Equest Users:
I'm modeling a single story building with 2 packaged rooftop HP units in Las Vegas. I have allowed the program to autosize airflow and cooling and heating capacities.
I'm guessing that the high amount of underheated hours is due to the fact that the PSZ system type uses only one of its assigned zones as the control for constant volume airflow. I have designated the control zone as the zone with the highest internal loads. This works OK in cooling mode, though, I still have about 400 undercooled hours. But in heating mode, the zone with the highest internal loads does not need as much heat as the other zones. Since it's a constant volume system and the control zone is saying "no heat needed", the rest of the zones are not getting any heat. Is that right??
Following through with the assumption above, I tried to increase the minimum cfm/sqft on zone level to 0.5, but my underheated hours went up.
I only manage to decrease unmet heating hours after I increase the cfm on the system level to about 5 times what Trane sizes.
Can someone help me out here?
Thank you!!
Linda Lam, LEED?? AP
You might want to look at your night cycle fan control:
NIGHT-CYCLE-CTRL
Takes a code-word that specifies whether the fans will cycle on at night or on weekends (implied by a 0 in FAN-SCHEDULE) to maintain the temperature setpoint. The fans are off when the hourly values in the schedule that is referenced by FAN-SCHEDULE are equal to 0. If the hourly schedule values are positive, the fans are on and if the hourly schedule values are negative, the fans are not permitted to be on under any circumstances. NIGHT-CYCLE-CTRL also cycles fans on when the temperature goes above the throttling range of the ZONE:COOL-TEMP-SCH. To lock out this feature you must input a -1 in the FAN-SCHEDULE for the summer (cooling) period. NIGHT-CYCLE-CTRL only affects the fan operation. Once the fans have cycled on, the availability of heating or cooling is controlled by HEATING-SCHEDULE and COOLING-SCHEDULE. Allowed values of NIGHT-CYCLE-CTRL are:
STAY-OFF The fans stay off regardless of conditions.
CYCLE-ON-ANY The fans are cycled on for the hour if the temperature of any zone in the system falls below the throttling range for heating,
CYCLE-ON-FIRST The fans are cycled on for the hour if the temperature in the system's control zone falls below the throttling range for heating.
ZONE-FANS-ONLY For a PIU system, the main or central system PIU fan will remain off but the individual zone terminal fans will cycle on separately to satisfy the heating setback temperature for each zone.
The default is to stay off. It sound like in your situation you want it to cycle on any.
Vikram Sami, LEED AP
Linda,
Try changing your control zone to different zones to reduce your underheated and undercooled hours. Instead of zones not getting enough heat, it could be that they are getting overcooled. Check your SS-R and SS-O reports. You might also need to adjust supply flow in specific zones instead of on a system-wide basis.
William Bishop, EIT, LEED? AP
Whenever I have multiple zones served by a single zone AHU this is what I do:
In Detailed Edit -->"Air-Side HVAC" tab--> Select a space--> "Spreadsheet" Tab --> restore to default (right click) any zones that are not the control zone in the "Heat Temp and Cool Temp Schedule" columns. You can also set one zone to be the Cool Temp control zone and another to be the Heat Temp control zone. This eliminates the issue when the zone with the highest cooling demand has the lowest heating demand and vice versa.
Hope this helps!
Seth Spangler, EIT, LEED? AP
Seth
Thanks for your suggestion.
I just wanted to confirm that, by setting all non-control zones to an undefined heating and cooling schedule, I'm basically telling the program, I don't care what the temperatures are in those spaces, so that the unmet heating/cooling hours don't show up. Is that kosher per USGBC?
Where do I find the option to set one zone as cool temp control zone and another zone as heat temp control zone?
Best,
Linda Lam, LEED? AP