HVAC Equipment Can Overpower Wind and Stack Effect
Mechanical pressure usually beats natural pressure
Mechanical forces created by fans — either ventilation fans or furnace fans — can completely overwhelm wind and the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.. “If we blow air into a building, we pressurize it,” Straube says. “Everywhere. If I have a 10 Pascal pressurization, it will spread through the whole building and everything will be pressurized. And if I negatively pressurize, everywhere around the building will be negatively pressurized.”
High-end houses with mammoth downdraft range hoods often have powerful exhaust fans. Exhausting cooking smoke and odors requires a huge air flow, so downdraft range hoods tend to have fans rated at 1,000 cfm and up.
Big fans can suck air through the walls and floor
“[These are] very powerful fans,” Straube says. “You turn them on and there’s this big wind-up, like a Boeing jet on the runway, and you’ve got to keep the kids away or they’re gonna get sucked onto the stove.”
If you don’t provide makeup air for such a large volume of exhaust, the whole house becomes strongly negatively pressurized. “You start sucking on the garage, and you’ll [breathe] air sucked backwards through your water heater vent. It’ll suck air backwards down your fireplace,” Straube says. “People have died—people continue to die every year—because of things like this.”
If you want to avoid sucking backward through the combustion appliances in your home, you must provide makeup air for your dryer, range hood, and other exhaust appliances.
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