There are a few reasons why Sherlock Holmes notices this ventilator.
First of all, it seems very odd that the bell rope is attached to the ventilator and not to anything else.
Second, it seems strange that you would put in a ventilation duct and make it go and connect to another room. You would think that you would want your ventilation to connect to the outside so you could have fresh air. This would be easier too.
Finally, it seems suspicious that Roylott put this ventilator in without being asked and then (after Julia is dead) demands that Helen move into the room with the ventilator as well.