ASHRAE Journal - January 2021 - 41

TECHNICAL FEATURE

FIGURE 4 Potential results of a vacuum decay test. In a system with a medium or
large leak (solid red line), the vacuum will decay rapidly in near-linear fashion. In
a system that is free of leaks but in which significant moisture remains (dashed
red line), the vacuum will decay rapidly at first but will stabilize above the decay
target. A tight, dry system (green line) will stabilize below the decay target and
remain there for the duration of the test. Some systems with very small leaks
(yellow line) may also pass the vacuum decay test.
900

800
700

Wet System
Medium Leak

600
Microns

(°F [°C] that the liquid line is below high-side saturation
temperature).
On modern inverter-driven heat pumps, like other
critical-charge systems, refrigerant adjustments
cannot be made based on ﬁeld measurements of
temperature and pressure. Because the compressor,
controlled by microprocessors with feedback from sensors throughout the system, operates across a range
of frequencies, there is no ﬁxed relationship between
charge and superheat/subcool. Systems come charged
from the factory for line sets up to a certain length; for
each foot beyond that length, a calculated quantity of
refrigerant must be added at start-up. Once a system
has been put in operation, the only way to conﬁrm that
it has the correct charge is to remove and weigh the
refrigerant.†
Each time gauges are attached to a heat pump, a small
but unknown amount of refrigerant is lost. Refrigerant
escapes whenever a gauge or hose is connected to a
valve, and additional refrigerant is stranded in the hoses
and manifold after they are disconnected. Although
the amounts are small (and are considered allowable de
minimis releases under EPA regulations), they eventually
add up, so that, after a series of careful but unnecessary gauge attachments, ounces of refrigerant could be
lost. Because connecting gauges to inverter-driven heat
pumps provides little useful information and eventually
hurts performance, we recommend avoiding hooking
gauges to these systems.
Noninvasive tests are a better choice for inverter-based
systems. The most basic noninvasive test involves running the system at maximum output and measuring
∆T across the heat exchanger at steady state. A more
thorough test involves measuring delivered capacity,
which requires measuring ∆T (heating), ∆H (cooling)
and airﬂow. A complete procedure for performing this
test on ductless systems is described elsewhere.6 If
these tests reveal a ∆T or delivered capacity outside the
manufacturer's recommended range, a leak check must
be performed, and refrigerant must be recovered and
weighed. Leaks can be found before recovery using an
electronic leak detector or after recovery using nitrogen
and bubble solution. Once the leaks are ﬁxed and the

500

Decay Target

400

Small Leak

300

Tight, Dry System

200

Evacuation Target

100
0

100

200

300
Time (s)

400

500

600

system tested, the correct charge can be weighed back
into the system.
Electronic leak detectors, useful for ﬁnding known
leaks, can also become part of a standard preventive
maintenance. Good-quality leak detectors are available,
many for under $500. An electronic detector can be used
to make a quick pass over accessible line sets, ﬁttings,
and heat exchangers, allowing the technician to ﬁnd
and ﬁx small leaks before they lead to complete loss of
charge.

Strengthening Leak Prevention Standards
Proper recovery, recycling or disposal of refrigerants
during repairs and replacement are also key to reducing climate impacts. Section 608 of the Federal Clean
Air Act, written originally to prevent release of ozonedepleting compounds, also regulates non-ozone depleting HFCs. The act prohibits venting (deliberate release
of all but de minimis amounts of refrigerant) and provides
standards for recovery/reclamation equipment. It also
requires technicians working with refrigerants to pass
a certiﬁcation test and keep records of refrigerant disposal. While the act spells out large ﬁnes for violations,

†Some manufacturers incorporate built-in pressure sensors along with diagnostic tools that allow the technician to read these data
without attaching gauges. Multiple thermistors also provide detailed data on the state of refrigerants throughout the system. We suspect
that these data, paired with information on compressor speed and graduated opening of electronic expansion valves, will ultimately allow
technicians to make charge adjustments without weighing out all the refrigerant in the system. However, to our knowledge, no manufacturers currently offer this functionality.
JAN UARY 2021

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ASHRAE Journal - January 2021

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