Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
The trouble with autosizing equipment is that you don't know how the equipment will respond to actual conditions encountered during the simulation. For this reason, users enter a supply air temperature (and humidity ratio) in the Sizing:Zone object. The supply air temperature and zone set point temperatures are used "size" the zone air flow rate needed to meet the zone load. Using this air flow rate and the corresponding coil inlet air (mixed) condition (which the simulation tracks during sizing) along with the SAT conditions entered in the Sizing:Zone (or Sizing:System depending on the equipment type), the coil capacity can be calculated.
From the simulation, look at the zone entering air temperature(s) and compare that temperature to values entered in Sizing:Zone and/or Sizing:System. Adjust the sizing SAT values as necessary. This can be an iterative process since changing these values will change the zone air flow rate, and changing the zone air flow rate can change the mixed air temperature if OA is included in the simulation.
| | 2 | No.2 Revision |
The trouble with autosizing equipment is that you don't know how the equipment will respond to actual conditions encountered during the simulation. For this reason, users enter a supply air temperature (and humidity ratio) in the Sizing:Zone object. The supply air temperature and zone set point temperatures are used "size" the zone air flow rate needed to meet the zone load. Using this air flow rate and the corresponding coil inlet air (mixed) condition (which the simulation tracks during sizing) along with the SAT conditions entered in the Sizing:Zone (or Sizing:System depending on the equipment type), the coil capacity can be calculated.
From the simulation, look at the zone entering air temperature(s) and compare that temperature to values entered in Sizing:Zone and/or Sizing:System. Adjust the sizing SAT values as necessary. This can be an iterative process since changing these values will change the zone air flow rate, and changing the zone air flow rate can change the mixed air temperature if OA is included in the simulation.
When the chiller and boiler are sized, all water coil equipment is sized (similar to description above) and then the required coil water flow rate is reported to the plant. These values are summed and used to size the chiller/boiler using the plant delta T specified in the Sizing:Plant object. Since all coils will likely not be at the peak load at the same time, the boiler will be oversized. You could add a sizing factor in the chiller and boiler as, for example, 0.75 for a diversity factor since you know the capacity is oversized since all coils do not peak at the same time.
| | 3 | No.3 Revision |
The trouble with autosizing equipment is that you don't know how the equipment will respond to actual conditions encountered during the simulation. For this reason, users enter a supply air temperature (and humidity ratio) in the Sizing:Zone object. The supply air temperature and zone set point temperatures are used to "size" the zone air flow rate needed to meet the zone load. Using this air flow rate and the corresponding coil inlet air (mixed) condition (which the simulation tracks during sizing) along with the SAT conditions entered in the Sizing:Zone (or Sizing:System depending on the equipment type), the coil capacity can be calculated.
From the simulation, look at the zone entering air temperature(s) and compare that temperature to values entered in Sizing:Zone and/or Sizing:System. Adjust the sizing SAT values as necessary. This can be an iterative process since changing these values will change the zone air flow rate, and changing the zone air flow rate can change the mixed air temperature if OA is included in the simulation.
When the chiller and boiler are sized, all water coil equipment is sized (similar to description above) and then the required coil water flow rate is reported to the plant. These values are summed and used to size the chiller/boiler using the plant delta T specified in the Sizing:Plant object. Since all coils will likely not be at the peak load at the same time, the boiler will be oversized. You could add a sizing factor in the chiller and boiler as, for example, 0.75 for a diversity factor since you know the capacity is oversized since all coils do not peak at the same time.
