It is one of the most common calls we get, and almost nobody describes it as a fault. They describe it as a room. "The master bedroom is freezing. The other room never really gets there." Same system, same remote, same temperature set on both, and one room simply refuses to catch up.
This is not usually a broken machine. It is usually one of six things, and five of them can be narrowed down at home this evening without anybody coming to your flat. Here is how to work out which one you have, so that when you do call somebody you are asking for the right job instead of paying for a guess.
First, the thing nobody tells you when they sell you a System 3
A System 3, 4 or 5 is not three, four or five air conditioners. It is one outdoor compressor feeding several indoor fan coils through a shared refrigerant circuit. The outdoor unit has a fixed capacity, and that capacity is shared out among whichever indoor units are calling for cooling at the time.
Run one fan coil on its own and it has the whole machine behind it. Run three at once on a hot afternoon and each one is working with a share. If the system was specified on the assumption that you would rarely run everything together — and many are — then the room that loses is normally the one furthest along the pipe run, or the one with the smallest indoor unit, or simply the one whose door gets opened most.
That is why the single most useful test is also the simplest.
The 30-minute test to do tonight
Switch off every indoor unit except the problem one. Close that room's door and windows. Set it to the temperature you normally use, leave the fan on auto or high, and leave it alone for half an hour.
- If it cools properly on its own, the fan coil, the coil and the airflow in that room are basically fine. What you have is a capacity-sharing or sizing question, not a repair. Skip to the sizing section below.
- If it still will not get there on its own, the problem belongs to that unit or that room, and the next few checks will narrow it further.
This one test separates the two halves of the problem, and it costs nothing. Do it before you agree to anything.
Cause 1: that fan coil cannot move enough air any more
The most common single cause, and the cheapest to fix. Air is pulled through the filter, across the cold coil, and thrown back into the room by a long cylindrical blower wheel sitting behind the coil. Every one of those three can silt up. When they do, less air crosses the coil, less heat is carried out of the room, and the machine runs and runs without the room ever arriving.
You can check the first two yourself. Slide the filters out and hold them to the light. Then put a hand at the outlet — if the air feels thin compared with a unit in another room, you have found something. The blower wheel is the one you cannot check without opening the unit, and it is also the one a general service does not reach, which is exactly the trap we set out in what a general service does not reach.
Two other tells worth knowing: a fouled unit often runs noticeably louder for the amount of air it produces, and it is usually the same unit that smells first when it starts up. If one room is both the warm one and the smelly one, stop looking anywhere else.
Cause 2: the room was never going to be cooled by that unit
Rooms in the same flat are not equal loads. A bedroom taking the western sun through a full-height window, with an external wall on two sides and a flat roof or a ceiling void above it, can carry a heat load several times that of an internal room of the same floor area. If the installer put the same size fan coil in both — and matched fan coils across bedrooms are extremely common — the hot room is simply outgunned.
The honest fix here is not a repair. It is either a larger indoor unit on that room, or changing the load: a blackout or reflective blind on the west-facing glass, closing the door, and not treating the room as a general storage space full of things that keep the air still. Our guide to how many BTU a room actually needs covers what to look at before anyone quotes you a unit, and which system fits your flat covers the outdoor side of the same question.
Cause 3: the charge is marginal, and the far unit shows it first
When a system is slightly low on refrigerant, it does not usually stop working. It gets weaker, and it gets weaker unevenly — the indoor unit at the end of the longest pipe run, or the one with the greatest height difference to the outdoor unit, tends to feel it before the others do.
So a system that has been quietly losing gas for a year can present exactly as one room is not cold, which is why owners so often get told they need a top-up when what they actually have is a leak that will empty the system again. The distinction matters and it costs real money — we set it out in gas top-up or leak repair and in where refrigerant actually leaks.
The clue you can look for yourself is at the outdoor unit while the system is running: both copper lines should be cold, and the larger insulated one should be genuinely cold to the touch rather than merely cool. Heavy frost or ice anywhere on the pipework is not a sign of good cooling — it is a sign something is wrong, and it is covered in why an aircon ices up.
Cause 4: the air has nowhere to go
A fan coil throws cold air out and needs to draw room air back in. In a small bedroom with the door shut, a bed and wardrobe placed under the unit, and the airflow blades pointed straight down at the mattress, the cold air short-circuits: it drops, returns to the unit, and the far corner of the room never joins in. The unit reads its own supply air, decides the room is cold, and throttles back.
Three things worth trying before spending anything: point the blades more horizontally so the air throws across the room rather than down; make sure nothing tall sits directly under or in front of the unit; and if you keep the door closed all night, check there is a gap under it. It sounds trivial. It routinely accounts for two or three degrees of difference between one end of a room and the other.
Cause 5: the ceiling and the pipework between the units
On a concealed or long trunking run, the refrigerant pipes are insulated along their whole length. When that insulation is thin, crushed at a bend, or has degraded over the years, the pipe picks up heat in a hot ceiling void before it ever reaches the far bedroom. The unit at the end of that run gets colder gas than the compressor is producing and warmer gas than the ones closer to it.
This is not something to guess at from the floor. What you can look for is the visible evidence in the room: staining on the trunking or ceiling near the run, which points at condensation forming where insulation has failed. That is the same failure described in trunking condensation and ceiling stains, and it is worth taking seriously because the water has to go somewhere.
Cause 6: the unit is reading the wrong air
The sensor that decides whether the room has reached temperature normally sits in the return air path of the indoor unit, not in the middle of the room. If that unit is sited above a doorway, next to a light fitting, in the path of another unit's discharge, or has a fouled filter blanketing it, it will read a temperature that is not the room's temperature and behave accordingly.
You will spot this as a room that swings — properly cold, then noticeably warm, then cold again — rather than one that is simply never cold. A room that never gets there is a capacity or airflow problem. A room that cannot hold steady is usually a sensing or cycling one.
What to ask for when you do call someone
The difference between a diagnosis and a guess is whether anything was measured. Reasonable things to ask for, and to expect an answer to:
- The temperature of the air going into the problem fan coil and the air coming out of it, measured, with the same figures taken on a unit you are happy with for comparison.
- Whether the airflow at that unit is visibly weaker than the others, and if so, whether the blower wheel has been inspected rather than just the filters.
- Whether the operating pressures were checked at the outdoor unit, and whether the reading is consistent with the number of units running at the time.
- Whether the fault follows the unit or the room — which is what your 30-minute test already established, so say so up front.
Be wary of a visit that begins and ends with "need to top up gas" without anything being measured and without anybody asking why the gas went. That is the most expensive way to be wrong about this, because it comes back.
The short version
- Run the problem unit alone for half an hour. That single test splits the problem in two.
- Cools fine alone means it is capacity sharing or sizing, not a fault.
- Still warm alone means airflow, charge, insulation or the room itself.
- Weak airflow at the outlet points at filters and the blower wheel first, and that is the cheapest thing on this list.
- One room going warm across a whole system can be a slow refrigerant loss showing up at the far end of the pipe run.
- Ask for measurements. A number you can compare is worth more than an opinion.
If you would rather someone worked through it with you, WhatsApp us on 9107 2601 — tell us the property type, the system you have and which room is the stubborn one, and we will tell you what to check before anybody quotes anything. What is fixed and what is quoted after a site check is set out on our published price list, and if it does turn out to be a fouled coil, what a chemical wash actually does explains the difference from a general service.