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Why It Works?

Larson Fan Company ~ Save money on your heating and cooling bills. The damper feature in our products is the reason why. Move your mouse over the image below to see what happens. The average bathroom has one outlet from the heating and central air conditioning equipment in a typical forced air system. When that equipment is running, the same amount of air that is entering that room is exiting that room to equalize the air pressure. This forced air, like electricity and water, will take the path of least resistance, and when the bathroom door is closed; most of it will be through the gap between the door and the floor, but not without some resistance. That resistance will cause a slight increase in air pressure inside the bathroom that will easily open the back draft flap mounted just outside of your current ventilation fan housing and become another exit pathway for the air pressure in the bathroom to equalize. In the winter when the forced air furnace is running, you are pushing some of the warmest air in the bathroom straight outside thought the exhaust vent. In the summer, when the central air is running, again you are pushing the cool, dry air inside your house outside. Most back draft flaps do not seal very well (see pictures below) and when it is hot and humid outside, they let the moisture of the humid air outside to wick into your house, causing your air conditioning equipment to work longer and more often. Also when it is windy outside, the wind blowing across the exhaust port of the ventilation fan exhaust duct(s) will create a Venture effect, and will draw more air from inside your home outside. The stronger the wind, the more inside air will be pulled out of your house.

Larson Fan Company Larson Fan Company

Most back draft flaps hang vertically in the ductwork connection terminal when the fan is not running, like the one on the left (back lit to highlight the gaps), closing most of the opening, but not sealing it. The recently purchased NuTone model using 9 x 9 housing on the right has a back draft flap that at rest hangs open as shown.

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