Tuesday, July 29, 2008

natural ventilation = money saved

The time has come to finalize and order a whole house fan, which will help to naturally circulate the air within the house and reduce the need to run the air conditioning system. "Whole house fans" are often confused with "attic fans", but there's a big difference between the two. While attic fans merely exhaust hot air from an unfinished attic space, whole house fans draw cool, outside air into the house through operable windows on the facade and exhaust the hot, overheated air through the exhaust fan directly to the outside. It's important to make sure that you have enough ventilation (open area) when operating a whole house fan. Usually you need to open at least 1-2 windows (preferably on a shaded side of the house) to ensure that enough air can be drawn in and cycle through smoothly. Because these fans are really only effective when the outside air is cooler than the inside air, they're unfortunately not going to help offset those unbearably hot & muggy Philly summer days. They will help the most during the spring and fall swing seasons when the temperature drops at night. By cooling down the house and fully exchanging the stale indoor air with fresh outside air, whole house fans reduce the need for mechanical cooling during the day and- depending on your climate- can allow the elimination of some or all mechanical cooling at night.

Two important things to keep in mind when selecting a whole house fan are:

CFM (Cubic Feet per Minute) - Basically, how much air can it move? In order to determine whether a certain CFM is enough, you'll first need to calculate the approximate amount of cubic feet in the building (SF x avg clg ht). We have roughly 15,000 cubic feet of space in our project. The fan we selected is 800 CFM which means that (after a bit of math) we found out that it would be able to empty the entire house of the inside, stale air in about 20 minutes. Which means that after 20 min of running the fan, theoretically, the house will be totally filled with outside, fresh air. Crazy, huh? Now this is all a little subjective for the ideal "CFM per SF" for one project might be different than another project. You should consult an expert, if necessary. All I can say about our situation is that we could only find one fan to fit that would work and it only has one speed, so there wasn't much of a choice this time. Fortunately, it will work very well for this size house.

and the other factor is...

Sones (unit of perceived loudness) - Essentially, how much noise does it make? To put this in perspective...1 sone = about 40 decibels. But it gets tricky here because the ratio is not 1:1 (i.e. 2 sones do not equal 80 dB) when determining how "loud" a specific sone rating is. "Loudness" ranges from 0 dB (inaudible) to around 200 dB (loudest sound possible). "A quiet library" is around 30 dB and "normal conversation" is around 60 dB. The fan we selected is 3 sones, which actually equals about 48 dB - right in between the two. So, for a fan that moves that much air...well, that's pretty darn quiet! BTW...how loud is the 'loudest sound possible'?

Most whole house fans are designed for sloped roof homes that include a traditional attic space above the conditioned area. Because the fans are not directly exposed to the elements, it's not necessary for them to be weather-tight. In our case, since we have no attic, we are planning to mount the fan in the wall at the top of the stair at the third level which means that it will be oriented vertically rather than horizontally. We will also need the fan to be weather-tight or at least capable of being somehow retro-fitted to prevent cool air loss during the warm months and heat loss throughout the winter months. Through our exhaustive (no pun intended) research (or at least according to Google) there only seem to be three commonly used whole house fan manufacturers. Of these brands, there is only one fan that we've come across that is designed for installation in a location that directly meets the exterior. This model (by RE Williams) includes exterior insulated panels that prevent air leakage and provide a rating of R-6. While our exterior walls will be achieving an R-25 rating on average, this does represent a large reduction in performance which we will have to address in some fashion. Maybe with an additional removable panel on the interior? Oh, and the price of the fan is in the $800 range- a big jump over the typical $300-400 mark. While the "payback" might not be as good as we had hoped, we feel the benefits are worth the extra money.

Another thing to keep in mind is that whole house fans aren't for everyone. They may not work well for individuals who have allergies, as they draw in air directly from the outside (unless, of course, maybe some sort of HEPA filtration is put into place at the point of air intake.) HVAC systems filter the outside air and therefore have a better ability to reduce particulate matter. Whole house fans don't do much to control humidity, either. Some HVAC units integrate dehumidification systems into their cooling systems which can be a big help during the summer. We are currently looking into how much it would be to add this feature onto our system. During times of high humidity, whole house fans can create a cooling effect by creating a light, constant breeze during operation, and as they won't help with the actual humidity, they do increase the overall IAQ (indoor air quality) by circulating fresh air through the entire home. You can also easily run the on/off switch directly through a thermostat that kicks the fan on when it gets up to a certain temp and then turns it off automatically, in order to save electricity.

If you've ever experienced Grandma's whole house fan that almost took your socks off , you'll be happy to know that today's fans are much improved. Think quieter, gentler, smarter, and more efficient than the days of old. Sound like anyone you know?

In the end, we're convinced that a whole house fan will still save money in the long run, despite a higher up-front cost. And while it's by no means the most technologically advanced option out there, it's a least a time-tested strategy that relies on good ol' Mother Nature to provide comfort. For a row house with a relatively small footprint, we think it will make a big difference. One of the toughest things about the long, narrow shape of a row house is getting good cross ventilation since the only openings are usually only at the front and rear. While we're fortunate to be able to have additional windows on one side (due to our lot's placement next to an adjacent utility easement) the fan can only help to promote more of those rare South Philly breezes. And hopefully that's a good thing. :)


Chad said...

Great post with a lot of info. This tactic should be used more often. It would save a lot of A/C bills. With a home sealed as well as yours, that cool air brought in at night will keep the home cool for a long time once it's turned off and sealed back up.

I have seen people create simple boxes for the incoming air where one or more filters can be installed and sometimes a small fan to facilitate intake air. Can't find the link at the moment.

Tanya Seaman said...

Hello -- I'm so glad you're finding a way to cool your house without A/C. My renovation involved improving air circulation (larger rear windows and a skylight) and using ceiling fans in every room. I felt pretty comfortable all summer, and I have no A/C. Certainly mine was lower-tech than yours.