Aircon refrigerant restriction: when gas is not the fault

A restriction is anything that narrows the passage refrigerant has to travel through. The loop was built with a designed bore, and every part fitted onto it was sized to pass a certain volume. Squeeze one place and the whole loop is limited by that place, however healthy everything else looks.

Four things produce one in practice. A metering device jammed part closed. A drier so loaded with what it collected that it now chokes the line it was fitted to protect. A length of pipe crushed or kinked, during installation or during renovation work carried out around it. Debris that has drifted inside the pipework until a bend caught it.

Geometry is what unites them, not quantity. Nothing escaped, nothing is missing, and the weight sealed inside is whatever it was before the trouble started. Refrigerant simply cannot get past one place at the rate the design assumed.

The repair follows from that difference. A shortage is put right by returning refrigerant to the loop. A narrowing is put right by taking away whatever is doing the narrowing, and gas takes nothing away.

Both faults arrive at the same destination, which is an indoor coil fed less than it was built to receive. The coil has no way of knowing why. It boils whatever reaches it, runs dry partway along its length, and returns less cooling than the machine was rated for.

Suction pressure sinks in both cases, and that is the first figure a quick visit takes. Gauges go onto the service port, the reading sits under where it belongs, and the shape of it matches every account of lost gas the attending technician has met before. Either fault can end in a low pressure trip for that same reason.

The flat gives nothing away either. Air keeps coming out of the grille with force while failing to feel properly cold. Rooms take longer to settle than they used to. Frost can show on the pipework whichever fault is present, because a coil starved by a blockage ices for the reason a coil starved by loss does.

So this is rarely a careless reading of the symptom. It is an accurate reading of a symptom that two faults share, followed by whichever cause turns up more often. Leaks are common. Narrowed pipework is not. Under time pressure the frequent answer wins, and it wins before anything has been measured that could have told them apart.

An undercharged loop is thin in every part of itself at once. Less sits in the outdoor coil, less travels down the liquid line, less reaches the room. Nowhere holds a surplus to make up the difference, because the total quantity inside has dropped.

A restricted loop holds its full charge and cannot shift it. Refrigerant banks up behind the narrow point and thins out in front of it. One circuit is therefore oversupplied on one side of a single place and starved on the other, which is a state no shortage can create.

Readings pick that up directly. Superheat runs high on both faults, because a dry coil is a dry coil regardless of the reason, so it settles nothing by itself. Subcooling is the figure that splits them, since it reports what the outdoor coil is holding. A loop genuinely short of refrigerant cannot fill that coil, so the number falls. A loop backed up behind an obstruction fills it further, so the number climbs.

The obstruction frequently marks its own position as well. Refrigerant squeezed through a narrow gap loses pressure, and losing pressure makes it cold. A liquid line that feels warm going in and noticeably cool coming out has been metered at a spot that was never meant to meter anything. Sweat beads or a ring of frost sitting there is the same evidence in a form anybody can see.

Extra refrigerant does not widen anything. How much gets past the tight spot is decided by the size of the gap, not by how much is queued behind it, so the room coil receives no more than it did before the hose was connected. The cooling complaint survives the visit untouched.

The rest of the loop is left worse off. It now carries more than the maker specified, and every gram of the surplus gathers on the side where refrigerant was already banking. Liquid stands in the condenser instead, taking up surface that has to release heat into the air. Head pressure rises, the compressor pushes harder against it, and current draw goes up for cooling that never arrived.

None of that is visible while the technician is still in the flat. A blocked system carrying too much refrigerant still puts cool air through the grille, only feebly, and nothing on the day shows the extra load just placed on the compressor. An overcharge does its damage quietly and out of reach.

One further risk waits further out. Clear the narrowing later, whether by repair or because trapped debris finally shifts, and the loop is suddenly free to circulate an amount it was never charged to carry. Liquid can then travel back to the compressor inlet, and a compressor handles vapour rather than liquid.

• •The tight spot passes the same amount whatever is queued up behind it
• •Surplus collects upstream, on the side that was already overfull
• •Head pressure rises, so the compressor draws more current for less cooling
• •Clearing the obstruction later frees a quantity the loop was never sized around
• •Nothing on paper says how much refrigerant was inside before the top-up

Cooling that did not improve after refrigerant went in is evidence about the diagnosis, not about the dose. A system truly short of gas answers immediately and obviously. Where the room feels the same afterwards as it did before, what was added was not what was missing.

A brief improvement carries the same message with one detail attached. Filling a blocked loop raises the pressure behind the gap, and higher pressure does force a little more through it, so the flat can feel better for a spell. The gain is small and it does not last, because the gap itself never changed. That fade is not the signature of a fresh leak.

Two further facts strengthen the case and both belong to the household rather than the technician. The first is whether an escape point was ever actually named on any previous visit. Refrigerant that has gone missing left through somewhere specific, so repeated gas visits with no location identified describe a loop that is losing nothing. The second is whether weak cooling started right after somebody opened the pipework. A valve left part shut, a flattened line and debris shut inside all date from that day.

Ask for subcooling by name. Superheat reads high on either fault and cannot separate them, which is why a report carrying only that figure has not finished the work. Subcooling says whether the outdoor coil is starving or backing up. It is also the reading most often absent from a report that concluded low gas, and a technician who took it can say which of these two faults was present.