Modelling VRF system with exhaust through fume hoods and Treated fresh air supplied through an ERV

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Hi All,

Have been working on this project since a few months. The building is in a
warm and humid climate zone.

The HVAC system adopted is a VRF system designed at 80% diversity. I am
facing an issue in trying to combine the TFA system with the exhaust
ducting. These are the points to be considered while modeling:
1. VRF system supplies conditioned air at 78 deg F to the zones.
2. The exhaust is through a ducted system: There are two types of exhaust
systems- fans in the zones and fume hoods in laboratory facilities which
are manually operated.
3. To supplement the air requirement created by the exhaust system, treated
fresh air will be supplied to the rooms. this fresh air will be cooled
using an ERV through which the exhaust air is ducted.
4. The treated fresh air is different from the supply air system of the
HVAC. also, the TFA system basically consists of a fan that propels air
into the ERV, it has no condenser/compressor.

The issues I am facing in modeling:
1. When I consider the VRF system as a PVVT system, I am getting very high
unmet hours. Is it wrong to consider it as a packaged multi-zone system, as
a multi-zone system enables individual controls for each zone thereby
fulfilling the basis of a VRF/VRV system.
2. If i do consider the system as a packaged multizone, the energy
performance increases multifold. Is this because the part-load curves are
not being taken into consideration? Is there any algorithm that i should
consider while modeling it into a Packaged multizone that is different from
the part-load performance curves used in the PVVT system.
3. Also, how do i model the ERV to work with the return of the VRF/VRV
system and external fresh air supply. If it'll work, should I model OA from
System and incorporate the TFA as a system, even if there is no system that
is providing the air as per design.

Really looking forward to great replies. Any help will be appreciated!


R Pushkala

Environmental Architect, India

MS Sustainable Design, Carnegie Mellon University

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