Split air conditioning systems
By Airva Editorial Team · Checked against published UK guidance · Updated
A split system is two boxes and a pipe. One box goes in the room, one goes outside, and a pair of insulated copper lines carries refrigerant between them. That is the whole architecture, and it is why splits dominate UK home installations: putting the compressor outside is what makes the indoor unit quiet enough to sleep next to.
The equipment is largely a commodity decision. The pipe between the two boxes is the project — it determines what your installation costs, where the units can go, and whether the system performs as specified.
What "split" actually means
The word describes where the components sit, not what the system does.
- The indoor unit holds a heat exchanger coil and a fan. It is the quiet half: no compressor, so the noise floor is fan noise, which on low speed on a modern unit is close to inaudible.
- The outdoor unit holds the compressor and condenser coil. This is the half that makes noise and rejects heat, and it is why it lives outside.
- The refrigerant lines run between them — two copper pipes, insulated, plus a control cable and a condensate drain from the indoor unit.
Reverse the cycle and the same hardware heats the room instead, which is why virtually every modern split is a reversible air-to-air heat pump. That matters legally as well as practically: the permitted development right for outdoor units in England is written for heat pumps and excludes equipment used solely for cooling [3], so a reversible system is the one with planning rights attached. Our planning permission guide covers the rest.
A "single split" is one indoor unit on one outdoor unit. Several indoor units on one outdoor unit is a multi-split, which is a different design with its own constraints.
The pipe run is the project
Almost every cost surprise and performance disappointment in a split installation traces back to the refrigerant line.
Length costs capacity. Every manufacturer publishes a maximum pipe length and a maximum height difference between indoor and outdoor units, and performance degrades before you reach either. A long run to a bedroom at the far corner of the house is not just more copper — it is measurably less cooling delivered.
The route decides the price. Surface-mounted trunking down an external wall is fast and cheap. Chasing the pipework into walls, or threading it through floor voids and a redundant chimney, takes far longer and costs accordingly. Both are legitimate; they are not the same quote.
Condensate has to go somewhere. The indoor unit produces water whenever it cools. Gravity drainage to a suitable point is ideal. Where the geometry does not allow it, a condensate pump is needed — typically £120–£250 — and it should be in the quote rather than discovered on the day.
Access decides the rest. A condenser at ground level beside the back door is a routine day. The same unit two storeys up above a drop needs a tower or scaffold, and that cost recurs at every future service.
The refrigerant rules changed, and single splits were the target
If you are comparing older advice with current equipment, this is what changed.
Since 2025, Great Britain rules prohibit placing on the market single split systems containing less than 3kg of refrigerant where the F gas has a global warming potential above 750 [1]. The guidance defines a single split as a system with one cooling coil connected to a remote condensing unit — which is precisely the bedroom or living-room system most households buy.
What that means in practice:
| Refrigerant | GWP | Position |
|---|---|---|
| R-410A | 2,088 [2] | Above the threshold — no longer sold in new small single splits |
| R-32 | 675 [2] | Below the threshold — the current mainstream choice |
| R-290 (propane) | Very low | Appearing in a growing number of products |
Three things follow for a buyer:
- A new single split will be R-32 or similar. If a quote offers R-410A in a small new single split, ask why.
- Your existing R-410A system is not illegal. The rule restricts what may be placed on the market, not what may be run. Existing systems remain legal to operate and service.
- The ban is narrower than it sounds. Larger split systems and chillers are excepted, as are air-to-water single splits and units used solely for cooling products [1] — the guidance was updated in August 2026 to spell those exceptions out.
Choosing the indoor unit
The outdoor half is broadly fixed by capacity. The indoor half is where you have choices:
| Type | Best for | Trade-off |
|---|---|---|
| Wall-mounted | Most rooms; the default | Visible; needs clear wall high up |
| Floor-mounted | Rooms with no usable high wall, under windows | Airflow can be blocked by furniture |
| Ceiling cassette | Rooms with a suitable ceiling void | Needs the void; more builder's work |
| Ducted | Where nothing should be visible | Most expensive and most disruptive to retrofit |
For most homes a wall-mounted head is the right answer and the money is better spent on correct sizing and a tidy installation than on a more discreet unit.
What a split system costs
| Scope | Indicative installed cost |
|---|---|
| Single room, wall-mounted | £1,500–£3,500 |
| Two rooms (multi-split) | £2,800–£4,500 |
Common extras: a dedicated electrical circuit at £400–£700, a condensate pump at £120–£250, and access equipment where the condenser is out of ladder reach. Our cost guide covers the full picture, and running costs works out what it costs to operate.
Treat a single-room quote materially below £1,500 with suspicion. Refrigerant work is legally restricted to F-Gas certified engineers, and that is a real cost in a real quote.
When a split is the wrong answer
- No usable external position. A flat with no balcony, no private external wall, or a freeholder who will not consent needs a different approach — see air conditioning without an outdoor unit.
- Three or more rooms. Past two rooms the economics usually favour a multi-split, because you find one condenser position and pay access costs once.
- A few weeks of use a year. For a spare room used three weeks each summer, a portable unit is genuinely the sensible purchase — see the portable comparison.
- Rooms with opposite seasonal needs. If one room needs cooling while others need heat, separate systems preserve that independence.
Specifying one properly
- A heat-load calculation for the room — floor area alone is not a specification. A west-facing bay carries far more late-afternoon solar gain than the same square metres facing north.
- The pipe route and length, stated.
- The condenser position on a drawing, including service access in ten years.
- Electrical scope, including whether a dedicated circuit is needed.
- Commissioning paperwork recording the refrigerant type and charge in kg and tCO₂e. Keep it — you will want it at sale, and it determines whether any leak-check duty applies [4].
Frequently asked questions
How does a split air conditioning system work? An indoor coil absorbs heat from the room, refrigerant carries it through insulated copper lines to the outdoor unit, and the outdoor coil rejects it. Reversing the cycle heats the room instead.
Why is the compressor outside? Because it is the noisy component. Putting it outside is what makes the indoor unit quiet enough for a bedroom — see our quiet air conditioning guide.
Is R-32 better than R-410A? For new small single splits it is the compliant choice, because R-410A's GWP of 2,088 is above the 750 threshold that applies from 2025, while R-32 sits at 675 [1][2]. R-32 systems also typically need less refrigerant charge for the same duty.
Do I need to replace my R-410A system? No. The rules restrict placing new equipment on the market, not running or servicing existing systems [1].
How long can the pipe run be? It depends on the model — every manufacturer publishes a maximum length and height difference. Performance falls off before the limit, so shorter is better. Ask for the assumed length in the quote.
Do I need planning permission? For a house in England, usually not, provided the system is reversible and the Class G conditions are met [3]. Cooling-only units, flats and listed buildings are the exceptions.
Get one specified for your room
The equipment is the easy part. Tell us about the room and the building and we will match you with one vetted installer who will calculate the load, plan the pipe route and quote against both: get an installer match.