Aircon Refrigerant Circuit: The Sealed Loop Explained

The refrigerant circuit is a single closed run of tubing that finishes where it starts. It leaves the outdoor unit, crosses to the indoor unit through one pipe, returns through another, and passes the same components in the same order. That is what makes it a circuit rather than a supply.

One quantity of refrigerant lives inside it permanently. Whatever went in at commissioning travels round it now, shifting between liquid and vapour as it goes. That cycle carries heat out of a room, but this page is about the container, not the physics inside it.

Two copper lines carry the traffic between the units. The larger, insulated one returns cool vapour from the room to the compressor; the thinner one sends warm liquid out. In a Singapore flat both travel together inside trunking or above a false ceiling.

The loop is the machine, not plumbing bolted to a machine. A homeowner pictures the outdoor box as the aircon and the pipework as the connection between two boxes. Copper running through a wall is as much a working part as the compressor inside the casing.

Six things sit on the circuit, and the value is in seeing them as one list. A fault in any one of them makes the same demand on the job that follows.

• •The compressor drives refrigerant round the loop. It lives in the outdoor unit inside a welded shell that is itself part of the sealed volume.
• •The condenser coil is where heat leaves the refrigerant: a long folded run of tubing threaded through fins, every centimetre of it circuit.
• •The metering device drops the pressure ahead of the indoor coil: a capillary tube on many systems, an expansion valve on others.
• •The evaporator coil is where heat enters the refrigerant. Same construction as its outdoor counterpart, sitting behind the indoor unit's filters.
• •The two connecting lines run between the units and hold a genuine share of the total refrigerant, more on a long routed run.
• •The service valves and the joints are where the loop was closed: flares at each unit, brazed joints along the run, valve cores behind the port caps.

The circuit is sealed, meaning nothing is supposed to cross its wall in either direction: no refrigerant out, and nothing from the room or the ledge in. Every rule about these faults falls out of that sentence.

Refrigerant getting out is a leak, and a leak has a location. A closed loop uses up none of what sits inside it, so a system holding less than it should has an opening somewhere on the run. That is a fault to be found and shut, not a level to be brought back up.

Anything getting in is contamination, and it behaves worse than most owners expect. Air, water vapour and fine debris arrive by the same door, an opening either made deliberately or failed unnoticed, and travel with the refrigerant around the loop.

Moisture is the one that does lasting harm. Water and refrigerant oil react, the blend turns corrosive, and the compressor washes itself in it every time it starts. Moisture can also ice up at the metering device and throttle the flow on and off.

Air is the quieter passenger. Refrigerant condenses and air does not, so air occupies coil surface out on the ledge that ought to be releasing heat. Pressure climbs across the loop and the compressor labours against it. The unit still cools, but spends more to deliver less.

Work that reaches inside the loop is not the same kind of job as work that stays outside it. Changing a fan motor means undoing fixings and swapping the part. Changing a component on the circuit means the refrigerant comes out first, the loop gets dried and proved, and a fresh charge goes back in.

Three steps wrap around every such repair, and none are discretionary. Recovery pulls whatever refrigerant remains into a bottle instead of letting it escape off the ledge. Evacuation clears out the air and water vapour that entered while the loop stood open. Recharging returns the correct quantity.

This is why a quote climbs once the fault turns out to be inside the loop. Recovery equipment has to come to the flat, refrigerant has to be replaced rather than reused, and a whole sequence has to finish before the system runs again. One identical-sounding complaint carries two very different scopes.

Ask which side a quoted repair falls on before setting two prices against each other. Quotes are only comparable when both do the same class of work. A cheaper one that leaves out recovery and evacuation is a different job wearing the same description.

Nobody can look inside a sealed pipe, so nothing on the loop is diagnosed by eye. The refrigerant is invisible, under pressure. Every fault inside the circuit is established indirectly, from pressure and temperature at the points where instruments attach.

Those points are the service valves on the outdoor unit. Gauges connect there, and readings taken with the system running describe how the refrigerant is behaving. Pipe temperatures get read alongside, because a pressure alone says too little. The pairing turns a number into a statement about one stage of the loop.

This marks a real difference from the rest of the machine. A seized fan is visible, a blocked drain announces itself in water. A restriction in the liquid line is inferred from a temperature difference across pipe that looks like every other stretch.

What follows is straightforward. A verdict about the loop has to arrive with readings attached: figures taken at the valves and what they were weighed against. A conclusion resting on how the air felt at the vent has not been established.

Four fault names cover most of what goes wrong inside the loop, and they usually get presented as unrelated problems. Each describes a different way the same closed system has stopped behaving like one, established from readings at the same two valves.

A leak is the loop open where it should be shut; a low charge is the state that follows from one. A restriction is the loop obstructed where it should be clear, and from the room side that produces cooling loss indistinguishable from a shortage. High pressure is the loop unable to shed heat as fast as it takes heat in.

Reading them as one family changes what a quote ought to contain. All four sit within the sealed volume, so all four bring the recovery and evacuation sequence with them if the repair reaches in. All four are established from readings rather than sight.

The table below sets what an owner typically gets told against what the statement actually claims about the circuit.