Not much point depressurizing your house with a box fan if you cant quantify exactly how much air is leaking into your house, either your not going to depressurize enough to feel anything, or if you have a tiny house you over depressurize and youll never stop sealing beyond reason. You may feel a bunch of air coming out of an outlet but the breach in your air barrier might be 5 stud bays down traveling through your wiring holes. Dont know unless you have a thermal camera.

If you value your time, you will be better off hiring someone with a blower door and thermal camera, find major leaks in an hour, fix and move on.

If you can squeeze the test after drywall hanging but before taping you can pull off a piece of drywall if something major is going on, usually no worries if you have a simple house, problems begin with chases, framing transitions, tub bump outs, soffits, and trades not sealing their pipes and vents, ect.

If your lucky and have a cool drywaller like me they will hang the lid only then let me come in and test for a day before starting walls, otherwise you run the risk of pulling the poly off the ceiling if you do that in your area, although if you mind your pressure you can do it with ceiling poly only, just need to watch it carefully as you ramp the fan up.

It will need to be done after sheetrock, if I do it all. I am almost 2000 miles away and can't just run over and play around in between ceiling and walls, which starts in less than 10 days. I will not return until next summer, when I move to the area permanently.

The walls are filled with R24.5 Demelec open cell foam, and after spending most of September working there, I am reasonably confident there aren't too many air leaks in the walls or first floor, if any. Again, my biggest breach will be range hood, fireplace and 3 bath fans.

I have looked at chimney dampers but most of those are for mason chimneys and are square. I have a 13" round Heatilator brand chimney and have not found much that would fit that particular chimney, but I will continue looking.

hopefully the Heatilator has airtight doors