A few years ago I installed a whole-house fan in the ceiling of an upstairs closet of my home, hoping to ventilate the house when it was cool outside. The property was surrounded by giant trees and there was rarely a good enough breeze to circulate air through the house, so I opted to do it mechanically.
I thought I was being so clever, putting the noisy fan inside a closet to muffle the noise. Well, the sound was indeed muted ~ but I never used the fan! Turns out, Vicksburg, Mississippi is extremely humid all year long, even in the fall and spring. It might feel cool enough outside to flush out the house with clean air, but – the humidity!
Humidity is a big issue with air conditioning. You can’t cool down air until the humidity is lowered. Or: at least half of the energy needed to cool down hot and humid air is used to wring out the humidity. So, my setup replaced all the air in the house with outside air well enough. However, when it got hot outside and I closed all the windows, the air conditioning had to work twice as hard to cool down the air.
I should have listened to my realtor friend, a long-time resident of Vicksburg: “We can only open the windows for a few weeks every year. A few days in the spring and a few days in the fall.” That’s hardly enough operating hours for an all-house fan to be cost-effective.
The only way you can really tell (especially if you live along the Gulf Coast or Deep South) if it’s okay to flush your house air with outside air is a humidistat. There are many inexpensive digital models available, typically packaged with a thermometer. Test the humidity of the air in your house, and then the outside air. If the outside air is cooler and dryer ~ go for it!
If you live in an arid region, like the southwest or west, and the summers are dry, a whole-house ventilation fan can be used almost daily in the warm season. Even if it gets very hot outside during the day, the diurnal swing is very large in an arid climate. (i.e., it gets very cool at night.) Running a house fan for a few hours each night when the temperature is much cooler is an easy way to save AC costs. Plug the fan into a digital timer to automate the process.
Think about de-humidification in terms of the weather. When a cold front moves in during the hot, humid summer ~ it rains, sometimes very hard, before the air physically cools down. Dehumidification on a grand scale!
Otherwise, if your home has a high thermal mass, it may take all day for the inside temperature to get uncomfortable (dry air always feels cooler), so you can get by without the AC altogether. In fact, my present house in North Carolina has so much thermal mass on the outside walls (books, lots of books), I run the AC very cool at night when it’s cool outside and the system operates more efficiently – then turn the AC off all day, and the temperature inside doesn’t become uncomfortable until very late in the day.
“Thermal mass” projects function on the notion that the air inside a building doesn’t physically increase until all the objects inside the building envelope are warmed up: books, walls, furnishings, appliances, etc. Similarly, for cooling down at night during the winter months.
__________
Excerpted from Going Green, available as an eBook at this list of eBook retailers online.