Reversible air conditioning: the 7 faults that show up at the October restart (and how to avoid them)
L'équipe Proclimo
5 Oct 2026 - 08 min read
There's always that October evening when you press the little sun icon on the remote for the first time in six months. The unit starts up, blows lukewarm air for four minutes, then falls silent. Or it displays three cryptic characters on the indoor unit. Or a smell of hot dust fills the living room and refuses to go away. Refrigeration engineers' call-out diaries always fill up at the same two moments: the first week of a heatwave, and the fortnight following the first real cold snap.
This is no coincidence. A reversible air conditioner switching to heating mode doesn't simply do "the opposite" of cooling in a symmetrical way: it reverses its refrigeration cycle, calls on components that have sat idle all summer, produces water where it previously produced none, and runs the compressor at far higher loads. Latent faults that went unnoticed in August declare themselves in October.
What physically changes when the unit switches to heating
On an air-to-air heat pump, a four-way valve reverses the direction in which the refrigerant circulates. The indoor unit, which was the evaporator (cold), becomes the condenser (hot). The outdoor unit does the opposite: it becomes the cold point of the system, and its coil regularly drops below 0°C.
Three very concrete consequences:
- the outdoor unit ices up and has to defrost periodically by briefly reversing its cycle. That's normal, noisy, and accompanied by a plume of vapour;
- the condensate switches sides: it is now the outdoor unit that produces it, several litres on a cold, damp day;
- the compressor works harder. The temperature gap between outdoor air at 3°C and supply air at 40°C is far more brutal than in summer. A worn bearing, a capacitor nearing the end of its life or a slightly low refrigerant charge will show up immediately.
That's why a unit that "worked perfectly this summer" can fail on the very first evening of heating without this being in any way contradictory.
The 7 most common faults at the autumn restart
1. The unit blows cold air and never warms up
In nine cases out of ten, this isn't a fault. First check which mode is actually selected: many remotes have an AUTO mode that, depending on factory settings, switches to fan-only as long as the gap to the setpoint stays small. Heating mode has to be forced explicitly (sun icon, or HEAT).
Next, the cold-air prevention delay: when heating starts, the indoor fan deliberately stays off for two to five minutes while the heat exchanger warms up. If nothing happens after ten minutes and the outdoor unit stays silent, the problem lies elsewhere.
If, on the other hand, the supply air is distinctly cold while the outdoor unit is running, there's a good chance the four-way valve is stuck in the cooling position. That's a part costing €150–300 fitted, but professional diagnosis is essential — the "tap the coil with a mallet" advice doing the rounds on forums is best forgotten.
2. An error code on the indoor unit display
Manufacturers don't use the same nomenclature, but the logic is shared: the first letter identifies the fault family.
| Family | General meaning | Manufacturer examples |
|---|---|---|
| Sensor / probe | Thermistor disconnected or out of tolerance | Daikin A5/C4, Mitsubishi P8, Toshiba F04 |
| Communication | Indoor/outdoor link broken | Daikin U4, LG CH05, Atlantic E1 |
| Pressure / charge | Abnormal high or low pressure | Daikin E1/E6, Mitsubishi U2 |
| Fan / motor | Motor blocked, rotor forced | Daikin A6, Samsung E154 |
| Drainage | High water level in the pan | Mitsubishi P5, Daikin A3 |
Write the code down exactly (letter + digits) before cutting the power, and photograph the display. Many codes are wiped by a reset and the technician then loses half an hour trying to reproduce the fault. Service manuals, often downloadable from manufacturers' websites, give the precise correspondence for your model — failing that, a heating and air conditioning service logbook in which you record codes, dates and interventions stops you telling the repair engineer three different versions of events.
A "communication" code is very often linked to the electrical link between the two units: a loose terminal, rodent damage, or water ingress in the cable run. That's electrical work, so off-limits for DIY.
3. The outdoor unit stays encased in ice
A normal defrost lasts 5 to 12 minutes and triggers every 40 to 90 minutes depending on humidity. If the outdoor coil remains a permanent block of ice, or if defrost cycles run back to back without ever freeing the unit, the classic causes are:
- dead leaves pinned against the coil by autumn winds, blocking all air exchange. By far the number one cause in October;
- a blocked defrost water drain under the unit: the water stagnates, refreezes, and traps the bottom of the coil;
- an insufficient refrigerant charge (a micro-leak that appeared during the season), which lowers evaporating pressure and speeds up icing;
- a faulty defrost sensor that no longer detects the end of the cycle.
The first two can be sorted in twenty minutes. Clear 50 cm of free space in front of the coil, remove the leaves by hand, then finish with a soft brush — never a screwdriver, as the aluminium fins bend under the slightest pressure and a flattened fin permanently reduces airflow. A low-pressure washer is tempting but crushes the fins: better to rinse with a gentle garden hose, with the unit switched off at the breaker.
4. An unpleasant smell within the first few minutes
Two smells need to be carefully distinguished.
The hot dust smell in the first few days is perfectly ordinary: dust that settled on the indoor heat exchanger burns off against a coil that is now hot. It disappears after two or three hours of operation.
The musty, cellar-like or sweaty-sock smell, on the other hand, points to microbial colonisation of the drain pan and the fan wheel, a legacy of summer humidity. It isn't just a comfort issue: ANSES has pointed out on several occasions that poorly maintained air conditioning systems can contribute to the dispersal of bioaerosols in indoor air. The first step is still to clean the filters in warm soapy water, dry them completely in the shade, then apply an air conditioner disinfectant spray to the accessible part of the exchanger grille. If the smell persists, the biofilm is in the fan wheel or the pan: only a deep clean by a technician, with partial dismantling, will get rid of it.
Be careful, though: an electrical burning smell, like hot plastic, means switching off immediately at the breaker. You don't restart it "to see if it comes back".
5. The indoor unit's condensate pan overflows
This is the fault that surprises people most in heating mode, since the indoor unit should no longer be producing water. Except that the drain pipe has spent the summer carrying water laden with dust and biofilm. Once it stops, that deposit dries and hardens, forming a blockage. At the slightest cooling or dehumidification cycle in the shoulder season, the water can no longer get through and backs up into the pan.
The signs: dripping down the wall, gurgling, or a "high water level" fault that shuts the unit down. If the installation has a condensate pump, the float can also seize up after six months of inactivity — you then hear it running continuously without ever stopping.
The preventive step, to be carried out at the end of the cold season as well as at the end of the hot season: flush the line. On an accessible gravity drain, a little clean water poured into the pan is enough to check the flow. For blockages that have already formed, a manual condensate vacuum pump lets you suck out the deposit from the end of the pipe without dismantling anything. Never inject chemical drain cleaner, however: the pans are made of polystyrene or ABS and won't tolerate it.
6. Consumption soars with nothing apparently broken
A reversible air conditioner that heats but uses twice as much as the previous winter is faulty, even without an error code. The usual suspects:
- clogged filters: 20 to 30% less airflow, so a compressor that runs longer for the same result;
- low refrigerant charge: the coefficient of performance collapses well before a pressure fault appears;
- setpoint too high: each extra degree costs around 7% more consumption, according to the orders of magnitude used by ADEME;
- fan set to minimum speed, which seems quieter but degrades heat exchange.
To get hard facts, nothing beats measurement. A plug-in power meter on a single-split system, or a monitoring module at the electrical panel on a multi-split installation, gives real consumption in kWh day by day. It is also the only way to check that a change of settings has genuinely cut the bill. Remember that since summer 2026, the shift in off-peak tariff windows has also changed the optimal time to run the system: programming a heating boost during the midday off-peak period no longer makes the same economic sense as it once did.
7. The unit starts, then shuts down and restarts over and over
These short cycles, known as short-cycling, wear the compressor out very quickly. The most common causes at the autumn restart are an indoor temperature sensor sitting in a direct stream of warm air, a unit that is heavily oversized for the room, or a tired start capacitor on non-inverter models.
One point often overlooked: measuring the room's actual temperature and humidity. Many owners set 23°C believing it is 19°C in the room, when the unit's internal sensor, up against the ceiling, is reading 26°C. An indoor thermo-hygrometer placed at seated height, away from the unit, will reveal differences of 3 to 4°C in a single evening — enough on their own to explain erratic behaviour.
What you can do yourself, and what is prohibited
The dividing line is legal, not just technical. Any intervention that involves opening the refrigerant circuit — connections, charging, refrigerant recovery — is reserved for holders of a certificate of competence working for a company holding an operating certification, in line with the F-Gas framework and the national rules applying to refrigerants such as R-32. A private individual who handles or releases refrigerant risks penalties, and their insurer may refuse to cover any resulting damage.
Allowed without a professional: cleaning the filters, dusting the grilles, clearing and gently rinsing the outdoor coil, flushing the accessible condensate line, replacing remote control batteries, resetting at the breaker.
Prohibited: opening the refrigerant circuit, recharging refrigerant, working on the power cabling between units, removing the cover of a live outdoor unit.
For everything else, the logic is economic as much as regulatory: a diagnosis billed at €90–150 often prevents compressor failure, which costs between €900 and €2,000 on a domestic split system and frequently writes the unit off.
The autumn routine that prevents 80% of these call-outs
It takes an hour, once a year, ideally in late September or early October before the first real demand for heating.
- Switch off the power at the dedicated breaker before touching anything.
- Remove and wash the filters of each indoor unit, drying them completely before refitting.
- Dust the supply grille and the swing flap, which is often full of fluff.
- Clear around the outdoor unit: leaves, vegetation that grew back over the summer, boxes stored in front of it. Check that the bottom drain is clear.
- Flush the condensate line and test the flow.
- Run it in heating mode for 30 minutes in broad daylight, at a high setpoint, to pick up any fault before the evening when you really need it.
- Note the date and any codes in the service logbook.
For installations containing 2 kg of refrigerant or more, remember that a maintenance visit by a professional is mandatory at regular intervals in any case, and that it also serves as evidence in the event of a dispute with an insurer or a building management company. In a block of flats, this is also the moment to check that the outdoor unit's mounting hasn't worked loose: a bracket that has come away from the wall transmits vibrations that, come November, turn into a registered letter from the neighbour downstairs.
One last piece of common sense: don't run your heating test on a Sunday evening. If a fault shows up, you'll be calling out an engineer at emergency rates for a part that a technician could have replaced at leisure the following Tuesday.
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