Registers sit inside what is called a 'boot', which joins the duct work to each space or room in the home. The 'boots' dimensions match the inner dimensions of the inserted vents. While the majority of 'boots' have room for Alea vents, there are some 'boots' that do not have enough depth to support the protruding Alea vent module (which houses all of the electronics and sensors). All of the Alea vents have a depth (sideview) of approximately 2 3/4".
In the image below, you can see a standard boot which has adequate space for Alea (the boot lip is deeper than 2 3/4").
The image below shows a boot that does not support the depth requirements for Alea. You can see that there is no lip on this style boot and the walls taper inwards. While uncommon, it is important to check to see if your boot has flat edges and depth of 2 3/4 inches. This boot lip depth is a requirement for Alea vents.