Bench Degree·REFRIGERATIONchapter

Chapter 18: Split Systems

Cut the machine in half, put the noisy hot part outside, and run two copper pipes between them. Almost everything that goes wrong with a split system goes wrong in those two pipes.

Chapter 17 ended on the two structural faults of a window unit: the compressor is half a metre from your head, and the partition between hot and cold is a thin sheet of steel with holes in it.

A split system fixes both by cutting the machine in two at the two refrigerant lines and putting the halves in different places.

Indoors, the air handler: evaporator coil, blower, filter, drain, and the metering device.

Outdoors, the condensing unit: compressor, condenser coil, condenser fan.

Between them, the line set: two copper pipes and a control cable.

That is the entire idea, and every consequence in this chapter follows from the fact that the two halves are now separated by distance.

Section 1: What the Split Buys

Noise. The compressor is the loudest thing in the system and it is now outside, on the ground, on rubber mounts. An indoor unit contains a fan and nothing else that makes noise. This alone sold the architecture.

A proper thermal boundary. No hole in the wall except two small pipes and a cable, all of which can be sealed and insulated properly. Chapter 17’s conductive partition is gone.

Better efficiency. Both coils can be sized generously because neither has to fit on a shared chassis. Larger coils mean smaller temperature differences between refrigerant and air, which by Chapter 13’s argument means the compressor works across a narrower gap. This is why a split system’s SEER is roughly double a window unit’s.

Zoning and flexibility. One outdoor unit can serve several indoor units, which is what a multi-split or a VRF system is.

And better filtration, because a full-size filter can be fitted in a return duct rather than a 10 mm slot behind a plastic grille.

Section 2: The Line Set, and Why It Is the Problem

Two copper pipes. They look trivial. They are where the faults live.

The suction line is the larger of the two, typically 16 to 22 mm (5/8 to 7/8 in) outside diameter, carrying cold low-pressure vapour from the evaporator back to the compressor. It is always insulated, for two reasons: to stop it picking up heat from the surroundings, which is wasted capacity, and to stop it sweating, because a pipe below the dew point of Chapter 14 condenses water down the wall.

The liquid line is the smaller, typically 6 to 10 mm (1/4 to 3/8 in), carrying warm high-pressure liquid from the condenser to the metering device. It is not normally insulated, because a little heat loss from it is beneficial: it adds subcooling, which Chapter 10 showed buys capacity.

Both are sized from tables published by the manufacturer against the equipment capacity and the total line length. Getting the size wrong is not a small error.

Suction line too small produces excessive pressure drop, which lowers the pressure at the compressor inlet, which lowers the evaporating temperature, which reduces capacity and can ice the coil.

Suction line too large lowers the vapour velocity, and here the problem is not refrigerant at all: it is oil. Oil circulates with the refrigerant as a mist, and it only returns to the compressor if the vapour is moving fast enough to carry it. Below about 5 m/s (1,000 ft/min) in a horizontal run, and considerably more in a vertical riser, oil begins to lie in the pipe. The compressor slowly starves of lubricant and fails months later, and the fault looks like a compressor defect rather than a pipe-sizing error.

Liquid line too small produces pressure drop that can cause the liquid to flash to vapour before it reaches the metering device, which destroys capacity exactly as Chapter 10 Section 5 described.

IN PLAIN ENGLISH: The suction line has to be big enough not to choke the refrigerant and small enough to keep the oil moving. Those two requirements pull in opposite directions, which is why the pipe sizes come out of a table and not out of a hunch.

Section 3: Length, Lift and the Numbers That Actually Bite

Two more line-set constraints, and both are routinely violated.

Total equivalent length. Every metre of pipe and every fitting costs pressure. Manufacturers publish a maximum, commonly 15 to 30 m (50 to 100 ft) for residential equipment, with longer runs permitted only with a larger line size and additional refrigerant charge.

And beyond a stated length, the system needs extra refrigerant, typically 20 to 30 g per metre (0.2 to 0.3 oz per foot) of liquid line beyond the factory charge. Skip that and the machine runs undercharged forever, showing exactly the high superheat and low subcooling signature of Chapter 12, and it will be misdiagnosed as a leak.

Vertical lift. If the outdoor unit is above the indoor unit, the compressor must lift liquid, and gravity opposes it. If the outdoor unit is below, oil must be lifted back up the suction riser against gravity, which is harder. Maximum lifts are published, usually 15 to 20 m (50 to 65 ft), and a P-trap at the bottom of a long vertical suction riser is often required to collect oil and let vapour velocity carry it up in slugs rather than losing it.

SLOW DOWN. Check Your Understanding: An installer runs a 30 m (100 ft) line set on a system rated for a 15 m (50 ft) standard charge, and does not add refrigerant. Predict the superheat and subcooling readings a technician will find a year later, and what that technician will probably do wrong. Think before reading on.

High superheat, low subcooling, which is the Chapter 12 signature of undercharge. And the likely error is that the technician adds refrigerant to correct the readings without asking why the charge was low, finds no leak, and puts it down to a slow leak that does not exist. The readings will improve, so the diagnosis appears confirmed. The right first question on any undercharged system is not “where is the leak” but “was it ever charged correctly?”

Section 4: Commissioning, Which Is Where Systems Are Made or Ruined

A split system arrives as two boxes with no refrigerant in the pipes between them. Getting from there to a working machine is a procedure, and every step exists because of something in Part III of this book.

Braze the joints under a nitrogen purge. Copper at brazing temperature in air forms a black scale of copper oxide on the inside of the pipe. That scale flakes off, travels with the refrigerant and blocks the metering device. Flowing a trickle of nitrogen through the pipe while brazing displaces the oxygen and prevents it entirely.

Pressure test with nitrogen, typically to 2,750 kPa (400 psig), and leave it for an hour. Nitrogen because it is inert, dry and cheap, and because you are looking for leaks before introducing anything you would have to recover.

Evacuate to below 500 microns, and prove it with a micron gauge rather than the compound gauge on a manifold, which cannot read that low with any accuracy. Then close the valve and watch: if the pressure rises and stabilises, there is moisture still boiling off. If it rises without stopping, there is a leak. That distinction is diagnostic and it is why the standing test matters.

Why 500 microns. At that pressure water boils at about 0 °C (32 °F), so any liquid water left in the system is being actively boiled away at ambient temperature. Chapter 7’s saturation curve, used as a manufacturing tool. Water left inside will freeze at the metering device and will form acid with the POE oil of Chapter 8.

Weigh in the charge. For a fixed-orifice system the factory charge plus the line-length adjustment, measured on a scale to within 30 g (1 oz). Charging by pressure alone is guesswork, because pressure depends on load and ambient as well as on charge.

Then verify with superheat and subcooling per Chapter 12, after fifteen minutes of steady running.

The step that gets skipped is the evacuation, because it is the slow one and nothing visibly bad happens if you cut it short. The consequences arrive in year three.

Section 5: Fixed Orifice or TXV, and Why It Changes the Charging Method

Chapter 9 introduced the metering devices. On a split system the choice has a direct practical consequence.

A fixed orifice or piston cannot adapt. The charge determines the superheat, so superheat is the charging measurement, and manufacturers publish charging charts indexed by indoor wet-bulb and outdoor dry-bulb temperature. Which means you cannot correctly charge a fixed-orifice system without a psychrometer, and Chapter 14 is therefore a prerequisite for a job that looks purely mechanical.

A TXV or EEV actively holds superheat at its setpoint, so superheat reads normal across a range of charges and tells you nothing about the charge. Subcooling becomes the charging measurement, and the target is usually 5 to 11 °C (10 to 20 °F).

Getting this backwards is the classic field error, and it is worth stating as a rule: find out which metering device you are looking at before you believe either number.

Section 6: Turning It Round, and the Reversing Valve

A split system’s outdoor unit contains everything needed to be a heat pump, which is why almost all of them now are one. Chapter 19 covered the physics and the defrost cycle in full; here is the hardware detail specific to a split.

The reversing valve is a brass cylinder near the compressor with four copper tubes and a solenoid coil. It swaps which coil receives hot discharge vapour. In cooling, the outdoor coil is the condenser. In heating, the indoor coil is.

Two hardware consequences follow, and they are why a heat pump is not simply an air conditioner with a valve added.

The metering device must work in both directions. A fixed orifice with a check valve bypass in each direction, or a bi-flow TXV, or an EEV which does it natively.

And an accumulator becomes essential. In heating mode the outdoor coil is the evaporator, its refrigerant charge distribution is different, and every defrost cycle throws liquid back down the suction line. The accumulator of Chapter 9 sits before the compressor, catches that liquid, and meters it back slowly. A heat pump without one destroys its compressor, which is why they are standard on heat pumps and optional on cooling-only equipment.

Section 7: Mini-Splits and Multi-Splits

The ductless mini-split is the same architecture with the air handler mounted on the wall of the room it serves, and no ductwork at all.

Which removes the single largest loss in a conventional split system. Duct losses in a typical house run 20 to 30 percent, from leakage, from conduction through ducts running in unconditioned attics, and from the static pressure the fan must overcome. A mini-split has none of it, which is most of why its rated efficiency is so much higher.

Nearly all are inverter driven, which matters more than the ductwork. A variable-speed compressor can run at 20 to 120 percent of nominal, so instead of cycling on and off it modulates. Three consequences, and the third is the one Chapter 14 predicted:

• Temperature is held far more steadily, because the machine matches the load instead of overshooting and coasting.
• Efficiency at part load is much higher, because a compressor running slowly works across a smaller pressure lift.
• And dehumidification is far better, because the coil stays cold continuously instead of in six-minute bursts. This is the direct fix for the oversizing problem of Chapter 14 Section 7, and it is a comfort argument rather than an energy one.

A multi-split runs several indoor heads from one outdoor unit, which brings a new problem: the outdoor unit must apportion refrigerant among heads whose loads vary independently. That is done with electronic expansion valves at each head and a controller deciding how to divide capacity. VRF, variable refrigerant flow, is the commercial-scale version, and it can heat some zones while cooling others by moving heat between them rather than rejecting it outdoors.

Section 8: What Goes Wrong, Specific to Splits

Six faults that are characteristic of this architecture rather than of refrigeration generally.

Line set never purged with nitrogen during brazing. Oxide scale blocks the metering device. Presents as high superheat and high subcooling, the restriction signature of Chapter 12.

Inadequate evacuation. Moisture freezes at the metering device intermittently, so the machine works, then does not, then works. Acid formation destroys the compressor in year three.

Charge not adjusted for line length. Permanent undercharge, misdiagnosed forever as a leak.

Suction line uninsulated or insulation degraded. Capacity loss plus water running down a wall. Extremely common on installations more than ten years old, because the foam perishes in sunlight.

Suction line oversized on a vertical riser. Oil logging, compressor failure much later, root cause almost never identified.

Outdoor unit installed too close to a wall or under a deck. The condenser recirculates its own hot discharge air, so the air entering the coil is far hotter than ambient, condensing temperature rises, head pressure rises, and efficiency and capacity both fall. Manufacturers specify clearances and they are routinely ignored.

ON THE BENCH: Inspect a real installation

Parts: your eyes, a tape measure, an infrared thermometer, and access to any residential split system. Yours, or ask a neighbour. Cost: nothing. Time: 30 minutes. Hazards: do not open electrical panels or remove service valve caps. This is an inspection, not a service call. Check, and write down what you find: 1. Is the suction line insulated over its entire length, including where it passes through the wall and right up to the service valve? Photograph any gaps. 2. Clearances around the outdoor unit. Measure to the nearest wall, fence and overhang. Compare against the nameplate or manual, typically 300 to 600 mm (12 to 24 in) at the sides and 1.5 m (5 ft) above. 3. Is the outdoor unit level and clear of debris? Look into the coil from the side with a flashlight and see whether it is packed with grass clippings or cottonwood. 4. With the unit running, measure the air temperature entering the condenser and compare it with air temperature in open shade 3 m (10 ft) away. A difference of more than 2 to 3 °C (4 to 5 °F) means recirculation, and it is costing capacity every hour the machine runs. 5. Find the metering device type, from the nameplate or the presence of a TXV bulb strapped to the suction line at the indoor coil. What you will probably find: at least two of the six faults above. These are not rare; they are the normal condition of installed equipment, which is why a reader who can spot them is genuinely useful.

Section 9: What This Chapter Bought You

A split system is a window unit cut in two at the refrigerant lines, which buys quiet, a proper thermal boundary and roughly double the efficiency.

The line set is where the faults live. Too small chokes the refrigerant, too large strands the oil, too long starves the charge, uninsulated wastes capacity and wets the wall.

Commissioning is the whole game: nitrogen purge while brazing, pressure test, evacuate below 500 microns and prove it with a micron gauge, weigh the charge, verify with the Chapter 12 numbers. The skipped step is always the evacuation and the bill arrives three years later.

Which measurement charges the machine depends on the metering device. Fixed orifice: superheat, which requires a psychrometer. TXV: subcooling.

A heat pump needs a bi-directional metering device and an accumulator, not merely a valve.

And inverter-driven mini-splits fix Chapter 14’s oversizing problem by modulating instead of cycling, which is a comfort argument first and an efficiency argument second.

Chapter 19 was the heat pump, written earlier. Chapter 20 goes commercial: three-phase power, scroll and screw compressors, and the machines that need a plant room rather than a garden.

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