When Passive Strategies Aren't Enough: Page 2 of 2

More Efficient Active Systems

Inside this Article

The minisplit interior unit
Small and fairly unobtrusive minisplit indoor units require no ducts.
The minisplit exterior unit
The minisplit exterior unit dumps the heat pulled out of the interior. In winter, the unit can run in reverse to produce heat.
A whole-house fan
A whole-house fan exhausts hot air out through the attic space, or directly to the outside. Cooler air is drawn into the home through open windows.
Evaporative cooler
Evaporative coolers work well in hot, dry climates.
The minisplit interior unit
The minisplit exterior unit
A whole-house fan
Evaporative cooler

Attics are the preferred locations for whole-house fans for several reasons. Attics are vented and usually have space for equipment. They also provide a space for hot air to collect. The fan itself is installed in a centrally located hallway ceiling that opens into the attic floor. For houses without attics, homeowners may choose alternate fan locations, exhausting to a garage or crawl space. Most whole-house fans are designed to sit on top of the ceiling joists. Typically, cutting joists is unnecessary—the area directly below the whole-house fan is framed to form a channel for the air and support for the grille or backdraft damper. These self-closing dampers close by gravity and open when the fan’s moving air pushes against them.

While backdraft dampers offer basic protection against debris (and rodents) from entering the house, they have no insulating qualities. These fans can be manually insulated in winter. A more expensive alternative is a whole-house fan with insulated doors. These use electric motors to open and close the doors.

Some whole-house fans use a remote fan connected to the plenum box with flexible ducts, which is a great acoustical attenuator. Pulling air through a duct consumes extra electricity, a consideration when you’re weighing energy use. However, with careful selection of motors, duct size, and fan blades, this energy cost can be minimized.

There are many factors that affect human comfort, from temperature and humidity to individual sensitivity. Based upon user feedback collected by Neil Smith, mechanical engineer and owner of AirScape Fans, a fan that delivers a minimum airflow per bedroom of 500 to 700 cubic feet per minute (CFM) is generally recognized as sufficient.

Computer software can predict energy performance quite well, including how whole-house fans will cool a house. For more information, see “Cheaper, Efficient Cooling with Whole House Fans” in HP140. For more about attic ventilation and attic fans, see bit.ly/AtticFans.

Evaporative (Swamp) Coolers

Probably the oldest form of air cooling, the technique of evaporative cooling dates back to ancient Egypt. People would hang wet mats in their doorways, hoping to make the most of a cool breeze. The richest Egyptians would have slaves fan large clay pots of water to keep the air moist while they slept. Evaporative cooling was also widely practiced in the Americas from colonial times on as people hung wet sheets on their sleeping porches on hot nights.

Today, evaporative coolers work on the same principle. Outside air is drawn over water-saturated pads by a fan. The water evaporates, cooling the air by 15° to 40°F. Larger units can direct the cooled air through the home’s ductwork for effective whole-house cooling. To allow the warm air to escape, you’ll need to have at least one window open on the opposite side of the house.

Evaporative coolers are ideal for hot, dry climates. They cost about half as much to install as central air conditioners and use about one-quarter as much energy—for the fan and a small pump that keeps the pads saturated with water. The moistened pads filter the air to help keep dust and pollen out of the home. The disadvantages are that they require annual maintenance (changing pads), and are only suitable for arid climates.

Evaporative coolers may be installed with or without ductwork. Ducted systems can cool an entire house. If installed without ducts, evaporative coolers are usually placed in a central location as window units. Portable coolers aren’t as effective as whole-house units. But they can cool a room by 5°F to 15°F, and may be used to cut back on air conditioning use at certain times of the year.

Evaporative coolers are rated in cubic feet per minute (CFM). It’s easy to determine what CFM rating you need. First, calculate the square footage of the area you need to cool. Multiply this number by the height of your ceiling, then divide by two. The result is the CFM you’ll need for a complete exchange of air every one to three minutes.


Kathy Kelley writes about energy and environmental topics for the U.S. Navy. She is a regular contributor to Currents, the Navy’s energy and environmental magazine. Last summer, her family installed a minisplit system in their home.

Comments (5)

tyme2par4's picture

I can attest to the value of a mini-split. My parents have had a Daikin unit for a few years now, and I just installed a new Mitsubishi Hyper-Heat unit in my house. The 18 kBTU unit can cool the whole first floor and is almost completely silent. Even the outdoor unit is very quiet.

ewoodhouse's picture

Generally a good article, but I'm dismayed that the author recommends attic fans. Even a casual scan of the building science literature makes it pretty clear that attic fans depressurize an attic and draw conditioned air into it, in most cases increasing energy use. I'm surprised that Home Power would let something like this slip through.

zap101's picture

Attic fans work without hvac, if one opens a lower window can cool down a home in 15 minutes.

Another issue that seems to be passed over is attic fans installed without any additional soffit vents or gable vents ...

zap101's picture

Attic fans used to cool down the home not while using ac is very effective. Think a giant boost to the chimney effect. opening windows in the lower level or basement can make ac not needed.

s premena's picture

Good brief article - I'd like to mention a 'swamp cooler' that does NOT put the humid air into the house - has additional heat exchanger humid air to house air. www.coolerado.com - somewhat more expensive and complicated but keeps humidity and legionarie's bugs out of the house... I like my cool air dry. [I have no financial interest in or experience with the the Coolerado product it just looked like a good idea when I saw it years ago].

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