I am running a model for an operating room. My goal is to show the system savings for a night set back. I am using a VAV system that is currently running 24/7 at 100% flow. My goal is to show the benefit of running this system at different maximum airflow levels while maintaining constant flow during unoccupied times.
If my OR schedule is 5am to 5pm, I would like the fan to run at 20 ACH during occupied time, 10 ACH during the first 3 hours of unoccupied time, and 6 ACH for the remaining unoccupied time.
Can I achieve this by adjusting fan schedules, or fan parameters for different occupancy settings? Any advice that may point me in the right direction would be helpful.
If three different levels of fan speed are too much to ask, I would be happy with being able to run at 20 ACH during occ. And 6 ACH during unocc.
Crina Bosch
Crina:
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If your system is 100% OSA (whicI i believe it is), you can use the MIN-FLOW-SCH to set an hourly air flow(and thus outside airflow) for your space.
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John R. Aulbach, PE
If it is a recirc unit, a second option would be to use a zone min flow schedule.
Jeremy McClanathan, P.E., BEMP, HFDP, LEED? AP
The system is actually a mixed air system with 25% OA. The zone control is a good idea but I need to set up a maximum flow instead of a minimum. Any thoughts?
Crina Bosch
You can set a max flow with the min flow schedule. It should get you what you?re looking for.
Jeremy McClanathan, P.E., BEMP, HFDP, LEED? AP
The setback strategy you are describing sets a minimum airflow, not a maximum. I don?t believe the actual control sequence would limit the ACH to a maximum value ? if there was heat gain in the space, the system would ramp up to meet the demand. Because the OR suite is unoccupied, the 6 or 10 ACH requirement should set the requirement instead of cooling load.
Set a report variable to monitor the airflow quantity to verify that the system is behaving the way you expect ? if the airflow is limited to a low level according to ACH and not heat gain, then changing the minimum as suggested will model the actual system.
Similarly when the OR is occupied, both ACH and cooling requirements are higher ? most likely the airflow will still be limited by the 20 ACH.
David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
You might want to check on IAQ rules if you are talking a hospital
operating room. You may not be allowed to use mixed air. If you can
there will be restrictions on where it can come from. You will also
have restrictions on the minimums for the OA that will give you problems
when you get down to 6 ACH, ie you might have to run 100% OA. This is
why John assumed 100% OA earlier.
Bruce Easterbrook P.Eng.
What about using the NIGHT-VENT-CTRL - keywords to run the supply fans at
reduced volumes during the unoccupied periods? You would have to play a bit
and set up hourly reports to ensure it is giving you what you are looking
for but might work.
Thank you all for the great ideas, as always.
I ended up taking Jeremy M's advice by setting the min flow schedule at the reduced airflow. The minimum flows did meet the airflow requirements for most of the time. There was about 150 hours that the airflow was different which I assume was from residual heat gain from the space during occupied time. I had to make sure to set my internal loads to a reduced schedule during the first tier reduction so the boxes would back down quickly.
Crina Bosch
Thank you, Crina, for sharing your solution and results. Will use this on my next hospital.
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