Air Conditioning, Whole-House Fans

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Keeping Your Cool

A well-designed ventilation system helps you control indoor temperature and humidity, as you have read in Chap 16. With a heat recovery ventilator (HRV) or energy recovery ventilator (ERV) unit added, it can reduce your heating costs and improve overall health and comfort. But as you read in the last chapter, most HRV and ERV units work best with forced-air heating, ventilation, and air conditioning (HVAC) systems. They provide less of a benefit in houses with hydronic or radiant heating.

Houses without forced-air systems generally rely on ceiling fans and window air conditioners for cooling in the warmer months. Unfortunately, ceiling fans only affect the air in one room—and while window air conditioners can cool and dry more air, they are generally expensive to run. This is where a whole-house fan makes a huge difference.

Energy Efficiency

During warmer months, a well-sealed house holds in heat. As soon as the temperature outdoors is higher than it is inside, the house begins to store heat, and stays warm even after outdoor temperatures go down. Outdoor temperatures are generally cooler in the evening, at night, and in the early morning. These are the best times to draw in cooler air to replace stale, heated indoor air.

Depending on the difference between indoor and outdoor temperatures, and on your personal preferences, a whole-house fan

may be all you need to cool your house even at the height of summer. In the hottest climates, it can supplement your air conditioning to give you better cooling at lower cost

The chief benefit of using a whole-house fan is that it provides cooling for large areas at relatively little expense. The initial cost of a whole-house fan can be as little as $150; models with automatic features, higher flow rates, and advanced noise reduction cost more,
but are worth a look. Air conditioners cost at least $250 for small window-mounted models, and central air conditioners start in the thousands.

Operating costs for whole-house fans are much lower as well. Comparative studies of the two cooling methods have shown that air conditioners cost 4 to 20 times as much per hour to run as wholehouse fans. Granted, whole-house fans do not remove moisture the way air conditioners do, but the difference in operating cost remains significant. And whole-house fans may be better for your health.

Health Considerations

If you’re like most people, you spend most of your time indoors. And although you may notice outdoor airborne pollution such as smog, exhaust fumes, smoke, and so on, indoor air is more likely to contain substances that can harm your health. For example, the following pollutants are much more common indoors, and in greater concentrations
than outside:

• Asbestos
• Combustion gases
• Formaldehyde
• Lead
• Pesticides
• Radon
• Tobacco smoke

A publication from the U.S. Environmental Protection Agency (EPA) (www.epa.gov/iaq/pubs/insidest.html) includes this warning:

“Air inside our homes is up to 100 times more polluted than outdoor air. If too little outdoor air enters a dwelling, microorganisms can collect to levels that pose health and comfort problems. This problem can be especially serious in the energy efficient homes built
in the last twenty years. These homes may not allow enough air changes, keeping contaminated air in and fresh air out.”

In fact, the EPA lists indoor air quality as one of the five most urgent environmental risks to public health. Other organizations, including the National Academy of Sciences Institute of Medicine, also cite indoor air quality as a leading public health concern. While you can reduce the number and volume of these pollutants, the best way to handle them is to keep the air moving.


How Air Flows through a House

Older houses are generally not well sealed. That’s partly because traditional building techniques and materials were less airtight, and partly because of the way structures age. In warmer climates, where heating concerns have been less of a priority for homeowners, houses can be almost porous.

Houses like these allow outdoor air to enter in several ways. One kind of airflow is called infiltration. That’s where outdoor air enters the house through intentional and accidental openings, such as joints between walls, floors and ceilings, and around the edges of doors and windows. Another kind of airflow is called natural ventilation, which just means the movement of air through windows and doors as a result of wind, or differences in temperature and pressure. The third kind of airflow is mechanical ventilation—forced-air systems, air conditioners, exhaust fans, and so on.

If your house has many air leaks, you get less control over the indoor environment. But if your house is well sealed against air leaks, not much indoor air goes out, and not much outdoor air comes in, which results in stale, uncomfortable, and potentially unhealthy air.
The ideal balance is a “tight envelope” with a system in place to circulate the air on purpose.

Calculating Air Exchange Rates

The amount of time it takes to replace indoor air with outdoor air is called the air exchange rate. The Home Ventilation Institute (HVI) recommends installing a whole-house fan, also called a comfort ventilator, powerful enough to replace the air in any room within two minutes.

A simple formula for calculating this airflow is to multiply the square footage of your house, including unoccupied areas, by the height of the ceiling. In this example, that measurement is eight feet. Choose a fan that moves at least half that figure in cubic feet per minute at 0.1 inches static pressure. Here are three calculations using this formula:

• 1000 square feet × 8 feet = 8000 × 0.5 = 4000 cubic feet per minute (cfm)
• 2000 square feet × 8 feet = 16000 × 0.5 = 8000 cfm
• 3000 square feet × 8 feet = 24,000 × 0.5 = 12,000 cfm

As you can see, larger houses require much higher volumes of air to be moved. Some whole-house fans are available in these greater capacities, but their cost is generally higher as well. You can also supplement a smaller whole-house fan by using your existing ceiling
fans or an oscillating fan in each room.

Another consideration is where the indoor air will go. If you exhaust a whole-house fan into an attic space rather than directly outside, you will need somewhere for the air to go until it disperses through roof, gable, and soffit vents. To find the exhaust area you need in square feet, divide the capacity of your whole-house fan by 750. For example:

• 1000 cfm ÷ 750 = 1.50 square feet
• 2000 cfm ÷ 750 = 2.67 square feet
• 3000 cfm ÷ 750 = 4.00 square feet

If your whole-house fan vents directly outside, these figures will not matter. In fact, a powerful fan can create enough negative pressure within a house to make it uncomfortable. Be sure you have an adequate supply of incoming fresh air before you switch the fan on.

Features and Options

Traditional whole-house fans operate at one speed, and are controlled by a single switch. Newer models offer a range of choices, from multiple speeds to automatic controls and motorized dampers.

A fan with optional high-speed operation will let you cool a heated house quickly, then turn down the speed for normal circulation. A timer or thermostatic switch will let you choose whether to let the fan switch on or off by itself. And motorized dampers help prevent
air from flowing back through the fan when it is idle. All of these are impressive, but remember to choose the features you believe you will really use.