[MN Renovator]

I did a little test because of this thread. I took two box fans and mounted them in a second story bedroom and opened the kitchen window on the opposite side of the house. Turned on the fans and walked outside, good breeze coming out. So I closed the kitchen window and listened to the fans load up a bit since they weren't able to push the same amount of air. I went to the basement and grabbed a small wad of spider web and held it near the draft of the water heater and it was sucked right in. That's all it took to put pressure on my house. I have a total of four box fans that I usually use when trying to circulate a whole bunch of air in one side of the house and out the other when the temperature outside is far more favorable than the temperature inside and it works well. I figured I'd need all 4 to get good pressure but I'm thinking that it is probably not needed and I think I was underestimating what two cheap box fans can do with a house that tested out as 1500 cfm50. I'm still not sure how accurate two magnahelics, a tarp, a chart to do the math for CFM, and a fan that looks like it came out of a hardware store used on a house with atmospheric ventilation can be compared to a blower door that can use progressively smaller openings to get accurate results for less CFM. Makes me wonder how these can cost $3k when they are so simple.

Either way, your makeshift attic fan might do the trick to pressurize the house blowing in to watch a smoke pen suck the smoke out, if you can't see air getting sucked out of a known leaky area, add your furnace blower and turn it up until you can see the smoke exiting your known leaky spot and I think that should be fairly safe if you are careful about it.