joeq wrote:Maybe I'm visualizing this incorrectly, but isn't a negative # an actual reverse flow, or "down-draft" which would prohibit any flow outward or up?
In a natural drafting chimney (with no power vents, direct vents or combustion blowers) there are these two following scenarios :
To be technically correct, a negative number actually represents a vacuum in the flue pipe. This vacuum is created from rising warm air in the chimney which is trying to pull air thru the fire chamber.. Since the pressure difference (in this case vacuum) can't be satisfied with incoming primary or secondary air (or leaks in pipe joints or wherever), we get a reading on the manometer. This is of course, is what we want the chimney to do.
A positive number (to be technically correct) would represent pressure in the flue pipe. This positive pressure would be created when a chimney fails and gases in it try to sink back down thru it. Since the pressure difference (in this case positive pressure in the flue and stove) can't be satisfied with outgoing flue gases thru primary or secondary air controls (or leaks in pipe joints or wherever), we get a reading on the manometer the other way. There are many contributing factors that can cause this. This is bad since now carbon monoxide along with the flue gases have an opportunity to leak out of the pipe or stove..
The confusion occurs because many of us set up the Dwyer manometer "backwards" to read negative
draft pressure (vacuum) on the positive
side of the scale. We do this because the positive side of the scale reads higher than we will ever need, while the negative side of the scale only reads down to a -.05" WC.. And then to add to the confusion, sometimes when speaking in terms of negative draft pressure not everyone (including me) will put a "-" in front of their draft number to indicate (to be technically correct) a vacuum.
SO, usually its assumed that a .05" WC is actually meant to be negative pressure value, unless its defined in the post or thread otherwise.
That's kinda messy isn't it...
Sorry, I just woke up