I made some diagrams and had lots of coffee this morning.
I'm ready to take another stab at this.
I will probably re state a few things to keep train of thought moving in the right direction
How much coal is wasted per day thru a barometric damper? When Larry asked this, I assumed the waste of coal would be due to the need for more heat, since the baro would promote excess cold air infiltration into the home. So, it turns out that based on what Carbon12 posted, the amount of air required for the barometric could be much more than anticipated earlier in this thread. Its true that all
of the air that the stove and baro uses ultimately comes from cold air infiltration. Its also true that the house has its own cold air infiltration going on due to stack effect. There is warm air pushing against the ceiling making its way out thru gaps, cracks and every unsealed light fixture, attic door and so on, up high in the house. At the same time, cold air is infiltrating the bottom of the house in an equal
amount the same way.
of the air that the stove and baro use additional
air infiltration? Or could it be that only part
of the air that the stove uses is additional air infiltration? And if the latter question is true, where would the other part of the air that the stove uses come from?? These few questions I want to challenge since it very well could be a significant difference..
So here is what I've come up with:
The neutral pressure plane (NPP) is a point in the house where the pressure inside exactly equals the pressure outside. It makes a horizontal line somewhere in the house. A house that is equally sealed on top and bottom will have a NPP somewhere in the middle of it like in the diagram below. In this demonstration, the temperature difference between inside and outside (whatever they may be) causes a stack effect in the house with the pressures I listed on the movable plane.. (Please pardon the crudity I didn't have time to make it to scale lol) You can see we have a -.015 at the floor and a +.015 at the ceiling.
Now, lets turn on an exhaust fan or run a heating appliance that requires a chimney that will take air out of the house. Lets say the result is a drop in pressure in the entire house of a -.005 ... Since the NPP always equals exactly the pressure outside look what happens in the diagram below.
What happens is, the NPP moves up! So what right? Well, now we have a -.020 at the floor and a +.010 at the ceiling. So what does that mean? Since pressure dictates the volume of air coming in at the floor and also dictates volume of air going out at the ceiling we have more
cold air infiltrating than what is leaving the house at the ceiling. Time for another demonstration to explain how all this results.
For simplicity, lets say stack effect by itself in the house is turning over 5000 cubic feet per hour. There is 5000 cf/hour coming in at the floor and 5000 cf/hour leaving the house at the ceiling. Now lets fire up the coal stove. For simplicity lets assume the coal stove with its baro sends 1000 cf/hour up the chimney. Like an exhaust fan, the chimney (with its lighter air instead of rotating blades - same effect) is pumping air out of the house which drops the entire house's pressure and moves our NPP up higher. With our NPP higher, less air is leaving the house at the ceiling and more air is coming in at the floor. How can more air come in at the floor than what is leaving at the ceiling?? (This would violate Air in= Air out). The stove and baro is using it. Only part of the air that the stove is using is additional
cold air infiltration to the infiltration that's already happening due to stack effect in the house, the other part is air that would infiltrate regardless if the stove is in use or not. In this demonstration, our house is equally sealed on top and bottom. The stove's usage is 500 cf/hour that is additional
cold air infiltration (for a total of 5500 cf/hour coming in at the floor) because of the increased negative pressure there, and the other 500 cf/hour is air that was held back
from escaping (total of 4500 cf/hour leaving at the ceiling) due to the equal amount
of decreased positive pressure at that location. The part that is held back is the same part
that would leave the house when the stove is not being used.
And this is how I have come to the conclusion that half of the total air that the stove and baro use is additional cold air infiltration. Why precisely half? Because the deficient pressure effect caused by the stove and baro would be equally dispersed across the entire house envelope.
I'm gonna go put some coal on the fire now