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Old 01-26-2007, 04:27 PM   #1
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negative pressure


Can anyone explain negative pressure to me? I am in a condo building and the air flow just seems strange.

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Old 01-26-2007, 08:34 PM   #2
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negative pressure


I'm only familiar with negative pressure in a hospital setting, where one or more rooms have exhaust fans circulating the air to the outside of the structure. These are usually rooms where patients with infectious diseases are placed. What happens is that when the door to the negative pressure room is opened, air from the hallway flows into the room but none of the air in the room escapes into the hallway. [Keeps the little germs corraled.] The air from the neg pressure room is vented to the outdoors. I've been in public structures where the air circulation but never gave much thought about it.

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Old 01-26-2007, 10:36 PM   #3
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negative pressure


At least in some applications I believe negative pressure can be explained as a partial vacuum, or low pressure. In Shapeshifter's example, it would be a mechanism for controlling the direction of air flow. I think it's also the area above an airplane wing that produces lift and allows flight. I'm sure others on this board could enlarge on that.

Shapeshifter, let me take a wild guess. Are you a fan of Tony Hillerman? I note that you are a reader.
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Old 01-27-2007, 06:11 PM   #4
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negative pressure


To vapros, I'm not yet allowed to send a private email but no, I haven't read Hillerman. I am, however, a fan of Kathy Reichs and James Lee Burke.

Not doing much reading these days, my rookie projects are keeping me way busy.
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Old 01-29-2007, 02:20 PM   #5
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negative pressure


In the Pharma industry we use pressures to keep dust in or out of a room. Some rooms are negative to another or some rooms are positive... when you open a door does the air blow out (positive) or get sucked in (negative).
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Old 01-30-2007, 08:28 AM   #6
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negative pressure


The air gets sucked in.
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Old 02-08-2007, 08:02 PM   #7
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negative pressure


Novice, i'm not sure what they do for multi-family residential units, but negative pressure just doesn't seem right. In commercial buildings, they actually try to balance the air flow in with that going out, with maybe 2 or 3 percent positive pressure. So using a special hopper with a flow meter, they measure the air volume, velocity being pulled out by the HVAC return and get that eqaul to or a little less than the air volume, velocity being blown in through the HVAC ducts / diffusers. A building with negative pressure will leak rain in and the walls will sweat when the humidity is high because it pulls outside air in. If you truly want to get it looked into, contact a mechanical engineer. But before you do, because we don't know how old the building is, check your vents to see if they are open and replace the HVAC filter. It is also possible that that the air handler was wired wrong causing the fan motor to run backwards. Turn on your HVAC unit to see if you get air blowing out of the diffusers. Use a candle or a piece of thread to see the direction of the air flow.

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