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HAP model for VRF with DOAS vs. baseline system 5 (VAV) for LEED v4

asked 2018-04-19 02:52:56 -0600

updated 2018-04-19 10:09:23 -0600

Hi everyone, We have a question concerning performance of VRF unit vs. rooftop VAV using HAP v5.01 modeling. We are trying to document the EAp2/EAc2 credits in LEED v4 for 5-storey office building in northern Poland (climate zone 5A). Each of 4 office floors includes social part/restrooms in the floor middle (ca. 60 m2), and office open space around it (ca. 380 m2) with energy-efficient glazing facade (SGG COOL_LITE). Design includes two HVAC systems for every office floor: rooftop VAV with preheat only coil, and rooftop VRF. Social area is supplied with separate rooftop VAV unit. HAP model was made as follows: proposed case - terminal unit VRF with DOAS (preheat only); baseline case - system 5 (rooftop VAV with reheat). Cooling is modeled as: in proposed case - by indoor VRF terminal, in baseline case - by central cooling coil. Heating: proposed - by preheat and VRF, baseline - preheat coil and zone heating. Model shows that cooling load for central cooling coil in baseline system is ca. 3 times lower as for proposed VRF unit. In the same time, preheat coil load for baseline system is ca. 20 times higher... All adjustments required by ASHRAE 90.1-2010 for spaces, systems, etc. have been provided for proposed and baseline cases. Does anyone know, why VRF shows such a low cooling performance in HAP? And why preheat coil works so strange in the same conditions? Thank you.

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You can contact HAP helpdesk at software.systems@carrier.utc.com, they usually reply on the same day.

TiejunWu gravatar imageTiejunWu ( 2018-04-25 09:28:11 -0600 )edit

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answered 2018-04-25 17:39:05 -0600

Cox311 gravatar image

Are you comparing the total VRF cooling load against the VAV AHU cooling load? If you run the system sizing report that just gives you the size of the DOAS cooling coil, not the VRF cooling load.
On the discrepancy on the VAV preheat coil, it could be coming from your leaving temperature set point. You might want to compare the coil leaving temperature to the set point for the DOAS.

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Asked: 2018-04-19 02:52:56 -0600

Seen: 122 times

Last updated: Apr 25 '18