If the air volume is constant, the division into stages or groups enables the output and the heating energy introduced to be varied.

If the air volume is variable or the air volume is reduced, it is not possible to adjust the heating output by switching off a proportional portion of the heating elements.

The reason for this is that the remaining active heating elements continue to give off 100% of their output and therefore 100% of the cooling or heat removal from the air flow is required.

If the air volume is variable, the power must be varied accordingly with a thyristor or power controller.