The short answer
Two insulated copper pipes carry refrigerant between the indoor and outdoor units. They are joined at each unit, tested for leaks and cleared of air before use, and their length and route affect how the job looks and whether extra refrigerant is needed.
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Pipework is the part of an installation that's hardest to judge once it's finished, yet it decides how well the system runs for years. Slow leaks, poor performance and early failures can often be traced back to a badly made joint, a kinked pipe or moisture left inside. Knowing what good pipework involves helps you ask the right questions.
What runs between the units
A wall-mounted split is linked to its outdoor unit by a bundle of parts, usually taped together where they pass through the wall.
| Part | What it does |
|---|---|
| Smaller copper pipe | The liquid line, one half of the refrigerant loop |
| Larger copper pipe | The gas line, the other half |
| Insulation | Stops each pipe sweating or losing heat |
| Drain hose | Takes water away from the indoor unit |
| Connecting cable | Carries power and signals between the units |
Refrigerant goes out along one pipe and back along the other. It takes up far more room as a gas than as a liquid, which is why the gas line is wider. In heating mode a valve in the outdoor unit reverses the flow, and depending on the mode, both pipes can be much colder or hotter than the air around them. The refrigeration cycle explains what happens inside the loop.
Pipe sizes are set by the manufacturer for each model, and larger units usually need a wider gas line.
Insulation and covers
Each pipe is insulated separately along its whole length, joints included, usually in closed-cell foam. Without it, a pipe would drip with condensation in cooling mode and waste heat in heating mode. The two pipes run at different temperatures, so they shouldn't share one sleeve.
Outside, many foam insulations break down in sunlight. Plastic pipe covers fixed to the wall are the neatest protection and also guard against knocks. Some installers wrap the insulation in UV-resistant tape instead.
Joints
Joints are where leaks are most likely, so a good installer keeps them to a minimum, ideally only where the pipes meet the units. From the moment a pipe is cut until it's connected, its ends should be capped or taped to keep out dirt and moisture.
Flare joints are the usual way to connect to the units. The pipe is cut square and the burr removed, with the end held facing down so filings fall out rather than in. A flaring tool spreads the end into a cone, and a nut clamps it onto the connection, tightened with a torque wrench to the manufacturer's figure.
Too loose and a flare joint leaks; too tight and the flare can crack. Installation instructions typically require flare joints to stay accessible for maintenance, so they shouldn't be sealed inside a wall or floor.
Brazed joints are used where pipes are joined along their length. The copper is heated and joined with a filler metal while nitrogen flows gently through the pipe. This stops scale forming inside, which could otherwise break loose and block small parts of the system.
Pipe routes
The route affects how the finished job looks, how long it takes and how the condensate drains.
| Route | How it looks | Things to know |
|---|---|---|
| Straight through the wall behind the unit | Pipes hidden indoors | Shortest and simplest |
| Along or down the outside wall | Covers on the outside | Common for upstairs rooms |
| Along an inside wall | Covers in the room | For rooms without a suitable outside wall |
| Through lofts, floors or cupboards | Mostly hidden | More work; drainage needs planning |
Whatever the route, pipes should be bent smoothly, supported along their length and kept free of kinks and flattened sections.
Length, height and extra refrigerant
Many small systems leave the factory holding enough refrigerant for a set pipe length, often somewhere between 5 and 15 metres depending on the model. A longer run needs refrigerant added on site, weighed in by the engineer and recorded with the system's details.
Every model also has a maximum pipe length and a maximum height difference between the indoor and outdoor units, and some have a minimum length too. Staying within them matters for performance and reliability, and warranties can depend on it. In general, shorter and simpler routes mean fewer bends, fewer joints and less refrigerant.
Testing before the refrigerant goes in
Once everything is joined, the pipework is pressure-tested with dry nitrogen to find any leak. A vacuum pump then draws out air and moisture. Moisture left inside can react with the refrigerant and oil to form acids that attack parts of the system over time.
Only when the pipework holds pressure and then holds a vacuum are the outdoor unit's valves opened to let the refrigerant through. Refrigerant must never be released into the air. Home systems commonly use R32, which is mildly flammable, so it is handled by qualified engineers following the manufacturer's instructions. The whole sequence is set out in what happens on installation day.
Related guides
- What happens on installation dayInstallers fix the indoor and outdoor units, link them with insulated pipes through a hole in the wall, test the pipework for leaks and clear it of air, then connect the power, run the system and show you how to use it.
- The refrigeration cycle, step by stepRefrigerant boils in a cold coil, taking in heat; a compressor squeezes the vapour, making it hot; it condenses in a second coil, giving the heat away; and an expansion valve drops its pressure so it can boil again.
- R32 refrigerantR32 is the refrigerant in most new wall-mounted systems. It carries heat efficiently, usually needs less of it than R410A, and has about a third of R410A's global warming potential. It is mildly flammable, so it's handled with specific precautions.
- Where the water goes: condensate drainageWater that condenses in the indoor unit as it cools runs through a drain hose to a suitable outlet, with a small pump where it can't fall naturally. In heating mode the outdoor unit produces water too.