Diagnosing compressor, condenser and evaporator problems

Dana gave Theo a stack of six service tickets from the past summer, with the customer names covered and only the readings left: suction and discharge pressures, superheat, subcooling, compressor current, outdoor and indoor temperatures, and a note on the system type. "Each of these was a different problem," she said. "Every one of them shows up on the gauges and thermometers if you know what normal looks like and you read all the numbers together. The mistake is reading one number and grabbing a cylinder." By the end of the afternoon, Theo had a page of notes, a table of patterns, and a new respect for the techs who could look at four numbers and know where to walk next.

This lesson pulls together the compressor, condenser, evaporator and accessory knowledge of the course into a diagnostic method. It explains what to measure and how to work out what normal is for a given system and conditions, the characteristic patterns of the most common faults in the three main components and the refrigerant circuit, and how to confirm a diagnosis before replacing parts. Charging methods themselves are covered in detail in RAC-220; this lesson concentrates on recognizing the faults that are not charge problems, and on recognizing charge problems so that they are not blamed on components.

A method: measure, compare, think, confirm

• Ask and look. What does the customer report? When did it start? Is the equipment running? Look for obvious problems: dirty filters and coils, iced coils, failed fans, oil stains that mark leaks, damaged wiring, blocked airflow and missing panels.
• Measure the full set. Suction and discharge or liquid pressures and their saturation temperatures; suction line temperature at the evaporator outlet or compressor for superheat; liquid line temperature for subcooling; air or water temperatures in and out of each coil; compressor and fan currents; voltage. Record outdoor and indoor conditions.
• Compare with normal. Normal depends on the system type, the refrigerant, the metering device, the load and the outdoor temperature. Use the manufacturer's performance data or charging charts where available, the design condenser and evaporator TDs from lessons 6 and 8, and the history in previous service records.
• Think through the pattern. Which readings are high, which are low, which are normal? Match the pattern with the possible causes.
• Confirm before replacing. Test the suspected component directly, or make a reversible change, such as cleaning a filter, and watch whether the readings move as expected.

The four key numbers

Most refrigerant-side diagnosis rests on four numbers, read together:

• Evaporating temperature, from suction pressure. Compared with the air or water entering the evaporator, it gives the evaporator TD.
• Condensing temperature, from discharge or liquid pressure. Compared with the air or water entering the condenser, it gives the condenser split or approach.
• Superheat at the evaporator outlet, or total superheat at the compressor. It shows whether the evaporator is getting the right amount of refrigerant: high superheat means starved, low means flooded.
• Subcooling at the condenser outlet. It shows how much liquid is backed up in the condenser: high means a lot of liquid held there, low means little.

To these, the compressor current adds a measure of how hard the compressor is working, and the air or water temperature splits across the coils show how much heat is actually being moved. A useful habit is to write the readings in a fixed order on every call, so that comparisons with previous visits are easy.

Common fault patterns

A few points help to separate patterns that look alike.

• Low charge and a liquid line restriction both starve the evaporator, with low suction and high superheat. The difference is subcooling: with a low charge there is little liquid anywhere, so subcooling is low; with a restriction, liquid backs up in the condenser and receiver behind the restriction, so subcooling is normal or high. A temperature drop across the restriction, as in lesson 9, confirms it.
• Low evaporator airflow and low charge both lower the suction pressure. With low airflow, the evaporator cannot boil all its refrigerant, so superheat is low on fixed-metering systems and liquid may return to the compressor. The air temperature drop across the coil is large, because the little air that passes is cooled a lot. Filters, blowers, belts and dirty or iced coils are the usual causes.
• A dirty condenser and an overcharge both raise the head pressure. A dirty condenser raises the condenser split while subcooling stays normal or falls; an overcharge fills more of the condenser with liquid, so subcooling is high. Cleaning the coil first is the easiest confirmation.
• An inefficient compressor is the only common fault with high suction and low head together, because the compressor cannot hold the pressure difference. Current is low because the compressor is doing less work. Before condemning it, check that its capacity control is not stuck unloaded, that a scroll is not running backward, and that a reversing valve or hot gas bypass valve is not leaking high-side gas into the suction.
• Non-condensables raise the head pressure without a dirty coil. After the system has been off long enough to settle at a steady temperature, the pressure in the condenser or receiver is higher than the saturation pressure for that temperature, because the trapped gas adds its own pressure.

Electrical and mechanical clues from the compressor

The compressor gives its own evidence. Current compared with the manufacturer's data at the measured pressures tells whether it is doing the expected work. Current that is high at normal pressures can mean a mechanical problem such as tight bearings, a failing capacitor or low voltage; current that is low at normal pressures can mean a compressor that is not pumping. Shell and line temperatures help too: the discharge line should be hot, the suction line cool, and the compressor's lower shell on a suction-cooled compressor cool to the touch. A hot compressor with a very hot discharge line, high superheat and a hot shell is overheating; a compressor with frost or heavy sweat on its shell and a cold crankcase is being flooded. Noises such as knocking, rattling, grinding or a high-pitched whine add information that should be noted.

A compressor that will not start at all is mainly an electrical problem, covered in RAC-130: check supply voltage, contactor, capacitor, start components, overload and windings. A compressor that starts and trips its internal overload may be overheating from a refrigerant-side problem, such as low charge or high head pressure, or from an electrical one. Internal overloads can take an hour or more to reset in a hot compressor, so a compressor that seems open between two terminals may simply be waiting for its overload to cool. A compressor shell too hot to hold a hand on is a clue; a measured open winding on a cool compressor is a failed winding.

A diagnostic helper

The program below turns the table into a simple scoring helper. It compares each reading with what would be normal, classifies it as high, low or normal, and scores each fault by how well its pattern matches. Real diagnosis needs judgement and confirmation, but the exercise shows how the patterns separate faults.

# Score fault patterns against observed readings (H = high, L = low, N = normal)
patterns = {
    "low charge":              {"suction": "L", "head": "L", "superheat": "H", "subcool": "L", "amps": "L"},
    "overcharge":              {"suction": "HN", "head": "H", "superheat": "L", "subcool": "H", "amps": "H"},
    "liquid line restriction": {"suction": "L", "head": "NL", "superheat": "H", "subcool": "H", "amps": "L"},
    "low evaporator airflow":  {"suction": "L", "head": "L", "superheat": "LN", "subcool": "NH", "amps": "L"},
    "dirty condenser":         {"suction": "H", "head": "H", "superheat": "NL", "subcool": "NL", "amps": "H"},
    "inefficient compressor":  {"suction": "H", "head": "L", "superheat": "HN", "subcool": "L", "amps": "L"},
    "non-condensables":        {"suction": "H", "head": "H", "superheat": "N", "subcool": "H", "amps": "H"},
}

def classify(measured, normal, band):
    if measured > normal + band: return "H"
    if measured < normal - band: return "L"
    return "N"

def diagnose(obs):
    scores = {}
    for fault, p in patterns.items():
        scores[fault] = sum(2 if obs[k] == p[k][0] else (1 if obs[k] in p[k] else 0) for k in p)
    return sorted(scores.items(), key=lambda kv: -kv[1])[:3]

tickets = {
    # name: {reading: (measured, normal, band)}  temperatures in degrees, current in amps
    "ticket A": {"suction": (2, 7, 2), "head": (38, 46, 3), "superheat": (22, 8, 3), "subcool": (1, 6, 2), "amps": (8.9, 11.0, 0.8)},
    "ticket B": {"suction": (1, 7, 2), "head": (45, 46, 3), "superheat": (20, 8, 3), "subcool": (13, 6, 2), "amps": (9.2, 11.0, 0.8)},
    "ticket C": {"suction": (11, 7, 2), "head": (40, 46, 3), "superheat": (14, 8, 3), "subcool": (2, 6, 2), "amps": (7.6, 11.0, 0.8)},
}
for name, readings in tickets.items():
    obs = {k: classify(*v) for k, v in readings.items()}
    print(name, obs, "->", diagnose(obs))

ticket A {'suction': 'L', 'head': 'L', 'superheat': 'H', 'subcool': 'L', 'amps': 'L'} -> [('low charge', 10), ('inefficient compressor', 8), ('liquid line restriction', 7)]
ticket B {'suction': 'L', 'head': 'N', 'superheat': 'H', 'subcool': 'H', 'amps': 'L'} -> [('liquid line restriction', 10), ('low charge', 6), ('low evaporator airflow', 5)]
ticket C {'suction': 'H', 'head': 'L', 'superheat': 'H', 'subcool': 'L', 'amps': 'L'} -> [('inefficient compressor', 10), ('low charge', 8), ('liquid line restriction', 5)]

The program's temperatures are saturation temperatures for suction and head, in degrees of either scale; only the comparison with normal matters. Ticket A matches a low charge, ticket B a liquid line restriction, and ticket C an inefficient compressor. In Dana's stack, ticket A was the attic heat pump with a leaking flare nut; ticket B was the clinic's plugged drier from lesson 9; and ticket C was the restaurant's reach-in from lesson 2, with its broken suction reed.

Worked diagnoses

Confirming before replacing

Parts cost money and replacing a compressor or coil involves recovering the charge, brazing, testing and evacuation, so diagnosis must be confirmed before work starts. Ways to confirm include:

• Making the simple corrections first, such as cleaning coils and filters, and rechecking.
• Measuring temperature or pressure drops across suspected restrictions: driers, solenoids, metering devices and distributors.
• Checking a compressor's current and pressures against the manufacturer's performance data at the actual conditions, rather than rules of thumb.
• Electrically testing windings, capacitors, contactors and protectors as in RAC-130.
• Looking for oil stains and testing for leaks with the methods of RAC-120 before adding refrigerant to a system that has lost charge. A system that is low on charge has a leak, and the leak must be found and repaired as the regulations require.
• Recording all readings and the conclusion on the service ticket, so the next technician, or the same technician next year, can compare.