System 8: VAV with PFP Boxes - High Reheat

asked 2019-07-01 15:46:10 -0500

BUSA gravatar image

I am modeling a large office building (~250,000 sqft) for LEED v4. Based on Table G3.1.1-4, Baseline System Descriptions, in ASHRAE 90.1-2013, my Baseline system type is System 8. VAV with PFP Boxes. However, in accordance with Note 2., I am using natural gas fired hot water boilers rather than electric resistance heating. After running the simulation, my Baseline model shows a very high amount of reheat, which is unusual for climate zone 2A. The heating peak demand is more than twice the cooling peak demand.

For each AHU, I am using a SetpointManager:Warmest, with a reset from 55°F to 60°F, as required by G3.1.3.12, so I understand the temperature coming from the AHUs is fairly cold for winter season. Is this the correct interpretation for G3.1.3.12? I've seen previous answers to similar questions bring up to vary the temperature reset parameters, but I want to ensure I am in compliance with G3.1.3.12 and something realistic (VAV AHUs may or may not have separate cooling/heating setpoints when the terminal units have reheat).

Lastly, the model runs with a small amount of unmet hours (<300) in occupied heating and none in occupied cooling, and these unmet heating hours occur mainly in interior zones. From my understanding, typically interior zones do not even have reheat due to buildings' thermal mass, so it is strange these interior zones are some of the more suspect zones.

I appreciate any insight from anyone with experience modeling this type of system.

  • interior zones

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