Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
@day9914, This measure was made when Air Mixing was added to OpenStudio, and to test and demonstrate an alternative approach to modeling zone boundaries in core and perimeter models where the zone boundary doesn't represent a physical wall. Traditionally there is just conductive heat transfer, but this measure instead changes the boundary condition to adiabatic and only uses air mixing to thermally connect the zones. This measure just applies a simple formula with a user adjustable coefficient (cross mixing coefficient) to apply a multiplier to the auto-calculated files. A number of factors that may not be evident from the model inputs will impact the actual realized air mixing in the building, and this measure does not attempt to determine a precise value.
The basic formula looks at the zone volume, area of the connecting surface, and the zone height. Using the default coefficient of 1, a space that is as high as it is deep (in distance away from the connecting wall) will have about 1 ACH of mixing. If you increase the depth by a factor of 4, the ACH will drop in half, although because of the volume there will still be twice as much CFM as with the original space. There could certainly be better formulas, but this was just something to start with.
In the linked documentation below, I did run a proottype medium office in Houston, and then swept from 0 to 10 for a air mixing coefficient. Using a coefficient of 0 (no air or conductive transfer) resulted in a 1% decrease of energy consumption versus the prototype building, while a coefficient of 10 resulted in a 2% increase in consumption. Somewhere between 1 and 2.5 for the coefficient had a resulted in a similar EUI as the un-altered prototype building. But in all cases the peak load was higher with air mixing than in the baseline model. The lowest increase in peak load is with a coefficient of 0.5 to 1.0. It would be nice to sweep across climate zones and target standards, as well as measured data, but I never got back to do that.
The measure is distributed with a PDF, but that had some inconsistencies and outdated values in it. I have a newer markdown file, which I have linked to below. This will eventually ship with the measure. The linked markdown document doesn't have images, so I have included them below.
https://www.dropbox.com/s/912w293aaerryav/README%20-%20AIr%20Wall%20Zone%20Mixing.pdf?dl=0 (looks like I can paste markdown here, so I'll put contents of linked file)
air mixing rate (CFM) = zone mixing coefficient * zone volume / sqrt(zone volume / (air wall area * zone height))
Example A:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 10 ft)) = 4000 ft^3 per hour = 1 ACH and 66.67 CFM
Example B:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (400 ft^2 * 10 ft)) = 8000 ft^3 per hour = 0.5 ACH and 133.3 CFM
Example C:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 40 ft)) = 8000 ft^3 per hour = 2 ACH and 133.3 CFM
Example D:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (1600 ft^2 * 40 ft)) = 32,000 ft^3 per hour = 2 ACH and 533.3 CFM
| | 2 | No.2 Revision |
@day9914, This measure was made when Air Mixing was added to OpenStudio, and to test and demonstrate an alternative approach to modeling zone boundaries in core and perimeter models where the zone boundary doesn't represent a physical wall. Traditionally there is just conductive heat transfer, but this measure instead changes the boundary condition to adiabatic and only uses air mixing to thermally connect the zones. This measure just applies a simple formula with a user adjustable coefficient (cross mixing coefficient) to apply a multiplier to the auto-calculated files. A number of factors that may not be evident from the model inputs will impact the actual realized air mixing in the building, and this measure does not attempt to determine a precise value.
The basic formula looks at the zone volume, area of the connecting surface, and the zone height. Using the default coefficient of 1, a space that is as high as it is deep (in distance away from the connecting wall) will have about 1 ACH of mixing. If you increase the depth by a factor of 4, the ACH will drop in half, although because of the volume there will still be twice as much CFM as with the original space. There could certainly be better formulas, but this was just something to start with.
In the linked documentation below, I did run a proottype medium office in Houston, and then swept from 0 to 10 for a air mixing coefficient. Using a coefficient of 0 (no air or conductive transfer) resulted in a 1% decrease of energy consumption versus the prototype building, while a coefficient of 10 resulted in a 2% increase in consumption. Somewhere between 1 and 2.5 for the coefficient had a resulted in a similar EUI as the un-altered prototype building. But in all cases the peak load was higher with air mixing than in the baseline model. The lowest increase in peak load is with a coefficient of 0.5 to 1.0. It would be nice to sweep across climate zones and target standards, as well as measured data, but I never got back to do that.
The measure is distributed with a PDF, but that had some inconsistencies and outdated values in it. I have a newer markdown file, which I have linked to below. This will eventually ship with the measure. The linked markdown document doesn't have images, so I have included them below.
https://www.dropbox.com/s/912w293aaerryav/README%20-%20AIr%20Wall%20Zone%20Mixing.pdf?dl=0 (looks like I can paste markdown here, so I'll put contents of linked file)
air mixing rate (CFM) = zone mixing coefficient * zone volume / sqrt(zone volume / (air wall area * zone height))
Example A:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 10 ft)) = 4000 ft^3 per hour = 1 ACH and 66.67 CFM
![image description]
Example B:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (400 ft^2 * 10 ft)) = 8000 ft^3 per hour = 0.5 ACH and 133.3 CFM
Example C:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 40 ft)) = 8000 ft^3 per hour = 2 ACH and 133.3 CFM
Example D:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (1600 ft^2 * 40 ft)) = 32,000 ft^3 per hour = 2 ACH and 533.3 CFM
| | 3 | No.3 Revision |
@day9914, This measure was made when Air Mixing was added to OpenStudio, and to test and demonstrate an alternative approach to modeling zone boundaries in core and perimeter models where the zone boundary doesn't represent a physical wall. Traditionally there is just conductive heat transfer, but this measure instead changes the boundary condition to adiabatic and only uses air mixing to thermally connect the zones. This measure just applies a simple formula with a user adjustable coefficient (cross mixing coefficient) to apply a multiplier to the auto-calculated files. value. A number of factors that may not be evident from the model inputs will impact the actual realized air mixing in the building, and this measure does not attempt to determine a precise value.
The basic formula looks at the zone volume, area of the connecting surface, and the zone height. Using the default coefficient of 1, a space that is as high as it is deep (in distance direction away from the connecting wall) will have about 1 ACH of mixing. If you increase the depth by a factor of 4, the ACH will drop in half, although because of the volume there will still be twice as much CFM as with the original space. There could certainly be better formulas, but this was just something to start with.
In the linked documentation below, I did run a proottype prototype medium office in Houston, and then swept from 0 to 10 for a air mixing coefficient. Using a coefficient of 0 (no air or conductive transfer) resulted in a 1% decrease of energy consumption versus the prototype building, while a coefficient of 10 resulted in a 2% increase in consumption. Somewhere between 1 and 2.5 for the coefficient had a resulted in a similar EUI as the un-altered prototype building. But in all cases the peak load was higher with air mixing than in the baseline model. The lowest increase in peak load is with a coefficient of 0.5 to 1.0. It would be have been nice to sweep across climate zones and target standards, as well as measured data, but I never got back to do to that.
The measure is distributed with a PDF, but that had some inconsistencies and outdated values in it. I have a newer markdown file, which I have linked to pasted below. This will eventually ship with the measure. The linked markdown document doesn't have images, so I have included them below.
https://www.dropbox.com/s/912w293aaerryav/README%20-%20AIr%20Wall%20Zone%20Mixing.pdf?dl=0 (looks like I can paste markdown here, so I'll put contents of linked file)As a note, while this measure focuses on cross mixing of equal values between zones, there is a different measure that deals with zone ventilation and air mixing in one direction from external zones to and then up through an atrium.
air mixing rate (CFM) = zone mixing coefficient * zone volume / sqrt(zone volume / (air wall area * zone height))
Example A:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 10 ft)) = 4000 ft^3 per hour = 1 ACH and 66.67 CFM
![image description]
Example B:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (400 ft^2 * 10 ft)) = 8000 ft^3 per hour = 0.5 ACH and 133.3 CFM
Example C:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 40 ft)) = 8000 ft^3 per hour = 2 ACH and 133.3 CFM
Example D:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (1600 ft^2 * 40 ft)) = 32,000 ft^3 per hour = 2 ACH and 533.3 CFM
| | 4 | No.4 Revision |
@day9914, This measure was made when Air Mixing air mixing was added to OpenStudio, and OpenStudio to test air mixing and demonstrate an alternative approach to modeling zone boundaries in core and perimeter models where the zone boundary doesn't represent a physical wall. Traditionally there is just conductive heat transfer, but this measure instead changes the boundary condition to adiabatic and only uses air mixing to thermally connect the zones. This measure just applies a simple formula with a user adjustable coefficient (cross mixing coefficient) to apply a multiplier to the auto-calculated value. A number of factors that may not be evident from the model inputs will impact the actual realized air mixing in the building, and this measure does not attempt to determine a precise value.
The basic formula looks at the zone volume, area of the connecting surface, and the zone height. Using the default coefficient of 1, a space that is as high as it is deep (in direction away from the connecting wall) will have about 1 ACH of mixing. If you increase the depth by a factor of 4, the ACH will drop in half, although because of the increased volume there will still be twice as much CFM as with the original space. There could certainly be better formulas, but this was just something to start with.
In the linked documentation below, I did run a prototype medium office in Houston, and then swept from 0 to 10 for a an air mixing coefficient. Using a coefficient of 0 (no air or conductive transfer) resulted in a 1% decrease of energy consumption versus the prototype building, while a coefficient of 10 resulted in a 2% increase in consumption. Somewhere between 1 and 2.5 for the coefficient resulted in a similar EUI as the un-altered prototype building. But in all cases the peak load was higher with air mixing than in the baseline model. The lowest increase in peak load is with a coefficient of 0.5 to 1.0. It would have been nice to sweep across climate zones and target standards, as well as measured data, but I never got to that.1.0.
The measure is distributed with a PDF, but that had some inconsistencies and outdated values in it. I have a newer markdown file, which I have pasted below. This will eventually ship with the measure. As a note, while this measure focuses on cross mixing of equal values between zones, there is a different measure that deals with zone ventilation and air mixing in one direction from external zones to and then up through an atrium.
air mixing rate (CFM) = zone mixing coefficient * zone volume / sqrt(zone volume / (air wall area * zone height))
Example A:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 10 ft)) = 4000 ft^3 per hour = 1 ACH and 66.67 CFM
![image description]
Example B:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (400 ft^2 * 10 ft)) = 8000 ft^3 per hour = 0.5 ACH and 133.3 CFM
Example C:
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 40 ft)) = 8000 ft^3 per hour = 2 ACH and 133.3 CFM
Example D:
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (1600 ft^2 * 40 ft)) = 32,000 ft^3 per hour = 2 ACH and 533.3 CFM
