Why the value of "Zone Ideal Loads Zone Total Cooling Energy" changes with OA flow rate

asked 2020-10-19 11:42:20 -0600

Dr. Fan
430 ●1 ●7

updated 2020-10-20 00:52:06 -0600

Hello everyone, I am using the ZoneHVAC:IdealLoadsAirSystem to calculate the zone cooling load for sizing a cooling coil. However, i have an issue on the output value of the "Zone Ideal Loads Zone Total Cooling Energy". I ran two simulation of the Ideal Loads Air System with different ventilation rate, and the ventilation rate of the second case is double of the first one. As expected, the output value of "Zone Ideal Loads Outdoor Air Total Cooling Energy" was doubled. However, what confused me was that the output value of "Zone Ideal Loads Zone Total Cooling Energy" had also nearly doubled. The two simulation had the same settings except the ventilation rate. My understanding is only the outdoor air load will change, while the Zone Ideal Loads Zone Total Cooling Energy should be unchanged. I have read the EnergyPlus document and know the several output variables related to the Ideal Load Air System follows the relation as below: Zone Ideal Loads Supply Air Total Cooling Energy + Zone Ideal Loads Heat Recovery Total Cooling Energy = Zone Ideal Loads Zone Total Cooling Energy+ Zone Ideal Loads Outdoor Air Total Cooling Energy I do not have heat recovery setting in my simulation. i cannot understand why the value of Zone Ideal Loads Zone Total Cooling Energy will change. The buidling is of single zone with 4 meters long and 4 meters wide with the the simulation performed under Guangzhou climate. The first simulation has OA flow rate of 10 m3/s and the load results are shown below: image description The second simulation has the OA flow rate of 20 m3/s and the load results are shown below: Hopefully you will give me some answers, Thank you guys.

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@Dr. Fan could you update your post to include the full text of the ZoneHVAC:IdealLoadsAirSystem object and all of its input fields that you've defined?

Aaron Boranian ( 2020-10-19 12:21:32 -0600 )edit

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answered 2020-10-21 09:38:32 -0600

rraustad

13804 ●79 ●12 http://www.fsec.ucf.edu/

The Zone Ideal Loads Zone Total Cooling Rate/Energy reports the delivered rate/energy to the zone based on the zone inlet condition (T, w) and air mass flow rate. For that report to change would mean that the zone node inlet condition or air mass flow rate changed. Since the OA flow rate increased between these 2 simulations, I suspect that the inlet condition changed. Most likely a change in humidity ratio, but possibly a change in T, w and/or air mass flow rate. Add these report variables for the zone inlet node (substitute the zone inlet node name for the wild card *) and compare the results:

Output:Variable,*, System Node Temperature, hourly;
Output:Variable,*, System Node Mass Flow Rate, hourly;
Output:Variable,*, System Node Humidity Ratio, hourly;