Since I already had my house blower door tested, I know how much air it takes to get to 50 pascals and have a vague idea of how much air was coming down through the open flue vent on my furnace when I put my hand inside. I thought of doing something similar but decided to use two box fans in each room, sealing off the rest of door space(s) or openings to the room and pressurizing the room to look for leaks.
It won't be a diagnostic tests without the duel magnahelic setup if you use a fan or blower so you really don't need a blower for this. If you use a furnace blower, you have the ability to put quite a lot of pressure on your house as they are designed to push well over 1 inch water column, which in itself is 249 pascals. If you don't have a way of measuring the pressure you could cause an issue. Also be sure that your furnace, water heater, etc gas appliances are all turned off(pilot on a modern water heater should be fine as long as you check it is still lit after you've used the blower door). If you have a fireplace, clean out ALL soot as you'll make a terrible mess as it backdrafts past the flue you though was sealed well.
My recommendation, don't do it. Use a standard axial fan setup (like two box fans taped on top of eachother with the rest of the door plastic sealed) because they can only push a small amount of pressure due to their design, that's all you need. A furnace blower can pull quite a bit of pressure with no real way of knowing how much pressure is really there.
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