In most instances the optimum mounting height is 20 – 100cm (depending on the model) below the ceiling, but the units can be bolted directly to the ceiling without impeding the airflow.
The only reason we don’t recommend this as a standard is because over time dust marks will start to show on the ceiling.
If the building has little to no insulation such as a single skin tin roof, it is best to hang the unit 120 – 300 cm (depending on the model) below the ceiling. This stops the units drawing cold air off the surface. The more powerful the unit, the further away it should be hung.