Hello Jorge.  Whereas the unit senses pressure differentially, to work as designed it requires both ports to be connected and the differential pressure between the two ports sensed.  When the differential pressure falls outside of the set range; 0.2-7" water colum than the SPDT switch actuates from its resting position.  On the Technical Data" tab of the product page we have the product data sheet and the Instruction Manual.  Neither answer your question directly. Whereas this part can have a crucial impact on safety of the equipment and environment it is protecting, and as each application use is different, we must refer you to these documents for more information.  Moreover, we strongly suggest that ONLY licensed and qualified technicians work of installing and adjusting such equipment.