The way to calculate this is to use the formula from ASHRAE 62.2 (2010). [Even though
the 2013 Standard has been out for some time, most programs are still using 2010.]
By this Standard, the number of cubic feet per
minute (CFM) of fresh air required for, say, a 3 bedroom, 2,000 square foot home is ...
CFM = (conditioned floor area x .01) + (7.5 x [# of bedrooms + 1])
CFM = (2,000 x .01) + (7.5 x 4)
CFM = 20 + 30 = 50 cubic feet per minute
Now you have the choice of 50 CFM for 60 minutes of each hour or 100 CFM for 30 minutes each hour. Most people go with the latter - "intermittent" ventilation.
What type of system do you want - exhaust, supply or balanced ?
Exhaust systems suck indoor air from the house and it is replaced by "fresh" air infiltrating to replace it. Bathroom exhaust fans are typically used.
Supply systems bring fresh air into the house, forcing out the funky indoor air.
Balanced systems use two fans, one to exhaust stale air from kitchens and bathrooms and another to supply an equal volume of fresh air.
So what are HVAC contractors using down here in the deep South ?
Exhaust systems, which are the least expensive (you've got to put bathroom fans in a new house anyway) are a bad idea. Since replacement air comes in by infiltration, you can't control the source, so you might be pulling in dirty, hot, humid air.
Balanced systems are the most expensive, so they are usually installed only if the homeowner demands one.
Supply systems are a good choice for hot, humid climates. A fresh air duct runs from a selected spot on the outside of the house (where the air is clean) and usually terminates at the return plenum.
The HVAC system fan pulls in the fresh air, which is then cooled and dried by the evaporator.
Two supply systems in common use locally are marketed by Aprilaire and by AirCycler - and they do pretty much the same thing.
The Aprilaire Model 8126A (shown here) has a motorized damper, which is incorporated into the fresh air duct, and a timer to control when it operates.
The volume of air taken in is determined by the number of minutes each hour you set it to run. Ventilation occurs during heating and cooling calls, but if the thermostat does not initiate heating or cooling for the required number of minutes, the controller will turn on the HVAC fan long enough to make up the difference.
The ventilation controller, purportedly, will not let outside air enter if it could potentially raise the humidity indoors to high levels. The controller can also prevent ventilation in extremely high or low temperatures.
A level of sophistication can be added by installing an ERV (Energy Recovery Ventilator) between the outside air and the HVAC return. Outside fresh air is pre-conditioned by the ERV, as it exchanges moisture and heat with the outgoing air.
And finally,how do you know how much fresh air is actually comingin each minute,
as you need to know this to set the timer correctly to satisfy the ASHRAE requirement ?
The Aprilaire website has a chart showing how many CFMs are entering, depending on duct length and return static pressure - BUT IT'S USELESS. You have to actually measure the real life air flow, using a manometer, to know how many CFMs are coming in. Then you just set the controller to the correct number of minutes per hour.