ASHRAE Journal - January 2021 - 40

TECHNICAL FEATURE

Changes in outdoor air temperatures can significantly alter nitrogen
pressures; for a system initially
pressurized to 500 psig (3447 kPa),
pressure will increase or decrease
by about 1 psi (6.9 kPa) for each 1°F
(0.6°C) of outdoor air temperature
change. In fluctuating temperature
conditions, this relationship can be
used to calculate a temperature correction. Before and after temperatures should be measured in a shady
location.
If the outdoor air temperature
increases, an increase in pressure could still indicate a leak. For
example, if outdoor air temperature
increases by 6°F (3.3°C), we would
expect an increase in pressure of 6 psi
(41 kPa). A smaller increase, for example 3 psi (21 kPa), indicates a likely
leak. If any deviation from expected
pressures is found, a leak should be
suspected, investigated and repaired.
Some manufacturers recommend
that the standing pressure test last a
minimum of 24 hours. It is our position that daily temperature fluctuations undermine the value of 24-hour
tests. In addition, a 24-hour test may
add an extra visit to what might otherwise be a one-day installation.
For both of these reasons, we have
moved toward a shorter standing
pressure test, about one hour in
duration, combined with rigorous bubble testing. We apply an
approved leak-testing solution (not
household dish detergent) to all
flares and other site-made connections (Figure 3). After about 10 minutes, we check each fitting for bubble formation, using a flashlight and
inspection mirror when necessary.
Once the system has been successfully pressure-tested, it must be evacuated. The purpose of evacuation is
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to remove air and moisture from the
system, but a high-resolution digital
micron gauge also allows evacuation
to be used as a secondary leak check.
Once the system is pulled down to a
deep vacuum below 200 microns, it
is isolated from the vacuum pump,
and changes in vacuum readings
are observed. If the system remains
below the decay target for the specified period, the system passes (our
decay target is 500 microns or less
after 10 minutes of isolation).
If the system fails, it may be because
air is entering through a leak or
because excess moisture remains in
the system. In a leaking system, the
vacuum will continue to decay in a
near-linear fashion, while, in a wet
system, the micron reading will rise
quickly, then level off (Figure 4).
Bluetooth-enabled micron gauges,
paired with mobile apps, can extrapolate the rate of vacuum decay, often
determining in less than a minute
whether a system will pass. These
apps can also provide a time-stamped
record of the test result.
While standing pressure and
bubble tests check for leaks under
a large, positive pressure differential (~500 psi [~3447 kPa]),
vacuum decay tests check for leaks
under a much smaller ΔP; the difference between atmospheric
pressure and deep vacuum is only
about 14.7 psi (101.4 kPa). In an
eye-opening video on his YouTube
channel, Zack Psioda shows an
evaporator coil with a known pinhole leak passing both a typical
standing pressure test (done with a
precise digital gauge) and a vacuum
decay test similar to that described
above.5 While some small leaks will
escape detection, the combination of
standing pressure test, bubble test,

JAN UARY 2021

FIGURE 3 Applying a leak-testing solution to newly
made flares.

and vacuum decay test, together
with a final check after charging (see
below), will catch all large leaks and
many smaller ones.
Once the system has been charged
with refrigerant, use a bubble solution or electronic leak detector to
make a final leak check on the service valves and charging ports; these
cannot be checked earlier in the
installation process.

Service and Decommissioning
Standard practice in traditional
HVAC is to hook up refrigerant pressure gauges (either individual shortstem gauges or a manifold gauge
set with hoses). For systems with
fixed-speed compressors, gauges
provide critical information on
operation of the system. Paired with
measurements of air and refrigerant temperatures, pressure readings
allow technicians to adjust charge,
adding or removing refrigerant as
needed. In cooling mode, systems
are charged to achieve the recommended level of superheat (°F [°C]
that the vapor line is above low-side
saturation temperature) or subcool

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ASHRAE Journal - January 2021

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