HVAC Equipment Can Overpower Wind and Stack Effect

Downdraft vent fans are very powerful.gif
Downdraft exhaust hoods have powerful fans that can depressurize a house.
[Click to enlarge]

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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5.
Sun, 12/18/2011 - 14:34

Another HUGE air-sucker you haven't mentioned.....
by Michael Scott

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.)


4.
Thu, 07/08/2010 - 18:44

heat exchangers
by amy amster

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.


3.
Thu, 02/11/2010 - 22:54

makeup air
by GH

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.


2.
Tue, 02/09/2010 - 11:32

Makeup air
by Martin Holladay, GBA Advisor

Sieb,
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:
http://www.sheltersupply.com/corporate/default.asp?cwpID=16


1.
Tue, 02/09/2010 - 11:13

makeup air
by sieb

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.


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