Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 6 — Introduction to System Maintenance
6.4 — Defects: Causes & Repairs
Identifying a defect is only half the job — a technician must also understand
why it occurred and carry out the correct repair procedure. This lesson links
specific component failures to their root causes and the standard repair sequences
recognised in 313A/313D Level 1 practice.
Jump to
6.4.1 — Bearings, Belts & Pulleys
Bearing Failures
Bearings support rotating shafts in motors, compressors, and fan assemblies.
Premature bearing failure is caused by:
Under-lubrication: Grease or oil film breaks down; metal-to-metal contact generates heat and wear
Over-lubrication: Excess grease churns, heats, and contaminates windings in sealed motors
Misalignment: Belt drives or direct-drive couplings that are out of alignment impose radial loads that bearings are not designed to carry
Contamination: Water, refrigerant, or acid in the oil/grease film accelerates pitting
Overloading: Operating outside rated speed or load reduces calculated bearing life
Diagnosis: listen for grinding, rumbling, or squealing. A stethoscope or vibration pen
placed on the bearing housing amplifies the sound. Measure motor amperage —
a worn bearing increases friction and amp draw before failure. Replace bearings
in matched pairs (both ends of the shaft) to prevent an imbalanced shaft load on
the new bearing.
V-Belt Defects & Tensioning
V-belts transmit power from motors to fan sheaves on belt-drive AHUs. A worn or
improperly tensioned belt slips under load, generates heat, reduces airflow, and
wears the sheave groove prematurely.
Defect
Symptom
Cause
Repair
Belt slipping
Squealing; reduced fan speed; belt glazed on sides
Undertension; worn sheave; oil contamination
Adjust tension; replace belt; clean or replace sheave
Belt cracking
Visible cracks on inner surface; belt pieces in housing
Age hardening; ozone; heat; misalignment
Replace belt; check alignment
Belt fraying
Fibres visible on belt edges
Sheave groove worn sharp; belt rubbing guard
Replace sheave; adjust belt guard clearance
Overtensioned belt
Excessive bearing load; premature bearing and belt failure
Tension set too high; belt not re-checked after run-in
Adjust to manufacturer deflection specification
Belt Tension Check Procedure
The deflection method: apply a perpendicular force at the midpoint of the belt span
equal to the manufacturer’s recommended force (typically listed in the O&M
manual or on a belt tension chart). Measure deflection; compare to the target
(typically 1/64 in. per inch of belt span, e.g. 24 in. span = 3/8 in. deflection).
Re-tension if outside the ±1/16 in. tolerance. Always re-check tension after
the first 24 hours of operation as new belts seat into the sheave groove and loosen slightly.
Pulley (Sheave) Wear & Alignment
A worn sheave groove becomes rounded (hourglass profile), reducing the belt’s
wedging action and causing slip even at correct tension. Check groove profile with a
sheave gauge or straightedge. Misaligned sheaves cause the belt to ride up on one
side, generating heat and uneven wear. Use a straightedge across both sheave faces
to check parallel and angular alignment simultaneously; maximum misalignment should
not exceed 1/16 in. per foot of shaft centre distance.
6.4.2 — Filter Driers & Incorrect Charge Repair
Dirty or Plugged Filter Driers
The filter drier protects the system from moisture and debris. A saturated or plugged
drier creates a pressure drop in the liquid line that promotes flash gas upstream of
the metering device, reducing capacity. Diagnosis: feel the inlet and outlet of the
drier — a significant temperature difference (more than 2–3 °F)
indicates a restriction. On some units, frost or condensation forms on the outlet
side of a plugged drier as the pressure drops enough to approach saturation temperature.
Always replace the filter drier when opening the refrigerant circuit
for any repair. A drier that has been exposed to atmospheric air has absorbed moisture
and is partially saturated before re-installation.
Replacement procedure:
Recover refrigerant per local regulations.
Cut or unsweat the old drier; note the direction of flow arrow on the shell.
Install the new drier with the flow arrow in the correct direction (liquid enters the inlet end).
Braze or flare connections; purge with dry nitrogen during brazing to prevent copper oxide scale.
Pressure-test, evacuate to ≤300 microns, verify vacuum hold, then recharge.
Incorrect Charge Repair Procedure
Adding refrigerant to a leaking system without finding and fixing the leak is not a
repair — it is a temporary measure that violates refrigerant management
regulations. The correct procedure:
Locate the leak: Use an electronic detector, UV dye (where already installed), or nitrogen pressure test. Document the leak location.
Recover the remaining charge to an approved recovery cylinder. Weigh the recovery to determine how much refrigerant was lost.
Repair the leak: Re-sweat the joint, replace the flare fitting, or replace the component as required.
Replace the filter drier (the circuit has been open).
Pressure-test with dry nitrogen to 1.1 × the maximum operating pressure. Hold for a minimum of 15 minutes with no drop.
Evacuate to ≤300 microns; measure with an electronic micron gauge (not a compound gauge needle). Isolate the vacuum pump and hold for 15 minutes — pressure rise of more than 100 microns indicates moisture or a leak.
Recharge to manufacturer’s subcooling or superheat specification. Record refrigerant type, amount added, and ambient conditions on the service record.
6.4.3 — Valve Defects & Operation
Leaking Compressor Valves
As discussed in 6.2, worn or broken compressor valves reduce volumetric efficiency.
The repair for leaking compressor valves in a reciprocating compressor is a
valve plate replacement, which requires opening the compressor head. In practice,
a compressor with confirmed valve damage is often replaced rather than repaired
in the field, particularly for hermetic and semi-hermetic units. Before condemning
a compressor:
Confirm the diagnosis with a valve efficiency test (equalisation rate test)
Verify that the symptom is not a reversing valve bypass (heat pump systems)
Check compressor age and remaining economic life before recommending replacement
On semi-hermetic compressors, verify that a valve kit is available before committing to an open repair
The TXV meters refrigerant to maintain a set suction superheat. TXV faults fall into
two categories: hunting (unstable) and stuck (fixed).
Fault
Symptom
Cause
Repair/Action
TXV hunting
Suction pressure oscillates; superheat swings widely; system short-cycles
Sensing bulb poorly clamped to suction line; bulb in wrong location; insulation missing
Re-clamp bulb; ensure bulb is at 4 or 8 o’clock position; insulate bulb; verify external equaliser is connected to correct port
TXV stuck open
Low suction superheat; liquid may reach compressor; high suction pressure
Debris under valve seat; loss of power element charge
Replace TXV; install new filter drier
TXV stuck closed
Very high superheat; low suction pressure; high discharge temp; system starved
Wax plugging valve orifice (wrong oil/refrigerant mix); ice blockage; loss of power element
Check for moisture and acid; replace TXV and drier; purge and recharge with correct refrigerant type
TXV underfeeding
Moderate-to-high superheat; capacity below design
Valve set too cold (adjustment); undersized for load; pressure drop in liquid line causing flash gas
Adjust superheat set point (CW = higher superheat; CCW = lower superheat); verify liquid line pressure drop; confirm correct valve model
Worked Example — TXV Hunting Diagnosis
Suction pressure on an R-410A system oscillates between 90 and 130 psig in a
45-second cycle, with corresponding swings in suction superheat from 3 °F to
25 °F. Head pressure is stable. Compressor cycles on the low-pressure switch
during each low-pressure event.
Inspection: the sensing bulb is clamped at the 12 o’clock position on the
suction line. At the top of the pipe, oil pooling can coat the bulb and slow
its thermal response, or the bulb senses liquid pooled from a previous flooded
condition. The bulb is re-positioned to the 4 o’clock position and insulated
with foam tape. After a 20-minute test run the system stabilises at 120 psig
suction and 10 °F superheat with no hunting.
Service Valve Operation
Schrader-type and ball-type service valves must seat properly to isolate the circuit
during service. Common defects:
Leaking Schrader core: Continuous refrigerant loss past the core seat. Replace the core with the system under pressure using a core-removal tool; no recovery required for a core swap.
Partially closed service valve: A back-seated service valve left in the mid position creates a restriction; suction valve restriction mimics undercharge. Always fully back-seat (open) service valves before leaving a system in operation.
Damaged ball valve: Replace; do not attempt to lap-seat a ball valve.
6.4.4 — Refrigerant Piping & Sight Glasses
Piping Defects & Repairs
Refrigerant piping failures are almost always traced to one of the following causes:
Vibration fatigue: Unsupported tubing resonates at compressor frequency; work-hardens and cracks. Fix by adding clamps with rubber isolation every 4–6 ft on copper suction and liquid lines; add a vibration loop at the compressor connection.
Formicary (ant-nest) corrosion: Pitting corrosion from formic acid in the air (from adhesives, cleaning products, MDF) forms fine pinholes in copper walls. Replace the affected section; improve ventilation to eliminate the acid source.
Mechanical damage: Kinked, crushed, or flattened tubing from improper handling. Damaged sections must be replaced — reshaping cold-worked copper increases the risk of hidden cracks.
Failed brazed joints: Incomplete penetration from insufficient heat or flux; improper alloy selection. Cut out and rebraze with correct alloy (stay-brite silver alloy or 15 % silver braze for copper-to-copper; 45 % silver alloy for copper-to-brass). Flow dry nitrogen through the line during all brazing operations.
Sight Glass Interpretation & Defects
The sight glass in the liquid line provides a visual indication of refrigerant charge
and moisture level. A properly charged system shows a clear, bubble-free window
under steady-state operating conditions.
Observation
Meaning
Action
Clear glass, no bubbles
Sufficient charge; liquid fully subcooled
Verify with superheat/subcooling measurements before adding refrigerant
Bubbles or foam streaming through
Flash gas in liquid line — undercharge or liquid line restriction or insufficient subcooling
Measure subcooling; check liquid line temperature drop; rule out restriction before adding charge
Moisture indicator yellow/orange
Moisture present in system
Replace filter drier; operate system; re-check indicator after 24 hours
Moisture indicator green
Dry condition confirmed
No action required for moisture
Oil streaks or dark fluid visible
Oil carryover; possible system contamination
Sample refrigerant and oil for acid and moisture; check compressor
Key point: A clear sight glass does not confirm a correct
charge — it only confirms there is no flash gas at that location. An overcharged
system also shows a clear glass. Always use subcooling and superheat measurements
to set final charge.
“Not cooling (or heating) enough” is the most common complaint in HVAC
service. A structured diagnostic approach prevents misdiagnosis and unnecessary
parts replacement:
Confirm the complaint: Measure return and supply air temperatures. Calculated sensible cooling = CFM × 1.08 × ΔT. Compare to the rated tonnage (12,000 BTU/h per ton).
Check airflow: Measure supply air velocity with an anemometer; confirm CFM matches system design (typically 400 CFM/ton for cooling).
Measure coil pressures and temperatures: Suction, discharge, superheat, subcooling. Identify which, if any, are out of range.
Check for load issues: Has building use changed? Are there new heat sources (IT equipment, additional occupants, glass exposure)? Is the thermostat calibrated?
Compare to design conditions: Systems are rated at ARI/AHRI conditions (95 °F outdoor / 80 °F, 67 °F WB indoor). A system running in 105 °F ambient will produce less capacity than its nameplate rating — this is normal, not a defect.
Fouled Heat Exchangers
A fouled heat exchanger surface reduces both sensible and latent capacity. Even a thin
biofilm on the evaporator fin surface reduces heat transfer coefficient by 20–30 %.
Chemical cleaning (acid descale) per manufacturer; record before/after temperatures
Worked Example — Fouled Evaporator & Capacity Recovery
A retail walk-in cooler (R-404A) is maintaining 42 °F instead of the design 35 °F.
Suction pressure is 22 psig (sat. temp. ≈17 °F); superheat 28 °F
(very high). The evaporator coil is coated in a thick layer of grey dust and debris.
After coil cleaning:
Suction pressure rises to 28 psig (sat. temp. ≈25 °F)
Superheat drops to 10 °F
Box temperature pulls down to 35 °F within 45 minutes
The fouled coil had been insulating the refrigerant from the box air, driving up
superheat and reducing evaporating temperature. No refrigerant was needed —
cleaning alone restored full capacity. This illustrates why coil condition must be
confirmed before adding refrigerant to a system with high superheat.