Aircon refrigerant circuit: the sealed loop explained
Leak, low charge, blockage and high pressure sound like four separate problems. They are four symptoms of one closed loop of pipe and components that was never meant to open. What that loop is decides how each of them gets found and priced.
By Team Snowflake | Updated 5 Aug 2026
One loop that runs between the two units
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 on every pass. There is no beginning to it and no end. That is what makes it a circuit rather than a supply.
One quantity of refrigerant lives inside it permanently. Whatever went in at commissioning is what travels round it now, shifting between liquid and vapour as it goes. How that shift carries heat out of a room is the refrigeration cycle explained elsewhere, and none of it gets repeated here. This page is about the container, not the physics running inside it.
Two copper lines carry the traffic between the units. The larger, insulated one returns cool vapour from the room back to the compressor. The thinner one sends warm liquid out to the room. In a Singapore flat both normally travel together inside trunking or above a false ceiling, taped as a pair, which is why most owners meet them as one bundle rather than as two separate pipes.
The loop is the machine, not plumbing bolted to a machine. A homeowner reasonably pictures the outdoor box as the aircon and the pipework as the connection between two boxes. The system does not work that way. Copper running through a wall is as much a working part of it as the compressor sitting inside the casing.
What is not on the loop
Most of an aircon sits outside the refrigerant circuit. Fans, filters, the control board, the wiring, the condensate tray and the pipe carrying water away all handle air, water or electricity. None of them touch refrigerant, and none of them oblige anyone to open the loop when they fail.
Drainage is the boundary people cross most often by mistake. Condensate forms on the outside of the indoor coil and runs off down a pipe with no connection whatsoever to what is inside the tubing. A dripping indoor unit is therefore rarely a refrigerant question. It is water that failed to leave by the route built for it.
What sits on the loop
Six things sit on the circuit, and the value here is in seeing them as one list rather than six separate parts. Each has its own page and its own failure story. Grouping them matters because 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, and that shell is itself part of the sealed volume.
- The condenser coil is where heat leaves the refrigerant. It is a long folded run of tubing threaded through fins, and every centimetre of that tubing is circuit.
- The metering device drops the pressure ahead of the indoor coil. On many systems that is a capillary tube. On others it is an expansion valve.
- 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 so 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, and valve cores tucked behind the port caps.
Length is a property of the loop, not a detail
Two of those six items are not really components at all. A coil is metres of narrow tubing folded into a small box, and the connecting lines can run the length of a flat. Most of the circuit, by surface area, is pipe rather than machinery.
That changes how a fault gets hunted. An opening at a flare is at a known fitting that a technician can put a hand on. An opening in a coil is somewhere across a long folded run buried behind fins, and it needs a different method to pin down. Each one is simply an opening in the same loop. Neither is the same size of search.
Loop size varies by system, and so does the search
Internal volume varies with the equipment and with the route it was given. A compact single-split serving one bedroom holds a modest amount of refrigerant in a short run. A multi-split feeding three rooms from one outdoor unit carries more tubing, more joints, and more refrigerant travelling round it.
More joints means more candidate points when something is escaping. That is the practical consequence for an owner weighing two quotes. A leak search across a multi-split covers a longer circuit with more places to check, so it is a bigger piece of work than the same search on a single system, even though the fault description reads the same.
Sealed is the one property everything else follows from
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 how these faults behave falls out of that one 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. Which quantity belongs in there, and how anyone arrives at that figure, gets settled on its own page.
Anything getting in is contamination, and it behaves worse than most owners expect. Air, water vapour and fine debris all arrive by the same door, an opening that was either made deliberately or failed unnoticed. Once inside, they travel with the refrigerant to every part on the loop, the compressor included.
Moisture is the one that does lasting harm. Water and refrigerant oil do not sit together quietly. They react, the blend turns corrosive, and the compressor ends up washing itself in that blend every time it starts. Moisture can also ice up at the metering device, where the pressure drop happens, and throttle the flow on and off. Cooling that comes and goes for no visible reason is one of its signatures.
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 whole loop as a result, and the compressor labours against it. The unit carries on cooling. It simply spends more to deliver less, and nothing in the room reports why.
Contamination cannot be cleaned out in place
There is no filter to change and no way to flush the loop while it runs. Some systems carry a filter drier on the liquid line, a small canister holding desiccant and a strainer, and it does catch what got in during a properly closed job. Its capacity is finite. It was never a rescue device for a circuit that was left open and then filled anyway.
Clearing contamination means emptying the circuit and drawing it down under vacuum until trapped water boils off and leaves. That cannot happen on a running system, and it cannot happen through a service port with refrigerant still inside. This is why technicians treat an opened loop as a commitment rather than a step. Once it is open, the whole sequence has to follow.
Opening the circuit is a different class of work
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, swapping the part, and putting the panels back. Changing a component that sits on the circuit means the refrigerant comes out first, the loop gets dried and proved, and a fresh charge goes back in behind the repair.
Three steps wrap around every such repair, and none of them 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. A part change only counts as finished once all three have happened.
This is why a quote climbs once the fault turns out to be inside the loop. The climb is not a mark-up. Recovery equipment has to come to the flat. Refrigerant has to be replaced rather than reused. A whole sequence has to finish before the system runs again. One identical-sounding complaint therefore carries two very different scopes. Which side of the wall the fault sits on is what separates them.
Ask which side a quoted repair falls on before setting two prices against each other. Quotes are only comparable when both are doing the same class of work. A cheaper one that leaves out recovery and evacuation is not a better deal on the same job. It is a different job wearing the same description.
| What the repair has to reach | Does the loop get opened | What the scope then has to carry |
|---|---|---|
| What the repair has to reachA choked drain line or a fouled coil surface | Does the loop get openedNo. The work happens on the outside of the tubing | What the scope then has to carryAccess, cleaning, reassembly, and a run test |
| What the repair has to reachA fan motor, a capacitor, or a control board | Does the loop get openedNo. Electrical parts sit off the circuit entirely | What the scope then has to carryThe part, the wiring, and proof it runs correctly |
| What the repair has to reachA weeping flare joint at the outdoor unit | Does the loop get openedYes. That fitting is part of the sealed wall | What the scope then has to carryRecovery, the joint remade, evacuation, then a measured charge |
| What the repair has to reachA metering device that has stuck | Does the loop get openedYes. It sits partway round the loop | What the scope then has to carryRecovery, the swap, evacuation, charge, then readings proving the new part feeds properly |
| What the repair has to reachA failed compressor or a pierced coil | Does the loop get openedYes, and the opening is a large one | What the scope then has to carryAll of the above, plus clearing whatever the failure pushed into the rest of the circuit |
Why a partial approach fails quietly
Cutting the sequence short produces no immediate failure, which is exactly what keeps the shortcut alive. A circuit that was opened, patched and refilled without a proper evacuation blows cold on the day the work is signed off. The damage accumulates out of sight and lands on the compressor well past the point where anyone would connect the two events.
The room reports none of this. No homeowner can tell a properly closed circuit from a rushed one by looking at the unit or feeling the air at the vent. What is available instead is the account of what was done. A repair described in steps that name recovery and evacuation was a different piece of work from one described as a patch and a refill.
Circuit faults are read, not seen
Nobody can look inside a sealed pipe, so nothing on the loop is diagnosed by eye. The refrigerant is invisible, it is under pressure, and the tubing around it is opaque metal. Every fault inside the circuit is established indirectly, from what it does to pressure and temperature at the few points where instruments can 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 as it travels. Pipe temperatures get read alongside, because a pressure on its own says too little. The pairing is what turns a number into a statement about one particular 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 dead capacitor is measured directly with the part in hand. A restriction in the liquid line offers none of that. It is inferred from a temperature difference across a stretch of pipe that looks identical to every other stretch.
What follows from that is straightforward. A verdict about the loop has to arrive with readings attached. Not a brand of instrument, and not a description of effort, but figures taken at the valves and what they were weighed against. A conclusion about the inside of the circuit that rests on how the air felt at the vent has not been established. It has been guessed at from the one place that cannot see in.
Where the readings run out
Readings narrow a fault to a stage, then physical methods take over to pin the exact point. A confirmed loss tells a technician the loop is open somewhere. It says nothing about where along the run. Finding that spot is a distinct piece of work carrying its own equipment and its own sequence.
Parts of the loop cannot be reached at all without opening finished work. Copper buried in a wall chase or sitting above a sealed ceiling is both invisible and untouchable, and a leak there turns the conversation from repair into access. Worth knowing before a search begins, because it decides whether the answer is a joint remade where it sits or a stretch of ceiling coming down.
Leak, low charge, restriction and high pressure are one family
Four fault names cover most of what goes wrong inside the loop, and they usually get presented as unrelated problems. They are not. Each describes a different way the same closed system has stopped behaving like one, and each is established from the same readings taken 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, and that can start on the circuit or in the air around the outdoor coil.
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 from sight. And all four raise the same opening question, which is what the loop is doing that a closed loop should not.
The table below sets what an owner typically gets told against what the statement actually claims about the circuit.
| What you were told | What that claims about the loop | What still has to be established |
|---|---|---|
| What you were toldThere is a gas leak in the system | What that claims about the loopRefrigerant is escaping at one or more points on the run | What still has to be establishedWhere the opening sits, before anything goes back in |
| What you were toldThe system is low on gas | What that claims about the loopThe loop holds less than the machine was built around | What still has to be establishedHow it got that way, since nothing in a shut system uses it up |
| What you were toldThere is a blockage in the pipework | What that claims about the loopFlow is obstructed somewhere along the circuit | What still has to be establishedWhich side of the obstruction the readings were taken on |
| What you were toldPressures are running high | What that claims about the loopHeat is going in faster than the loop can send it out | What still has to be establishedWhether the obstacle is inside the tubing or outside it at the fins |
| What you were toldAir got into the system | What that claims about the loopSomething entered a volume that should admit nothing | What still has to be establishedWhen it was last opened, and what was done before it was shut |
The frame survives the vocabulary
Suppliers describe these faults in different words, and some of those words are chosen to make the work sound smaller than it is. A top-up, a recharge, a pressure adjustment and a gas service can all refer to the same intervention on the same sealed volume. The naming shifts. The loop does not.
Holding the frame is what lets an owner ask the one question that cuts through the naming. Does this repair reach inside the circuit, and if it does, what is the plan for getting the refrigerant out, the moisture out, and the correct charge back in. Anything on the loop is either closed and clear or it is not, and every honest piece of work in there starts by settling which.
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