substance, for the prolonged release of this substance.