Unit must be placed in basement at the farthest point away from the stairs that lead to first floor. It draws air from the upper floors and down into basement, across basement floor and then vents out the side of house. This helps control humidity throughout the house, not just in basement. Size of unit is predicated on size of basement. Humidex brand and Wave are different companies but the same concept so look into both. As I said earlier, if you're handy you could build a unit like this pretty easily and save a lot of money. Hope this helps.
As a builder, I have built dozens of sealed, conditioned crawl spaces that typically have a lower Radon level than the surrounding environment, have zero mold or mildew issues, are warm year round in a relatively cold climate, and are as dry as a ghost fart. These buildings have poured concrete foundation walls, a thin concrete floor over plastic sheeting and crushed stone, and spray foam applied to the side walls, from the concrete floor, up the wall and covering about a foot of the wooden sub-floor. They also have a large dehumidifier, running 12 hours a day, on a timer, and draining to the stone below the floor. Finally, they have a remote read temp/humidity sensor to monitor conditions from the interior of the dwelling. My crawl spaces operate at less than 50% humidity and greater than 50*F temps. year round, with no supplemental heat. As for the humidifiers, they seem to cost about $12 a month to operate in my area. The only downside is that they are typically Chinese crap, and getting one to last more than three years is tough.
Your description of how you operate your "Humidex" system has several technical flaws, and potential life safety issues, and honestly, if I bought a home with anything like it, in operation, The very first thing I would do would be to pull the plug on that mess. First, there is a GIANT fire safety issue involved in this idea. Lets say you have a fire in the kitchen (typically located at the top of the basement steps) instead of the fire being contained to the room, you have negative pressure created at the basement door and you are essentially sucking the fire into the basement area. Would you prefer a controllable kitchen fire, or a structural collapse, and potential fatalities, as the entire first floor structure is burned from below? Second, since you have created a negative pressure atmosphere in the basement, you are not only pulling conditioned air from the structure above, but you may be pulling a vacuum on the sub-slab and any cracks in the foundation walls. This could be a great technique to really crank up Radon readings in a Radon prone region. Finally, you are spending the money to condition (heat, cool and dehumidify) air to make your home comfortable, and then pumping large volumes of this air out of the home, in an attempt to dehumidify the basement..............................Sorry, but this concept is a fail-fest, for so many reasons.