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HVAC Equipment Can Overpower Wind and Stack Effect

Downdraft vent fans are very powerful.gifDowndraft exhaust hoods have powerful fans that can depressurize a house.
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Mechanical pressure usually beats natural pressure

Mechanical forces created by fans — either ventilation fans or furnace fans — can completely overwhelm wind and the stack effect. “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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  1. sieb | | #1

    makeup air
    great article but I wished it would tell us how to go about getting makeup air into the house to offset air sucked out by fans.

  2. GBA Editor
    Martin Holladay | | #2

    Makeup air
    Here's my advice:

    1. A powerful range hood or downdraft exhaust fan is incompatible with an energy-efficient house, so don't install one. Install a range hood with a small fan -- ideally, 200 cfm or less -- and use it rarely.

    2. If you insist on installing a powerful exhaust fan, you'll either have to open a window every time you use it or install a makeup air supply. (By the way, it is essential from a safety standpoint that a house with a powerful exhaust fan have sealed-combustion appliances; no atmospherically vented appliances should be used.) Makeup air supply units are available from Shelter Supply:

  3. GH | | #3

    makeup air
    How about makeup air to replace the hot air going up the chimney? I have an oil fired hot air heating system and we often light a fire in the fireplace (mostly for looks, not heat). A recent energy audit showed significant negative pressure when the furnace is running.

  4. amy amster | | #4

    heat exchangers
    I have one less thing to worry about after Fluorotherm (FEP tubing) heat exchangers replaced our old tank(s) heating system. We are saving money. I will be happy to recommend their products.

  5. user-1069992 | | #5

    Another HUGE air-sucker you haven't mentioned.....
    Central Vacuum systems, when used in Winter withdraw monstrous volumes (between 300 and 600 cfm) of heated, moistened air that must be replaced by cold, dry air from whatever source is available, most often through leaks......and perhaps down the exhaust vents of gas, oil and wood-burning appliances.

    A 1500 sq ft home with 8ft ceilings can have its entire air content evacuated by just half an hour of vacuuming. (This entire comment, of course, is based on systems where the canister is located outside the heated area of the house, typically the garage.)

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