I’ve worked on this model some as well and wanted to expand on this discussion. In order to zone the capacities at the zone level, the sizing:zone object was
used with the TemperatureDifference Zone Cooling Design Supply Air Temperature Method using 20F dT shown below. You can see this from looking at the HVAC Sizing Summary Report for Zone Cooling under the Calculated dT determined from the Calculated Design
Load and Air Flow. The User Design Load is then 1.15 times the Calculated which makes sense since I have a 1.15 sizing factor. The interesting part comes in when looking at the User Design Air Flow. This was increased past the 1.15 sizing factor for a number
of the zones which resulted in a lower User Design dT shown in the below table. I determined this was from specifying design outdoor air conditions per person and per square foot. When I removed the design OA conditions, the User Design dT matched the Calculated
dT. The problem now is that the total airflow reduced significantly so the undercooled hours is now about 1,000, which tells me that without taking the outside air into
account for zone sizing, there isn’t enough airflow to cool the space during the year. My takeaway is that the outside air zone sizing parameter isn’t used for calculating the cooling capacity, only the airflow, which is what I think is causing difference
in temperature differences between the zones. My thinking is that to satisfy the baseline requirements for 90.1, the temperature difference should be specified at 20F which will properly size the cooling capacity and then the airflow can be increased if required
based on the OA design specifications. ASHRAE 90.1 App G Section G3.1.2.8 specifies the airflow be based on a 20F dT but I see no other way to do this than let the airflow be adjusted based on design OA even though it may reduce the design temperature difference.
I should also note that the required ventilation air is less than the User Design Air Flow for all the zones. The OA is just being accounted for after the cooling capacity is calculated instead of before. If I am missing anything or if anyone has any thoughts
on this, I’d love to get some feedback. Below are some screenshots showing the sizing:zone object and the HVAC Sizing Summary Report for Zone Cooling.
Thanks,
Matthew Larson, PE, CEM, BEMP, LEED AP BD+C
Project Engineer
Eaton Energy Solutions, Inc.
Eaton’s Electrical Services & Systems
143 Union Blvd, Suite 350
Lakewood, CO 80228
tel: +1 303 328-3419
mobile: +1 303 668-2511
fax: +1 303 974-1239
MatthewRLarson@xxxxxxxxx
www.eaton.com/energysolutions
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
On Behalf Of Jim Dirkes
Sent: Wednesday, June 26, 2013 4:54 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: [EnergyPlus_Support] Zone Level Airflows
Dear Celia,
The “Central Cooling Design Supply Temperature“ must be 11.1C below the zone cooling setpoint in order to achieve your goal. Since you
supply the air at 13.3C and serve 20 zones, ALL of the zone setpoints must be ~24.4 � andd they are not.
Similarly, the zone heating setpoints must be 11.1C lower than, in this case, the Zone Heating Design Supply Air Temperature (35C) in
Sizing:Zone � and they are not.
For an ASHRAE 90.1 energy model, each zone
on a particular system must use a uniform setpoint for heating or cooling! The fact that the actual design may use somewhat different zone setpoints is not relevant, although I suppose that you are allowed to specify a delta T
larger than 11.1C. The purpose of the Baseline � Proposed model is � the comparison betwetween a “fictitious” Baseline system and an almost-true Proposed system. In order to make the comparison consistent and understandable, some variables (like setpoints
and schedules) must be regulated.
I looked very quickly at your IDF and may have missed some things, but I think that you will see an improvement if you change these
setpoints.
James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
On Behalf Of Celia King-Scott
Sent: Wednesday, June 26, 2013 5:17 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Zone Level Airflows
Hi,
I was wondering if any can help me with an issue I am experiencing in EnergyPlus.
Initially when we looked at the User Design Airflow column under Zone Cooling Loads in the output I saw that the supply air-to-room air temperature was much less than 20F even though the room setpoint was at 75 and supply temperature
was 55F. (Following ASHRAE 90.1 2007 Appendix G requirements for Baseline model). We corrected this by adjusting the design outside air at the zone level sizing parameter however now the undercooled hours are about 1000 (much more than allowed). This tells
that without taking the outside air into account for zone sizing, there isn’t enough airflow to cool the space during the year. The outside air zone sizing parameter isn’t used for calculating the cooling capacity, only the airflow, which is what I think is
causing difference in temperature differences in the zone. However how do we meet the supply air-to-room air temperature delta T requirement and have less than 300 unmet load hours as also required?
I have attached the Baseline idf and output for your review.
Thank you for your time on this. Any help is greatly appreciated.
Celia
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