1A correctly charged system
The unit holds 0.8 kilograms of R-32, sealed in when it was made and released into the pipes when it was installed. Holding a 24 °C room it boils the refrigerant in the indoor coil at about 9 °C, nearly 10 bar on a gauge, and nothing ever needs topping up: a split system has no consumable refrigerant.
2A flare joint starts to weep
The four flare joints are the only joints made by hand on site: the installer flares the copper pipe into a cone and clamps it to a matching seat with a nut. This one was slightly under-tightened. Every on–off cycle cools and warms it by 15 to 20 degrees, the copper creeps, and refrigerant starts seeping out, a few grams a week. It carries a little compressor oil with it, which leaves an oily, dust-catching stain round the nut.
3The expansion valve hides the loss
With less refrigerant there is less liquid queued up in the condenser. The electronic expansion valve sees the gas leaving the indoor coil getting warmer (more superheat) and opens further to feed the coil. For a while nobody notices anything.
4The indoor coil is starved
With the valve wide open there still isn’t enough liquid: the refrigerant boils away in the first part of the coil and the rest only warms gas that has already boiled. The compressor pulls the pressure down, so the boiling temperature falls. The room warms, so the inverter speeds the compressor up, which pulls the pressure lower still. The returning gas is too warm to cool the compressor motor properly, and its discharge runs hotter.
5The coil drops below freezing
Once the refrigerant boils below 0 °C, the water that normally condenses and drains off the fins freezes onto them instead. Frost appears first on the first passes of the coil and on the small pipe, where the coldest refrigerant arrives.
6Ice blocks the airflow
Frost between the fins chokes the air, so less heat reaches the refrigerant, so the coil gets colder and frosts faster: a vicious circle. The coil turns into a block of ice, only a trickle of air gets through, and the room keeps warming although the compressor is working flat out.
7Anti-freeze protection stops the compressor
The thermistor on the coil has read below freezing for several minutes, so the control board stops the compressor and keeps the fan running to melt the ice. The meltwater arrives faster than the drain hose can take it, and the pan overflows: water drips down the wall.
8How it is caught and prevented
- A split system never “uses up” refrigerant: if it is low, there is a leak, and topping it up without finding the leak only restarts the clock.
- Symptoms: weaker cooling and longer running, ice on the coil or on the small pipe at the outdoor valves, water dripping from the indoor unit, an oily stain at a joint.
- Gauges on the service port show it: a low suction pressure, a high superheat (the gas leaving the coil far warmer than its boiling point) and little subcooling in the liquid.
- Finding it: an electronic leak detector, soapy water that bubbles at the joint, or pressurising the empty circuit with nitrogen and watching the gauge.
- Fixing it: recover what is left, remake the joint (or braze it), pull a deep vacuum to below about 500 microns to boil off moisture, and weigh in a fresh charge.
- Prevention: flares cut square, deburred, lightly oiled and tightened with a torque wrench, about 18 newton metres for quarter-inch and 42 for three-eighths-inch pipe, or brazed and press-fitted joints instead.
- The unit protects itself: the coil sensor stops it before ice reaches the fan, and the discharge sensor slows the compressor before it overheats. Many units then show an error code.