ASHRAE Journal - January 2023 - 20

COLUMN ENGINEER'S NOTEBOOK
PHOTO 2 Inside a distribution box.
correctly before pressure testing, and
later, evacuation. I can only speculate
how many incomplete pressure tests
took place because the distribution
box valves were not manually opened
prior to the test.
PHOTO 3 Installed distribution box.
(Photo 2). We've had occasions where
the whole box had to be pulled out
of the ceiling to effect repairs (imagine
getting adequate space to make
repairs on the distribution box or its
connecting piping in Photo 3).
Some more recent versions of the
distribution boxes have quit using
solenoid valves and are now using
stepper motor flow regulating valves.
A stepper motor approach not only
reduces the sound during heating/
cooling changeover; it also permits
the manufacturer to modulate flow
if they decide to do so as an operating
strategy. Either way, the presence
of distribution box valves impacts
installation. When pressure testing
or evacuating, the valves all need to
be open. Since all manufacturers
do not follow the same " normally
open " or " normally closed " approach,
somebody-often either a field piping
foreman or a field start-up foreman-has
to figure out how to do this
20
ASHRAE JOURNAL ashrae.o rg
Refrigerant Line Lengths
Another aspect to proper VRF
installations can be maddening.
Most designers of DX systems are
familiar with the issues that arise
from long refrigerant lines, so it generally
is not a surprise that there is a
maximum length limit to the refrigerant
piping between a distribution
box and a fan coil. But if you did not
realize that such lines can also be too
short, you will be in for a surprise.
A good VRF installation is an iterative
redesign process between the field foreman
and the project manager, with periodic reruns
through the manufacturer's software
based on actual distribution box and fan
coil locations. I kid you not-we once
had to go back and add 9 in. (yes,
just 9 in. [-229 mm]) to a set of fan
coil refrigerant lines that were too
short.
Refrigerant charge is completely
dependent upon actual piping as
installed. If a final, accurate as-built
of piping size, length, number of
fittings (everything!) is not properly
done and the final installation
is not re-run through the VRF
manufacturer's software, you don't
know three things: whether you've
met the minimum and maximum
line lengths; whether the final pipe
sizes are correct; and you also do not
know the proper refrigerant charge for the
system.
Simply marking up the original
design drawings to produce as built/
record drawings is not a viable solution,
though many contractors have
J A N U A RY 2023
apparently tried to do that based on
the installation problems I've seen.
I've found that usually less piping is
installed than what was shown on
the drawings. The reason is simple:
the pipe fitters will often find a
shorter distance between point A
and point B. The significance is that
if this is not accounted for in the
final refrigerant charge calculation,
the system will end up over-charged.
That is a bigger problem with VRF
than with a conventional DX system,
because a number of things in
most VRF systems will compensate
for an overcharge (including variable
speed condenser fans), and as
a result a system could easily be 20%
or more over-charged without any
alarms or operating problems unless
really hot, extreme conditions were
experienced. Most over-charged systems
I've seen were due to the lack of
a good piping as-built, resulting in a
faulty refrigerant charge calculation.
Running for a long time in an
over-charged condition is hard on
compressors. My observations over
the years suggest that significantly
under-charged systems will result in
alarms sooner and as a result, this is
less of a hidden problem, but I also
suspect that under-charged systems
seldom occur due to the normal
design/installation process (since
it would require more piping, not
less). Most under-charged systems
I've seen were due to refrigerant
leaks.
Refrigerant Isolation Valves
Bear in mind that there is a lot
of refrigerant in a VRF system. To
diagnose and repair leaks, all the
refrigerant must be removed from
the entire system unless there is a
good means to isolate portions of the
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ASHRAE Journal - January 2023

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