Simulate a pressure drop


I’m trying to simulate a pressure drop at the entrance and exit of a building (because it isn’t a hole but a grill that can’t relly be taken into the mesh as it is to small). You can take a look at the image to understand the situation (very simplified layout) :

(green are entrance, red exit). From what I’ve read I should use the boundary block, however I don’t really understand how it works and I wasn’t able to find more about it. Also the other issue I’m facing is the mesh not going inside the building as my in and out are simulate with blocks. Any thought on how to solve this issue to?