+/- pressures

[fishin]

someone please explain what positive and negitive pressure on a home is.

[177]

Turn off all fans, close all windows, stand at a door, blow some smoke, open the door. If smoke goes out house is in positive pressure, if smoke comes in house is in negitive pressure.

[fishin]

which do you you want? and why?

[4l530]

http://img292.imageshack.us/img292/2841/fishingpe0.gif

If there is too strong of negative pressure you will get drafts if the home is not well sealed, or gas appliances and fireplaces will backdraft into the home if the home is well sealed, my opinion is that you would want the home at a slight positive pressure in relation to outdoors to drive out unwanted moisture and unconditioned air.

Why do you ask?

[fishin]

just tring to learn, i have seen discussion about this and didn't know what they meant. which are you more likely to have with a tight built home?

[4l530]

With a newer or more tightly built home, unless someone thought to have a way to bring in make-up air I'd say you'd be more likely to be running a negative pressure. You have to consider, if you have gas appliances, you may or may not have
> clothes dryer
> furnace (if not sealed combustion)
> water heater
> Jenn-aire or similar, vent hood over the range
> vent fans in bathrooms
> attic fan
> gas log or fireplace
all pulling air out of your home. The clothes dryer will still exaust air if it is electric, electric furnaces and water heaters will not.
This air being exhausted from the home needs to be replaced somehow, if there is not a means in place for doing this, physics will take care of it by pulling outside air in down the chimneys (backdraft).

edit: There may not be so much air being pulled from the home to really cause problems, when only one or two of these appliances are running, but the home may become strongly negative if all of them are going at once.

[ryand]

or if there are duct leaks that also contributes. With a hole in the supply you are causing a negitive pressure and with a hole in the return a positive pressure

[davidr]

Can lights are also another factor in a home going negative.