Where the pipes go
Single-split or multi-split: how the unit on your wall connects to the one outside, what decides the route, and when one outdoor unit makes more sense than two.
The short version. Every indoor unit is joined to an outdoor unit by two insulated copper pipes, a cable and a drain, through one hole in the wall. The neatest route is straight through the wall behind the unit. Where that isn’t possible, the pipes go through a loft, a cupboard or under a floor, and outside only in trunking. A multi-split runs up to five rooms from one outdoor unit, and every room’s pipes run all the way back to it.
Five drawings, one house
Tap through them, or swipe. Each step has its own link, so you can send someone straight to the one that matches your house.
What runs between the two units
Three things, taped into one neat bundle:
- Two copper pipes, each insulated. The refrigerant goes out through one and comes back through the other, which is why they’re called a pair.
- A cable, so the two units can power and talk to each other.
- A drain, for the water the indoor unit takes out of the air.
They go through the wall in one hole, about 65 mm across, roughly the width of a drinks can. It’s drilled with the dust caught at the wall, then sleeved and sealed on both sides.
What decides the route
Three decisions, in this order.
1. Where the outdoor unit can go
Usually a side or rear wall, or the ground behind a hedge or garden wall: somewhere it can breathe, can be reached for servicing, and sits well away from your neighbours’ windows. Planning rules keep it off pitched roofs, and off a wall facing the road above the ground floor; in a conservation area, off the front altogether. The noise calculation settles the rest.
More on both: planning permission and noise and neighbours.
2. Where the indoor unit should go
High on a wall, where the air can reach the whole room without blowing on anyone: not over the bed or the sofa. An outside wall is easiest, because the pipes can go straight through behind it.
That’s where it performs best. In practice there’s plenty of flexibility: if furniture, a television or simply how the room looks matters more to you, most walls work, and we’ll show you what each choice means at the survey.
3. The route between them
Straight through the wall if the two are back to back. Otherwise, the shortest tidy route the house allows: up through the loft, down inside a cupboard, under a floor, or outside in trunking. The drain has to fall all the way or have a pump, and every run has to stay within the manufacturer’s limits.
How far can the pipes go?
Further than most people expect. Every outdoor unit has limits: how long each run can be, how long all the runs on a multi-split can add up to, and how far an indoor unit can sit above or below it. On a typical house they’re rarely what decides the route. We still measure every run at the survey and check it against the figures for the exact model, because going over them can void the warranty.
Longer runs do cost more: more copper, insulation and trunking, more time, and past a certain length, extra refrigerant, which we add, weigh and record.
One outdoor unit or two
If you’re doing more than one room, there are two ways to connect them, and neither suits every house.
| Separate systems | One multi-split | |
|---|---|---|
| Outside | One unit for each room | One unit for up to five rooms |
| If one stops | The other rooms carry on | Every room on it waits for the repair |
| Planning | Up to two without an application on a detached house; one on a semi or terrace | One unit, so within the count on any house |
| Pipe runs | Short, often straight through the wall | Longer: every room’s pipes go back to one place |
| Efficiency | Each system rated on its own | Rated as a combination, and your quote states it |
| Fitting | About a day for each room | Two or three days for a whole house |
| Adding a room later | A new system, if there’s space and the count allows | A spare connection can be fitted now, for a little more |
On a semi or a terrace the planning count often settles it: two rooms on separate systems would need a planning application, while one multi-split stays within the count. On a detached house it comes down to where the units can go, how you use the rooms, and the price. We’ll tell you which we’d choose for your house, and why.
Where the water goes
Cooling takes moisture out of the air, so every indoor unit has a drain. It runs with the pipes and falls to a gully or a downpipe outside. Water only runs downhill, so the drain has to fall the whole way. Where the pipes climb, into a loft for instance, a small pump lifts the water instead. A pump makes a faint sound when it runs, which is why we only fit one where the water really can’t fall.
The outdoor unit makes water too, when it’s heating: it drips, and in a cold spell it defrosts. So it stands on feet or brackets, over gravel or near a drain, and never where the water would freeze across a path.
What you’ll see outside
The outdoor unit and, wherever the pipes have to run outside, a line of trunking. We keep both as quiet as the house allows.
- Trunking runs straight, tucks into corners and follows lines that are already there, like a downpipe.
- It’s white, black or brown as standard, and we can look at other colours if none of those suits the wall.
- The outdoor unit goes at the side or the back, on brackets or on the ground.
If you’re building or renovating
Tell us early. While walls are open or floors are up, the pipes can go in before the plaster and the boards, and disappear completely. The same goes for a garden room: the route can be planned into the build rather than added after it.
What we do about it. At the survey we decide both positions with you, measure every route and check it against the limits for the exact model. Your quote names the route room by room, with photographs marked up to show where everything goes. Nothing about the route is decided on the day.
Questions people ask about the pipes
How big is the hole through the wall?
One for each indoor unit, about 65 mm across. The dust is caught as it’s drilled, and the hole is sleeved and sealed inside and out.
Can the pipes be hidden completely?
Often, indoors: through a loft, a cupboard or under a floor, and entirely if you’re renovating. Outside, they run in trunking. We’ll show you the neatest route your house allows at the survey.
How far from the room can the outdoor unit be?
Further than most people expect. Every model has limits on run length and height, and we check each run against them, but on a typical house the limits are rarely what decides it. The route is.
Will I need a condensate pump?
Only where the drain can’t fall all the way outside, usually when the pipes go up into a loft. We’ll tell you at the survey, and it’ll be on your quote.
Can I add another room later?
Yes. On a multi-split we can fit an outdoor unit with a spare connection now. It costs a little more, because an outdoor unit with more connections costs more. Otherwise it’s a separate system, if there’s somewhere for its outdoor unit and the planning count allows.
Is one outdoor unit cheaper than two?
Not automatically. It depends on the rooms, the sizes they need and where the units can go. We’ll tell you which we’d choose for your house, and why.
More guides
- Air conditioning and planning permission in the Chilterns
- Will the outdoor unit annoy my neighbours?
- Eleven questions to ask before you book any installer
This guide is general information. Every house is different, and your routes are decided at the survey. Last updated October 2026.
We work out the routes at the survey
Both positions and every route, measured and agreed with you before we quote, not on installation day.
Book a free survey