What would happen?
You have enough leaks under and around doors, or windows that aren’t perfectly shut, or maybe drafts in the attic, imperfectly sealed ventilation equipment, etc. that you’ll lose the pressure somehow.
A very rapid change in pressure (more rapid than a fan would give) can lead to a significant pressure differential between rooms. I’ve been trapped in an apartment before temporarily when a sudden onset of a storm made the pressure in my house significantly greater than the one in the shared hallway, to the point where I couldn’t open the door.
You’ll eventually overheat the attic fans motor. If you’re lucky, it will trip a thermal breaker in the motor. Otherwise, it could ruin the motor.
I have an attic fan and use it every spring and Fall. I use AC when it’s really hot. 
Nothing.
Unless you’re in South Korea, in which case you’ll die.
Had a WHF in my previous house. The fan can’t create enough vacuum to implode the windows. I don’t think it’s a matter of there being lots of leak paths in the house; it’s just the nature of the WHF that it can’t produce much vacuum at zero flow rate.
Note this is not true of all fans. Replace the WHF with a shop vac, and yes, now the leakage points throughout the house matter: if left unblocked, they will provide enough flow to prevent the development of a serious pressure differential. But if you could somehow plug all those leaks, yes, a shop vac could develop enough suction to shatter a window.
windows imploding is the least of your worries. it will suck the sewers dry through the toilets and flood your house.
i agree lack of air flow might lead to the motor overheating.
if you use it with the windows open there will be a breeze so anchor the kids and pets and use paper weights on top of things you don’t want run through your whole house shredder/fan.
using a whole house fan at night can eliminate the need to run air conditioning in moderate climates. at night you open windows and draw cool night air to cool the house and contents. then close the windows and shut off the fan in the morning (8AM), close shades to the south as much as can while still giving good light. your house may stay significantly cooler than the outside and be pleasant until late afternoon when you can open windows and use the fan (or small ones) to keep cool.
How? It’s fully contained in a closed system, and any suction it can develop would be offset by the pressure coming out of the vac’s exhaust. Unless that exhaust is vented to the outside, there’s going to be equilibrium, not broken windows.
My parents have a whole house fan in a hallway with doors on either end (plus bedroom doors and closets). If you run the fan with all of the doors closed, there’s enough negative pressure that it takes noticeably more force to open some of the doors again. It’s not a whole lot – no more than ten or twenty pounds of force – and it probably corresponds to a tiny pressure differential that’s incapable of breaking windows.
Unless you somehow seal everything and have a really powerful fan…
Can you clarify this? I’ve never heard that before.
Houses are designed to exchange air; they’re in no way sealed. I don’t see implosion happening. I think you’d just drastically step up the rate of air exchange.
If you make it a variable-pitch fan controlled by a voice coil, you get the world’s deepest subwoofer: http://www.youtube.com/watch?v=E7Bkrypxzs4
Sorry, I didn’t make my scenario clear. I was imagining replacing the WHF with a shop vac: suction end of vac inside house, exhaust end outside. Shop vac evacuates house, things go moobak.
johnpost raises a forgotten point: if you draw a sufficient vacuum inside your house, air will burble in through one or more of the plumbing traps on your sinks, showers and toilets. This may prevent buildup of sufficient vacuum to rupture windows, unless you regard those drains as leaks and plug them accordingly.
I say “may” because I’m not sure here. what is the maximum water column differential in a residential plumbing trap, maybe a couple of inches? If so, then you could draw a household vacuum of a couple inches of water before the trap limits development of further vacuum. OK, now imagine taking your largest household window, laying it horizontal, and pouring water on top of it to a depth of a couple of inches; that load will be the equivalent of what an installed window will experience when exposed to the aforementioned vacuum. If you’ve got a big window, that water’s gonna weigh a lot. Will the window crack? Maybe.
It makes a big whoooshing sound when it happens.
My folks have a huge whole-house fan in the attic. If you turn it on with all the windows closed what happens is: Nothing special. The louvers outside don’t open as far, but they still open, indicating that the fan is exhausting some air. But, as mentioned above, houses are very leaky and they aren’t going to implode. There’s even a commercial tester that fits in a doorway and measures that air leakage rate: photo here.
Also, a seriously doubt it a whole-house fan could pull water out of a trap. These fans are designed for high-volume, low pressure operation - they aren’t a vacuum pump.
:mad:
I was told that if you run the whole house fan without ventilation, there’s a good chance that you’ll get a nice ashy breeze coming in from the chimneys. I haven’t tried it out to test, though.
In the Prairies, it cools off fast at night, and my father would take advantage of that: open all the windows and the basement door overnight, and close everything early in the morning. Then we’d stay cool until afternoon and start the furnace fan later on to bring up that cool air from downstairs.
Never had trouble with the plumbing, though! 
Ah, that makes sense. I was picturing a shop vac sitting in the room, not replacing the WHF.
I was also picturing a smallish shop vac. Do they make them big enough to substitute for a WHF?