ASHRAE Journal - January 2023 - 24

COLUMN ENGINEER'S NOTEBOOK
leave the electronic expansion valve (EEV) in its position
when the power was cut. If refrigerant can circulate
through a partly open EEV, then the fan coil tonnage is
not considered to have been shed. Instead, the fan coils
must be removed from the system via the control system.
The fan coil tonnage needs to be removed from the system
within days, so prompt action is required.
Software and Firmware and Compatibility
One of the reasons often cited in greater Los Angeles
for the popularity of VRF with developers is that it permits
much of the system first cost to be transferred from
the initial shell and core building to the tenant development
phase. One of the aspects of the law of unintended
consequences with VRF has been that the rapid evolution
of control logic has led to many software and firmware
changes. As a result, it is commonplace to build
a building with one generation of software resident in
the condensing units, and then by the time the tenant
improvement work is later done, the fan coil units are a
generation or two removed from that software revision.
The fan coil units have generally been backward
compatible, but the condensing units have had to be
updated prior to start-up. That is not a big deal, but it
generally means allowing time in the schedule to get
the rep's limited tech support team lined up to visit the
site coincident with start-up. Bear in mind that this is
usually when every other trade's delays have caught up
with the air-conditioning start-up group, and pressure
to finish is intense. Be sure to allow time for this in any
schedule for which you are responsible.
The backward compatibility issue is more severe if the
distribution boxes have shifted from solenoid to stepper
motor style. There can be limitations in adding new
fan coils to an older system; for instance, the distribution
box may need to be replaced. Minimum initial fan
coil tonnages are also required for a condensing unit. A
general rule of thumb is that enough fan coils have to be
initially installed to account for over half of a condensing
unit's capacity, for at least two reasons. First, the
condensing unit controls will scan the system to see if
enough fan coils are present to account for at least half
load and refuse to do anything if there is not. Second,
though half of the fan coils are not required to be scheduled
on for the condensing unit to operate, that condensing
unit may need to use even off-line fan coils during
the oil return/oil management " rev up " phase. This
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J A N U A RY 2023
seems to most often come into play when a shell and
core project will have only a few fan coils initially covering
common areas and elevator lobbies while connected
to much larger condensing units. Check with your VRF
rep; this is a planning issue that can impact everything
from construction scheduling to operating schedule.
Unit Access
Because VRF was developed in countries where floor
space is at a real premium, unit access is typically
designed to be completed from one direction. While this
is handy from a service access perspective, it also means
that a unit with many circuit boards will have the circuit
boards stacked one behind the other, with access to one
near the back only being possible by sequentially removing
all of the ones in front of it. Replacement of a simple
circuit board can easily turn into an all-day process.
Decisions Based on Climate
In hot or cold climates, the design professional needs
to understand limits to entering air temperature for
both the condensing units and fan coils. If the climate
will exceed the limits, it may impact decisions regarding
condensing unit location and whether outdoor air
needs to be handled separately by a dedicated outdoor
air system (DOAS) or other method. With careful design
in a small VRF system, a small DOAS unit can often share
a condensing unit with zone fan coils, but this requires
careful modeling with the VRF rep.
VRF Integration with BMS
You need a good systems integrator to effectively integrate
a VRF control system with an overarching building
management system (BMS). The common controls
industry model of completing 95% of a project, then
having warranty or other service be slow or difficult to
schedule without a service agreement, runs into a lot of
problems on a VRF installation. The integration portion
has to be done before the building is turned over to the
owner, or there is a real chance it will never be properly
completed.
One area this is often recognized relates to alerts and
alarms. An informed discussion of what alerts and
alarms are desired (and why) should take place during
the design phase to ensure the BMS system has everything
needed. It may be necessary to install some duplicate
BMS sensors if the BMS cannot import the necessary
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ASHRAE Journal - January 2023

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