Aircon pipe run limits: how far the outdoor unit can sit

Every split system carries two published piping limits. One is the total length of refrigerant pipe running from the indoor head out to the outdoor unit. The other is the height difference between the two, measured vertically, whichever of them sits higher. Both belong to the specific model, not to the brand and not to the category.

Those figures are not interchangeable between units that look alike on a shelf. A larger unit in the same family usually gets a longer allowance than a smaller one, and a ducted unit sits somewhere else again. The place to find the pair is the installation manual for the exact model on the quote. A brochure, a spec summary, or a shop assistant's recollection is not the authority here.

Published allowances vary widely enough that a general figure is close to useless. Across residential split systems, total run allowances commonly land somewhere between roughly 15 and 30 metres. Height allowances tend to sit lower, often somewhere in the range of 8 to 15 metres. Treat both as a sense of scale rather than numbers to plan against, because a compact wall unit and a large ducted unit sit at opposite ends of that spread.

The height figure is the one buyers rarely think to check. In most flats both units sit on the same level and the vertical drop barely registers, so it never comes up. It becomes live in landed homes with a rooftop condenser, in maisonettes and duplex layouts, and anywhere the outdoor unit is dropped to a deck below. Length and height are separate ceilings, and clearing one buys no room on the other.

The limits protect the compressor, not the pipe. Refrigerant leaving the compressor carries a small amount of the oil that lubricates it. That oil travels the entire circuit and has to arrive back where it started to do its job again. The pipe run is the road it takes, and a longer road is a harder one.

Velocity is what keeps oil moving. Refrigerant loses pressure as it travels, so a long run slows the flow that the oil depends on. Oil that slows down settles into low points and long flat stretches and stays there. Whatever is lying in the pipework is not lying in the compressor, and the compressor is the only place it does any good.

Height applies the same pressure in one direction. Oil climbing a vertical rise fights gravity the whole way. A tall lift therefore needs more flow to clear it than a flat stretch of the same length. That is why the manual sets a separate height figure. The allowance often differs depending on whether the outdoor unit sits above the indoor one or below it.

A compressor running short of oil gives no warning. It wears at the bearings and the internal surfaces while the room carries on cooling normally, and the failure lands long after anyone would connect it back to install day. That delay is precisely why an over-long run gets signed off without argument. The consequence has no way of arriving in time to change the decision.

The cooling figure on a spec sheet is measured with the units piped at a reference length, and that reference is short. Capacity falls away from there as the run stretches. Pressure drops along the pipe, and less of what the outdoor unit produces arrives at the indoor coil to be used.

The loss is gradual, which is what makes it impossible to argue about afterwards. A room at the end of a long run still cools. It takes longer to get comfortable, slips backwards once the afternoon heat peaks, and draws more electricity for whatever it does deliver. None of that presents as a fault. It presents as a unit that seems slightly weak for the room.

Sizing and routing therefore stop being separate questions. A unit chosen to just about cover a room, then hung on the longest branch in the flat, has had part of its margin taken off before it is ever switched on. The honest fix is settled before the order goes in, either by shortening the route or by choosing capacity that accounts for the run it will sit on. After the pipe is in the wall, neither option is cheap.

A new outdoor unit arrives with refrigerant already sealed inside it. That factory charge covers the unit plus a set length of pipe, and the figure sits in the manual next to the limits. A run shorter than the pre-charged length needs nothing added. A run longer than it needs refrigerant put in, at a rate the manual states per metre of extra pipe.

Weighed is the word that matters. Additional refrigerant charge is measured on a scale against a figure calculated from the actual route, not judged by feel and not eyeballed off a gauge. A quote carrying a line for extra refrigerant on a long run is doing the correct thing, and that line should say how much and on what basis.

Skipping it is invisible on handover day. A system short of gas still blows cold air, and a fresh install gets the benefit of the doubt on everything. What it will not do is reach the cooling it was sized for, and the shortfall gets blamed on the room, the sun, or the brand on the box. Nothing about the symptom points back at a missing weight of refrigerant.

Undercharge is not a neutral state to leave a system in either. Less refrigerant moving means less oil returning with it, so a run that was already stretching oil return is now doing it on thinner circulation. The compressor absorbs that. A quote that stays silent about the charge on a long run is the one to question, because the same silence covers an honest oversight and a deliberate saving equally well.

A System 3 puts three indoor units on one condenser, and the piping constraint stops being a single number. The manual for a multi-split carries a set of them. One ceiling covers the longest individual branch. A separate ceiling covers the combined length of every branch added together. Height figures cover the gap to the outdoor unit and the gap between indoor units.

The combined figure is the one that catches people out. Three modest branches that each look comfortable on their own can total past the allowance while no single branch is anywhere near its own ceiling. Nothing about the individual routes looks wrong. The arithmetic is what fails, and it only fails once someone adds it up.

This is what decides whether a multi-split suits a given flat at all. Where the ledge is nowhere near the bedrooms, the branches to the far rooms carry the whole problem, and hanging a third or fourth head off the same condenser deepens it rather than spreading it. The alternatives are a second condenser or a different grouping of which rooms share which outdoor unit. Both change the quote, which is why neither gets raised late.

Branch balance is a separate concern from the totals. Very uneven branch lengths off one condenser make refrigerant distribution harder to get right, and the shortest branch tends to be favoured while the longest one underperforms. The room at the end of the longest branch is the room that gets complained about later. Naming that risk while the layout is still on paper costs nothing.

The condenser position locks everything downstream of it. Once it is chosen the routes are chosen, the lengths are set, and the limits either fit or they do not. It is also the cheapest thing in the whole job to move while it is still a mark on a drawing.

Ask about the routed length rather than the distance across a floor plan. Pipe does not travel in straight lines. It follows walls, turns corners, drops through a ceiling void, and climbs back up to the ledge, and the routed figure is routinely far larger than a plan suggests. Where the path is genuinely unclear, a site survey settles it by measuring the route instead of assuming it.

The questions below all have answers that exist on paper, which is the point of asking them. An installer working from the manual produces figures. One working from habit produces reassurance, and the two sound similar until you ask for the second number.