Aircon Refrigerant Charge: The Correct Amount Is a Weight

A refrigerant charge is a quantity, stated as a weight. The manufacturer fixes it for each model and prints it on the outdoor unit, in grams or kilograms. It is not a level to be judged by eye, or a setting anyone dials in on site.

The figure attaches to a model, and to nothing broader. A brand name settles it no more than a category does, because coil volume, pipe bore and compressor size shift across units that appear identical in a showroom. A machine built around one weight will not behave correctly holding a neighbour's weight, which is why no honest source publishes one number for aircon in general.

The nameplate settles it. That plate sits on the side or rear panel of the outdoor unit, beside the model number and the refrigerant type, and the installation manual repeats the same figure with its conditions. Both are published documents, so the number exists on paper long before anyone connects a hose.

What the plate states assumes a reference length of pipe. The factory seals the charge in before shipping, sized for the machine plus a stated run of pipework. Where the actual route is longer, the correct quantity is larger than the plate says, so the printed figure starts the calculation rather than finishing it.

Pressure inside a running system is not a count of what is in it. It reports the condition the refrigerant is in at that moment, and that condition moves with the weather, the room, and the compressor's effort. One correct charge reads high at noon on a west-facing ledge and lower at night in a shaded corridor, with nothing entering or leaving the circuit in between.

Heat load moves the reading just as much. A flat switched on after a hot afternoon presents a large load, and pressures sit high while that load comes down; the same machine holding a settled room reads lower on the same gauges. Both readings are correct, and neither is a quantity.

Weight holds still. Refrigerant does not thin out or evaporate away inside a sealed circuit, and does not weigh differently in the afternoon. A stated mass is a fixed target, and a fixed target is something a homeowner can check and find on an invoice afterwards.

None of this is an argument against gauges. Pressure readings taken alongside pipe temperatures produce superheat and subcooling, and that pair is how a technician judges whether a running system is fed properly. What gauges cannot do is count: they describe how the circuit is behaving, while only a scale establishes how much is inside it.

Pipework holds refrigerant, so a longer route needs more of it. The manual states a rate for exactly this, as a weight per metre past the pre-charged length. The sum is arithmetic: routed length, less the pre-charged length, times the rate, in grams.

Extra refrigerant itemised against a long route is a good sign rather than a suspicious one. That entry is not padding: the unit was designed around a reference length the flat did not provide, and the shortfall has to go in for the machine to deliver what it was rated at. The entry should name the weight and how it was arrived at.

Leaving it out puts the system short the day it is commissioned, and handover exposes none of that. Cold air comes out of the grille, the flat cools down eventually, and a new installation gets the benefit of every doubt. Months later the weak room gets blamed on afternoon sun, the ceiling height, or the brand on the box.

Whether the route is allowed at all is a separate question. Pipe run length limits govern how far the outdoor unit may sit from the room it serves, while the additional charge governs whether a permitted route was commissioned correctly. A run can clear its limit comfortably and still be under-filled, because the two get checked at different moments.

Both conditions cool badly and arrive there by opposite routes. Too little refrigerant starves the indoor coil; too much floods it. Which one is in front of you decides whether gas comes out or goes in, and a wrong guess costs either way.

The starved version is the familiar one. Pressure in the indoor coil falls, and the coil surface can drop below freezing and ice over, collapsing airflow so the room gets worse rather than slowly better. Circulation also carries heat away from the compressor, so a thin circuit leaves the machine running hotter than intended.

The flooded version is the one nobody expects, because more sounds as though it should mean colder. It does not: surplus refrigerant fills part of the outdoor coil that should be rejecting heat, so less heat leaves the system and pressures climb. The compressor pushes harder against that resistance for less cooling, and the electricity bill rises while comfort falls.

The heaviest consequence lands on the compressor itself. Liquid that should have boiled off indoors can reach the compressor inlet instead, and that machine was built to squeeze vapour. Repeated slugs of liquid batter valves and bearings, and the damage builds quietly behind a system that is still blowing cold air.

Neither condition separates itself at the vent. A flat that cools slowly feels identical in both cases, so the direction gets decided at the scale, not in the room.

Adding gas until the air feels cold aims at a sensation rather than a quantity. The sensation arrives before the correct weight does, and carries on arriving after the correct weight has passed; at no point does the system announce that it now holds enough.

Overshoot is the natural result. Cooling improves steadily while a short circuit fills, so the feedback keeps saying yes. Past the specified weight it goes quiet instead of reversing, because a mildly over-filled system still blows cold air that day.

The method also builds on an unknown base. Nobody in the flat knows what the circuit held before the hose was connected, so nobody knows the total afterwards. The next technician inherits a system with no established quantity, and the same guess starts over.

Why the circuit was low is the question that decides whether any of this repeats. Refrigerant is not used up by running, so a short system has lost it somewhere; restoring the weight without locating the opening settles nothing.

Putting the charge back after a confirmed repair is a different act, and it is the normal end of that job: the circuit gets emptied, the fault gets fixed, and the specified weight goes back in. That sequence contains a quantity from start to finish, while a blind top-up contains none.

A correct recharge begins by emptying the circuit rather than adding to it. Whatever is in there is unknown, and an unknown plus a known is still an unknown. Recovery draws the existing refrigerant into a cylinder, which also keeps it out of the atmosphere rather than venting it.

Pulling a vacuum comes next. Air and moisture get into any circuit that has been opened, and both have to come out before refrigerant goes in. The vacuum and pressure test is a subject in its own right, with its own readings and hold times, but what the quantity depends on is the order: an empty, dry, proven circuit is what makes a measured amount mean anything.

The charge goes in by weight after that. The cylinder sits on a scale, the target is the nameplate figure plus whatever the pipe route adds, and the scale decides when to stop. The number is settled before the hose is connected, and that is where charging a system and topping one up part company.

Verification comes last, and it never replaces the scale. Superheat and subcooling, read against the conditions on the day, confirm the machine is behaving as it should with that weight inside it. A reading that disagrees with a correctly weighed charge points at something else in the circuit, which is useful information rather than an invitation to add more gas.