A passive mechanical venting airflow can go either way, depending on prevailing
winds or structural layouts, etc., so 'in' and 'out' flows can be improved by installing
vents on either side of a roof , with trunking going to separate service points or both to
the same one.
In extreme conditions prevent condensation forming by lagging exposed trunking with
Keep the lengths of ductwork to a minimum by laying it as direct as possible.
Prevent kinks and drooping in flexible pipework to avoid pooling.