Bench Degree·REFRIGERATIONchapter

Chapter 22: How It Breaks

The chapter a working technician would keep. Twelve failure modes, what each one does to the numbers from Chapter 12, and the four that will end a compressor rather than merely annoy you.

Diagnosis is not memorising symptoms. It is knowing the mechanism well enough that the symptom is predictable, and every fault in this chapter is a consequence of something already established in Parts III and IV.

Two organising ideas before the list.

Almost every refrigeration fault is one of four things: not enough refrigerant, too much refrigerant, something restricting flow, or not enough air across a coil. The exotic failures are rare, and a technician who checks those four in order will solve most of what walks in the door.

And the single most expensive error in the trade is adding refrigerant to fix an airflow problem. It improves the readings, so it looks like a diagnosis. It leaves the machine overcharged, and Section 3 explains what that does over the following two years.

Section 1: Undercharge

By far the commonest fault, and it is almost always a leak, because refrigerant is not consumed. Chapter 3 Section 8: the fluid is a bucket, not a fuel. A system low on charge has lost it somewhere, and the correct response is to find the leak.

The signature, per Chapter 12:

Why. Not enough liquid arriving means the evaporator runs out of liquid early, so the last part of the coil merely warms vapour, so superheat climbs. The condenser has no surplus liquid to cool below saturation, so subcooling collapses. Both pressures fall because there is less mass in circulation.

What it looks like from the front. The machine runs continuously and does not satisfy. Capacity is down roughly in proportion to the missing charge. On a severe undercharge the evaporator may ice at the inlet end only, because the little refrigerant present boils very cold in the first few coils.

Where the leaks are, in order of likelihood. Schrader valve cores and caps. Brazed joints, particularly ones made in the field. Flare connections on mini-splits, which is the commonest of all on that equipment. Coil tubes, from formicary corrosion. The line set where it passes through a wall and has chafed. And on rooftop units, vibration fatigue at the compressor discharge.

Finding them. Electronic leak detector, then confirm with bubbles. Ultraviolet dye works but is a one-way decision, because once dye is in a system it is in it forever and some manufacturers void warranties over it. Nitrogen pressure test with the refrigerant recovered is the definitive method.

And the question to ask first, from Chapter 18 Section 3: on a system with a long line set, was it ever charged correctly? A machine short by the line-length adjustment has been undercharged since the day it was commissioned, and it has no leak at all. Hunting for one will waste a day.

Section 2: Overcharge

Less common, because it requires someone to have added refrigerant, which means it is usually iatrogenic: caused by the last technician.

The signature:

Why. Excess liquid backs up into the condenser, occupying coil that should be condensing vapour, so what liquid does leave is cooled further than necessary and subcooling climbs. Meanwhile too much liquid reaches the evaporator, boiling continues right to the outlet and beyond, and superheat falls toward zero.

Why it is dangerous rather than merely inefficient. Two mechanisms, both fatal to compressors.

Liquid reaching the compressor. Chapter 9’s one rule. Low superheat means the margin is gone.

And high head pressure, which raises compressor discharge temperature, raises current draw, and accelerates every wear mechanism in the machine. A compressor running at elevated head pressure does not fail today. It fails in year four instead of year twelve, and nobody connects that to a service visit eight years earlier.

Correcting it means recovering refrigerant into a recovery cylinder, weighing what comes out, and it is not optional to do it lawfully.

Section 3: Restriction

The fault with the signature that looks self-contradictory until you picture it.

The signature:

Both high, which is the giveaway. Nothing else does that.

Why. Liquid piles up upstream of the blockage, where it keeps getting cooler, so subcooling climbs. Downstream, the evaporator is starved, so superheat climbs. One fault, two coils, opposite symptoms, and the discontinuity between them is the location of the restriction.

Where restrictions come from:

A plugged filter drier, which is the commonest and the easiest. Often detectable by hand: a temperature drop across the drier means it is restricting. A drier should feel the same temperature on both sides.

Copper oxide scale, from brazing without a nitrogen purge. Chapter 18 Section 4. The flakes travel and lodge at the metering device.

Moisture freezing at the metering device, from inadequate evacuation. This one is intermittent and therefore maddening: the machine works, ices up, stops, thaws, works again. An intermittent restriction that clears when the machine sits is moisture, nearly always.

A kinked line, usually from installation.

A failed TXV. A valve whose sensing bulb has lost its charge closes and stays closed, which is a restriction in every practical sense.

And a crushed liquid line, from something being stood on it.

Section 4: Airflow, Which Causes More Trouble Than Refrigerant Does

Chapter 15 Section 5 established that most systems move less air than designed. Here is what that does.

Low airflow across the evaporator. Less heat arriving at the coil, so less refrigerant boils, so suction pressure falls and the coil gets colder. Below freezing it frosts, the frost blocks the fins, airflow drops further, and the machine ices itself solid in a runaway.

The signature is low suction pressure with low superheat, which distinguishes it from undercharge, because in this case the refrigerant is present and simply has no heat to absorb.

Causes, in order: filter, always check the filter first. Then a dirty evaporator coil, blocked returns, closed dampers, undersized or crushed flexible duct, a slipping belt on older air handlers, and a blower wheel loaded with dust, which reduces its output substantially and is invisible without removing it.

Low airflow across the condenser. Less heat rejected, so condensing temperature and head pressure rise. Compressor current rises, efficiency falls per Chapter 13, and eventually the high pressure cutout opens.

Causes: dirty condenser coil, which on a rooftop unit is usually packed with cottonwood or grass; a failed condenser fan motor or capacitor; recirculation from inadequate clearance per Chapter 18 Section 8; and on multi-fan units, one fan failed while the others mask it.

The diagnostic discipline that follows: check airflow before touching the charge. Every time. A machine with 30 percent low airflow shows low suction pressure, and a technician who reads that as undercharge and adds refrigerant produces an overcharged machine with an airflow problem, which is two faults where there was one.

Section 5: Iced Evaporator, Which Has Three Different Causes

Worth its own section because the symptom is unmistakable and the cause is not.

A coil ices when its surface goes below 0 °C (32 °F) and stays there. Three distinct routes:

Low airflow, per Section 4. Not enough heat arriving. The ice starts even across the coil face.

Undercharge. The little refrigerant present boils very cold in the first part of the coil. The ice starts at the inlet end only and the rest of the coil is clear, which is the distinguishing observation.

Low ambient, per Chapter 20 Section 6, on equipment running in cold weather without head-pressure control. Suction pressure collapses because the pressure difference across the metering device has gone.

And the trap. Ice blocks airflow, which lowers suction pressure further, which makes more ice. By the time you arrive, the machine is showing the symptoms of low airflow regardless of what started it. So the ice must be thawed completely, the machine restarted with the airflow verified, and only then can the readings be trusted. Diagnosing an iced machine from its readings while it is still iced is diagnosing the ice.

Section 6: Non-Condensables

Air or nitrogen in the system, always from inadequate evacuation or from a system opened and closed carelessly.

The signature: head pressure noticeably higher than the condensing temperature justifies. Take the liquid line temperature, look up its saturation pressure, and compare against the gauge. If the gauge reads higher than the temperature implies, the extra is partial pressure from a gas that will not condense.

Why it matters. The non-condensable occupies volume in the condenser without ever changing state, so it reduces effective coil area and adds its own partial pressure. Head pressure rises, compressor work rises, capacity falls.

The fix is recovery, evacuation and recharge. There is no way to purge only the air.

Section 7: The Four That End Compressors

Everything above costs efficiency or capacity. These four end the machine.

Slugging. Liquid refrigerant entering the compressor cylinder. Liquid does not compress, so something has to give: valve plates, connecting rods, or the crankshaft. This can happen in one revolution. Caused by overcharge, a flooded evaporator, a failed TXV passing liquid, or a defrost cycle returning liquid on a heat pump without an accumulator.

Flooded start. Chapter 20 Section 6. Refrigerant migrated into the compressor’s oil while it sat, and on start the oil foams, oil pressure is lost, and liquid slugs through. Prevented by a crankcase heater and by waiting after power has been off. The failure looks mechanical and the cause is a switched-off breaker.

Oil logging. Chapter 18 Section 2. An oversized suction line or a vertical riser with insufficient vapour velocity leaves oil lying in the pipe. The compressor starves of lubricant over months. The failure presents as a worn compressor and the root cause is a pipe size chosen years earlier, which is why it is so rarely identified.

Burnout and acid. A motor winding failure inside a hermetic shell puts the products of combustion into the refrigerant. With POE oil and any moisture present, the result is acid, which then attacks everything downstream. A burnout contaminates the whole system, and replacing the compressor without a proper acid clean-up, suction line filter drier and follow-up oil test simply destroys the new one. This is where corners are cut most often and it is the most expensive corner in the trade.

IN PLAIN ENGLISH: Three of the four compressor killers are liquid where vapour should be, and the fourth is contamination. Nothing in this list is about the compressor being worn out. Compressors are killed; they rarely die of old age.

Section 8: Short Cycling

Not a fault in itself. A symptom, and the one Chapter 21 built its monitoring around.

Why it matters more than it sounds. Starting is what wears a compressor. LRA is five to eight times RLA, per Chapter 20, so every start is a thermal and mechanical shock. Compressor life correlates with starts, not with hours. A machine running 12 hours in 6 long cycles will outlive an identical machine running 12 hours in 60 short ones.

And it destroys dehumidification, per Chapter 14 Section 7, because drying is a function of runtime.

Causes, and each points somewhere different:

Oversizing, which is the commonest and the least fixable. Chapter 15. Thermostat differential set too narrow, or the thermostat mounted somewhere unrepresentative such as in a draught or on an exterior wall. A safety tripping and auto-resetting, which is Chapter 20 Section 7 and produces a very distinctive rapid pattern. Low airflow, which drives suction pressure to the low pressure cutout repeatedly. Low charge, same route. And a failing run capacitor or contactor, where the machine attempts to start, fails, trips its overload and tries again.

How to tell them apart. Log the current. A tight regular cycle is a thermostat or sizing problem. An irregular pattern with very short runs is a safety tripping. The pattern itself is diagnostic before any gauge is connected.

Section 9: Electrical Faults

Six, in rough order of frequency.

Failed run or start capacitor. The commonest electrical fault on single-phase equipment. Compressor hums, draws locked-rotor current, trips its overload. A five dollar part, and a capacitor tester or a capacitance meter confirms it in seconds. Discharge it before touching it.

Contactor failure. Pitted or welded contacts from arcing under LRA. Welded contacts mean the compressor runs continuously regardless of the thermostat, which is a fault that mimics a control problem.

Open winding or shorted winding. Measured with a meter at the compressor terminals: winding resistance against specification, and each winding to ground for insulation. A compressor that reads a few hundred kilohms to ground is on its way out.

Single-phasing, on three-phase equipment. Losing one leg while running makes the remaining two carry enormous current and burns the winding in well under a minute. This is what a phase monitor exists to prevent, and it is why one is not optional on any three-phase compressor worth protecting.

Reversed rotation, on three-phase scrolls. Chapter 20 Section 3: two swapped conductors, the machine runs, makes no pressure differential, and destroys itself in minutes. Verify after any electrical work.

Voltage imbalance. More than about 2 percent between legs causes disproportionate current imbalance and localised winding heating. Above 5 percent shortens motor life materially. Measured with a meter, and it is a supply problem rather than an equipment problem.

Section 10: The Diagnostic Sequence

The whole chapter as an order of operations. Following it in this order is most of what separates a technician from a parts-changer.

1. Ask what changed. New installation, recent service, gradual decline, or sudden failure. A machine that never worked properly has a commissioning fault. A machine that worked for eight years and stopped has a component fault. These are different investigations and the answer to this question saves the most time of anything in the list.

2. Look and listen before instruments. Ice, oil stains, which is a leak, bent or blocked coils, filter condition, unusual noise, whether the fans run.

3. Verify airflow. Filter, coil cleanliness, blower operation, temperature split. Before the gauges.

4. Confirm electrical. Supply voltage, current per leg against RLA, leg-to-leg imbalance, capacitors and contactors.

5. Only now connect gauges, and only after fifteen minutes at steady state.

6. Take superheat and subcooling. Interpret with Chapter 12’s table.

7. Check the drier for a temperature drop and the sight glass for bubbles.

8. Verify against the manufacturer’s data for the actual conditions, not against remembered typical values.

9. Write it down. Chapter 21 Section 6: on equipment with auto-reset safeties, your notes are the fault history. Nothing else records it.

ON THE BENCH: Introduce faults deliberately

Parts: the scrap window unit from Chapter 17; gauges; thermocouples; cardboard; a clamp meter. Cost: nothing beyond the instruments. Time: two hours. Hazards: as Chapter 12. Scrap equipment only, gloves and eye protection, and never open the high side. Method. Establish and record a healthy baseline: both pressures, superheat, subcooling, temperature split, current. Then introduce one fault at a time, predict the readings before measuring, and record what actually happens. 1. Block half the condenser airflow. Predict head pressure and subcooling. 2. Restore. Block most of the return air. Predict suction pressure and superheat, and watch for ice starting evenly across the coil. 3. Restore, thaw completely, re-baseline. What you will learn that reading cannot teach: how fast each fault develops, what it sounds like, and how much a mild fault moves the numbers, which is far less than the textbook cases suggest. Real faults present mildly, and recognising a machine that is 15 percent wrong is the actual skill. Keep the table. A personal record of measured fault signatures on a machine you understand is worth more than any published chart.

Section 11: What This Chapter Bought You

Four faults cover most of the field: undercharge, overcharge, restriction, airflow. Chapter 12’s two numbers separate the first three, and airflow is checked before any of them.

High superheat with high subcooling is a restriction. Nothing else produces that pair.

Check airflow before touching the charge, always, because adding refrigerant to an airflow problem creates two faults out of one and the second one is permanent.

An iced coil has three possible causes and by the time you see it, it looks like all of them. Thaw first, then diagnose.

Three of the four compressor killers are liquid where vapour belongs, and the fourth is acid after a burnout. Compressors are killed rather than worn out.

Short cycling matters because starts wear compressors, not hours, and its pattern in logged current is diagnostic before a gauge is connected.

And on equipment with auto-reset safeties, your written notes are the only fault history that exists.

Chapter 23 asks whether the refrigerant can be dispensed with altogether.

Bench Degree

Get the degree without the diploma.

Learn the material, not how to pass the exam.