ASHRAE Journal - January 2023 - 22

COLUMN ENGINEER'S NOTEBOOK
system. For this reason, there should be refrigerant isolation
valves for every fan coil unit. Look at Figure 1. You
will see four positions noted.
Location 1 contains isolation valves as part of the condensing
unit. Location 2 is what I most commonly see
with shell and core office buildings, where the base
building work includes condensing units and refrigerant
main lines, but the tenant improvement (TI) work
will include the fan coils. Usually, I see the distribution
boxes provided along with the fan coils. In those cases,
Location 2 needs to either be capped or a valve included.
A valve is more expensive, but can save the labor
involved with evacuating the main riser piping headed
to the roof (significant in a relatively tall building).
As a practical matter, if the TI work is awarded to the
same contractor involved with the shell and core contract
and will follow later, a valve is usually used. On the
other hand, if the fan coils will be installed at the same
time as the risers, then there is no need for a valve at
Location 2. Location 3 is at the discharge of the distribution
box. This is arguably the best location between
choices 3 and 4, since it permits isolation of both the
refrigerant lines leading to the fan coil and the fan coil
itself. I know people who prefer Location 4, mostly due
to the likelihood of a leak being (by far) the highest at the
flare connection to the fan coil. However, my personal
preference is Location 3.
Temper this conversation with the fact that there are
also various viewpoints from the VRF manufacturers, so
this is a point to review with the rep of the brand of VRF
that is specified, and if a different brand will be installed,
it is a submittal issue to verify with the installed brand's
rep. Either way, do not omit isolation valves for the fan
coils, or all fan coils served by a distribution box will have
to be taken out of service to make repairs.
Condensate Pumps
The condensate pumps provided with high wall and
cassette fan coils are typically very low head (2 ft to
3 ft, 3 in. [0.6 m to 0.9 m]), intended only to move liquid
vertically a short distance to reach a gravity flow
line above a ceiling. They generally are not suitable for
pumping up a floor or more to (for example) get the
condensate to roof level. When that is necessary, either a
more robust condensate pump will be needed, or a second
pump arrangement in the ceiling will be required.
The strategy up front in the design process regarding
22
ASHRAE JOURNAL ashrae.o rg
J A N U A RY 2023
FIGURE 1 Possible isolation valve locations.
FC-1
FC-2
FC-3
FC-4
FC-5
4
where the condensate will go is, therefore, important to
later decisions, and given the occasional communication
difficulty involved during the mechanical and plumbing
phases of a design, this needs to be a " sooner rather than
later " decision.
What is also interesting is that the condensate pump
is usually powered from the fan coil. If the fan coils are
shut off at the disconnect because of remodeling, construction,
service or any other reason and the electronic
expansion valves (EEVs) were not directed to be closed
via the system controls prior to losing power, refrigerant
will continue to circulate through the fan coil and some
level of condensation will occur on the coil-without
benefit of an operating condensate pump. Many floors
have gotten wet due to this oversight.
Compressors
When VRF was first introduced, I was amazed at the
heating output available from the system. Eventually,
I figured out that the compressors could overspeed (in
some cases, double the compressor speed used in cooling
mode) to achieve peak heating performance. What I
did not learn until much later was that in some designs a
condensing unit with multiple compressors on a circuit
will automatically overspeed compressors even in cooling
mode if one compressor has failed, in order to meet
the fan coil demand. This can be brutal for the operating
compressors, and manufacturers have rules for how
long this can be permitted (days, not weeks).
If a compressor is not immediately available for replacement,
entire fan coil units need to be removed from the
system in proportion with the tonnage lost by the failed
compressor. And that does not simply mean shutting the
fan coil unit off at the disconnect; that approach would
Condensing
Unit
1
Distribution
Box
3
2
http://ashrae.org
ASHRAE Journal - January 2023

Table of Contents for the Digital Edition of ASHRAE Journal - January 2023
