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.