Posted By sailawayrb on 22 Jun 2013 06:26 PM

For a 2-story plan, you would probably want to open some lower story windows and then EITHER open the upper story clerestory windows (for passive cooling stack effect) or use forced exhaust ventilation located in the upper story (e.g., a whole house ventilation fan). In short, you need a way to get the hot air in the upper story out of the house and replace it with the cooler air entering the house from the open lower story windows. A HRV/ERV system designed with this in mind could get the job done too, but this would ultimately depend on the size of the building (and heat capacity of the furnishings in the building) and the HRV/ERV system air exchange capacity.

A 200cfm HRV/ERV in a 3,200 sqft home should be adequate to create good venting/cool stack effect. Also opening the windows on the 1st and 2nd floor on an open floor plan design should help a lot also.

When it comes to moving heat for an entire house, 200 CFM will have very little effect. 2000+ CFM will.

Yeah, the whole point is that stack effect moves MUCH more air than a fan and with no expenditure of energy. We had planned three clerestory windows to be openers, but upon calculation, we found that having just ONE would move over twice what was needed. You would have to have a very large fan to do the equivalent.

My windows are motorized. In other cases you might want to use motorized dampers, instead, so you still have a gadget, representing maintenance, but the energy necessary to open or close it is NOTHING compared to running a fan for even a short period.

If your design warrants, it's easy to utilize a window for the upper opening, but it could just as easily be a hidden damper high up in the side of a wall or a mushroom vent on the roof.

Stack effect generates very little pressure (say 1.5 pascals in the summer) as compared to a large fan (25+ pascals). In almost all cases, a whole house fan will generate far MORE pressure and flow (at greater cost) than stack effect. So much that you wouldn't consider running it through a single window. How much air either moves is very dependent on the sq feet of opening you have.

An efficient strategy is to use wind and stack effect and only add in a fan (moving air in the same direction) when they aren't sufficient. Then when that isn't sufficient, close the vents/windows and use A/C. This is exactly what I do, but it's all manual (which means it often doesn't get done very well and energy is wasted).

If you run the numbers you will quickly learn that stack effect ventilation rate can be much greater than exhaust fan ventilation rate for even a 2-story building with a significant indoor/outdoor temp differential. Wind effect can also be equally impressive, but you need to have wind for this to work...

There are a couple drawbacks associated with using stack effect ventilation that should first be considered. Stack ventilation rate quickly tends toward zero as the indoor temp approaches the outdoor temp. So you can typically only get the indoor temp to within about 5 F of the outside temp. You first must create a stack in your building in order to use stack effect, and this stack will increase the building heat loss during the heating season. If the less efficient building plan that you selected already results in a stack, you may as well take advantage of it for stack ventilation. However, there is always a significant economic and energy efficiency benefit in minimizing the quantity/size of windows (especially operable windows) which should never be ignored.

Using either stack effect ventilation or exhaust fan ventilation allows any outdoor contaminants (e.g., pollen) to enter the building. Some people with allergies do not like having pollen in their buildings. Some people simply do not like drafty buildings at all, even in the summer. Both of these reasons are why some people prefer passive solar and hydronic radiant floor heating. Most people do not like indoor temp swings and do not like having any extra work or expense to deal with this.

The best solution is to keep the building from overheating in the first place such that you do NOT need much ventilation to maintain the indoor comfort level. A well designed and well constructed/insulated passive solar building located in a climate with significant diurnal temp variation will not have an overheating issue. Nevertheless, if you are located in a non-diurnal climate where the temp or the humidity is continuously above your comfort level, you will still need AC to maintain the indoor comfort level.

An efficient strategy is to use wind and stack effect and only add in a fan

Yeah, you don't need a fan. Design it right. I get huge volume through my windows. It's way more than necessary. The temp that collects up high can easily be over 100F. With an outdoor temp of even 80F, it just erupts out of there when the window is opened, creating a huge draw below.