Air Conditioning Trunking and Pipework

Every split system needs pipes and a cable between the indoor and outdoor units. They are what let the noisy parts of the system sit outside, which is why a split system is quieter than a window unit or a portable one. How they are routed and covered makes a big difference to how neat the finished job looks.

What Runs Between the Units

• Two refrigerant pipes, one larger and one smaller, each with its own insulation.

• A condensate drain that carries water away from the indoor unit. It needs a steady fall, or a small pump.

• An interconnecting cable that carries power and signals between the units.

Together they usually pass through one hole in the wall, about 65 mm across.

The Neatest Route: Straight Through the Wall

Sometimes the outdoor unit can sit on the other side of the wall from the indoor unit. The pipes can then run straight out of the back of the indoor unit and through the wall to the outdoor unit, so very little pipework shows inside or out. This back-to-back arrangement is often the tidiest and the lowest-cost option. See wall-mounted air conditioning.

Trunking

Where pipework has to run along a wall, it is covered with trunking: a rigid plastic channel with a clip-on lid, and matching bends, corners and wall entry pieces. It protects the pipes and gives a clean, straight line. Trunking is usually white, and other colours are available to suit dark walls or brickwork.

• Plan the shortest, straightest route, following corners, skirting boards or ceiling lines where possible.

• Outside, use trunking made for outdoor use, which stands up to sunlight.

• Keep trunking clear of windows and downpipes, and leave access to the outdoor unit for servicing.

Hidden Routes

• Through a loft or ceiling void. This is common for upstairs rooms. The pipes rise from the indoor unit into the ceiling and run through the loft to the outdoor unit.

• Through cupboards and wardrobes. A cupboard on the route can hide a vertical run.

• Under floors. This is possible in some homes with suspended timber floors.

• Boxing in. A run can be boxed in and decorated to match the room.

Hidden routes are usually longer, so they must stay within the manufacturer's limits for pipe length. Many wall units allow up to 20 metres of pipework between the indoor and outdoor units.

Can Pipework Be Buried in the Wall?

In new builds and major renovations, it can be tempting to bury pipework in the walls before they are plastered. Industry guidance advises against routing refrigerant pipework where it cannot be reached, because pipework that cannot be reached cannot be checked for leaks. Joints, in particular, need to stay accessible. Where pipes need to be hidden, a route through a void, or a boxed-in run with access panels, keeps them within reach.

Protecting Pipework Outside

Pipe insulation left exposed to sunlight breaks down over time, so outdoor runs should be covered with trunking or a covering designed for outdoor use. Covered pipework is also less likely to be damaged by ladders, pets or garden tools.

The Condensate Drain

The drain needs a steady fall to the outside or to a suitable drain. If the indoor unit is on an inside wall, or the drain cannot fall, a small condensate pump lifts the water. Mini pumps can sit inside the trunking or behind the indoor unit. If the drain is not working, water can drip from the unit: see air conditioner leaking water.

What to Agree Before Installation

1. Where the indoor and outdoor units go.

2. The pipe route, inside and out, and how it will be covered.

3. Where the hole goes, and what is on the other side of the wall.

4. Where the condensate will drain, and whether a pump is needed.

5. The colour of the trunking.

We agree all of this with you at the survey, so there are no surprises on the day. See our guide to air conditioning outdoor units for where the outdoor unit can go.

Frequently Asked Questions

Can the pipes run down the outside of the house?

Yes. Pipes often run down or across an outside wall to the outdoor unit, covered in outdoor trunking.

How far can the pipes run?

It depends on the model. Larger units usually allow longer runs than small wall units, and every system has to be designed within the manufacturer's limits.

Can trunking go across a ceiling?

Yes, but most people prefer to keep it to walls, corners and coving lines, or to hide the run in the ceiling void.

Call 01772 288651 or use the form below to book a free survey. We will measure the room, recommend the right system and give you a clear written quote.

Last updated: November 2026.