Someone once told me you can tell if someone is an engineer by which direction they put their fan in their window. I'm not sure there's wisdom in this because there are a lot of factors that can effect the amount of air moved. If the window is high in the house and you're trying to remove heat, you'd want it to blow out so that you take advantage of convection. If you're windows are level and you have a cross wind, you'd set the fan to blow in the same direction of the wind.
One thing is for sure; if you put a fan in a window and seal it, you have changed it from just a fan to an air-exchanging fan (or exhaust fan or house fan). You're no longer just recirculating air inside a house, you're exchanging inside air for outside air. This can be used for great benefit in certain climates, including the one I live in now. The temperarature can get to high 90s or even crack 100 (F), but at night the temp will always drop to the 60s. If you have good insulation and sealing in your house, you can leverage this with an exhaust fan by trapping the cool air from the night and sealing up during the day. I always knew about this trick because my Dad did it, but the concept seems lost on a lot of people... even some engineers I know.
Image courtesy of Weatherspark. You can use this data to determine if exchanging air at night will work for you.
Performing this trick just takes timing. You open up the house and turn on the exhaust fan once the temperature outside drops to a comfortable temperature. You leave the exhaust fan on all night with the windows open. In the morning, once the temperature starts to rise, you turn off the fan and seal everything up. Using this method (warning: anecdote), my house will stay comfortable until mid to late afternoon on hot days. We run the AC for just a couple hours at most. I don't have a proper house fan so I'm using one of those crap window fans from target, but it still does the job. The annoying part? Taking the fan in and out, opening and closing windows with the right timing, etc. I assume this is why most people don't do it.
My question is: Why don't HVAC systems already do this? Why can't the whole process be automated? Can an existing HVAC system be modified to do this? It already has a powerful fan and air distribution system. There's a part you need that most home HVAC systems don't generally use and most people aren't familiar with. It's called a Damper and it's nothing more than a valve, but for air.
Here is a system that uses an HVAC system to exchange air between the inside and out. It uses three dampers.
"Outside" is a vent to the outside. Preferably this would be on the coolest side of the house. "Return" is the normal HVAC return vent (the place where you usually replace the filter). "Attic" is a vent to the attic.
With D1 and D3 open (blocking air) and D2 closed (passing air), the system works just like a normal HVAC. Air is blown through the vents into rooms and returns through the return vent and back to the HVAC in a closed circuit.
With D1 and D3 closed and D2 open, the system exchanges air between inside and out. Air is pulled in from the outside by the HVAC fan and blown into the rooms. This creates a positive pressure inside the house, which is then pushed through the return vent, through D3, and out to the attic. This is all without opening or closing windows.
The dampers are electronically controlled and can be automated. This means the entire system can be automated using timing, inside and out temperature measurements, and weather forecasting. Such automation controllers might not yet exist as a commodity, but what would the fun be if I didn't have to hack together a controller?
Pushing air out into the exhaust has another benefit; it cools down the attic space which can get extremely hot and, well, it's better than making another hole in the side of your house.
In order to build this you would need to make at least one large vent to the outside. If you look around your house, you might find it already has some vents to the attic and this is probably where you'd put such a vent. On the roof would be terrible because the roof is HOT. The goal is to pull in cool air. You'd also need a a bunch of ducting and, of course, three of the dampers. The filter would also need to move to a different spot. I'm widly guessing it could easily be done for less than $1,000.
There are a couple problems with the design. HVAC fans usually don't push as much CFM as what's typically desired from an exhaust fan. This design also needlessly pushes air across the HVAC heating and cooling elements, introducing some drag by some amount I don't know and additional dust. The dampers also don't close completely - they leak something like 2% at pressures found in HVAC systems. This isn't a problem except for D3, where attic air could be inadvertently pulled into the house. Not too big of a deal and could probably be offset by leaving D1 cracked slightly open to increase the pressure in the house. Filter placement also changes for hopefully obvious reasons. You want to filter all the air sources to the HVAC system, which puts the filter in the attic itself. I can imagine an access panel that would let you change the filter from inside the house, but this is probably also not a commodity part.
D3 could potentially be a one-way damper; it only allows air to travel in one direction and is passively operated. Anyone know what this is called? You see them in front of exhaust fans.
At this point, this is just an idea, probably not unique. But I do wonder why HVACs don't already do this in certain climates. The majority of people would see large energy savings as the automation would do all the work for them and let them get back to watching bad reality television.
If a house and HVAC system was built from the ground up, even more possibilities open up. Air could be pulled from ground level or even through the ground to take advantage of the massive heat sinking that the earth provides for us just a few meters deep. Check out the usage of a qanat on the Windcatcher Wikipedia. Apparently the persians have been cooling their buildings for thousands of years using windcatchers, solar chimneys, and the cool earth below. All of this without even using a fan. It seems we're in the dark ages of HVAC.
For your amusement, here is an HVAC system I made in sketchup that use convection, air exchange, and copious ducting. Probably not practical. An ideal system would be able to share one fan but bypass the heating and cooling elements. Lot's of possibilities.