End of Summer 2026: The Checklist for Switching Your Reversible Air Conditioner to Heating Mode

The summer of 2026 will go down in installers' memories as one of the busiest seasons in a decade. Weeks on end of near-continuous operation, outdoor units exposed to 40 °C in the shade, filters clogged first with pollen and then with dry dust: French reversible air conditioners are coming out of this period in a very different state from the one they were in back in May.

And that is precisely where the quality of your winter is decided. An air-to-air heat pump that has not been prepared for the seasonal switchover consumes more, defrosts more often, blows less pleasant air and eventually breaks down at the worst possible moment — namely during the first real cold snap, when refrigeration engineers' diaries are already full for three weeks ahead. Late August and September are the ideal window for taking action: temperatures are still mild, the unit is no longer being asked to cool, and it is not yet being asked to heat.

Why the cooling → heating switchover is anything but trivial

On paper, a reversible air-to-air heat pump simply reverses the direction in which the refrigerant circulates, thanks to a 4-way valve. In practice, the mechanical and thermal stresses are very different from one season to the next.

In cooling mode, the indoor unit acts as an evaporator: it condenses moisture out of the air, produces litres of condensate and remains permanently damp. In heating mode, it becomes a condenser: it dries out, heats up, and recirculates everything that settled over the summer in the fan barrel and on the heat exchanger. This is the source of the notorious "musty smell" that many users notice when they first restart in heating mode in the autumn.

Outdoors, the reversal is just as abrupt: the unit that spent three months rejecting heat will now be extracting warmth from cold, damp air, with frost forming on the coil and regular defrost cycles. An outdoor exchanger fouled by fluff, seeds and summer dust sees its efficiency fall, which multiplies those defrost cycles — and therefore electricity consumption.

Fouling of 20 % of the heat exchange surface can be enough to cost an air-to-air heat pump several COP points, according to field feedback from inspection bodies in the HVAC sector.

A reminder of the rules on mandatory servicing

Since decree no. 2020-912, any air conditioning system or heat pump with a nominal output between 4 and 70 kW must undergo mandatory servicing every two years by a professional. That inspection covers, in particular, checking the circuit for leaks, inspecting the filters, assessing efficiency and providing optimisation advice.

On top of that, for equipment containing more than 2 kg of refrigerant (or a charge exceeding 5 tonnes of CO₂ equivalent), there is the periodic leak check provided for by articles R.543-99 et seq. of the French Environment Code, under the F-Gas regulation. The vast majority of residential R-32 splits stay below this threshold, but high-output multi-splits and ducted systems can exceed it.

Legal servicing in no way removes the need for user maintenance: routine upkeep is a monthly or seasonal job.

Step 1: the indoor unit's filters

This is the single most rewarding task on the whole checklist, and the most often neglected. A filter clogged after an intensive cooling season reduces airflow, increases fan consumption, degrades air quality and encourages microbial growth on a heat exchanger that is still damp.

The procedure is simple:

1. Cut the power to the unit (remote control + dedicated circuit breaker if accessible).
2. Open the front panel of the wall-mounted unit — it usually swings upwards.
3. Remove the dust filters by unclipping them upwards then forwards.
4. Vacuum off the larger particles, then rinse in lukewarm water (never above 40 °C) with a little neutral soap.
5. Let them dry completely in the shade before refitting: a filter put back damp becomes a breeding ground for mould.

Many recent models also include additional filters: activated carbon, catechin, photocatalytic, or an electrostatic pre-filter. These elements cannot always be washed and are generally replaced every year. Check the exact part number in your model's manual before ordering a compatible set of activated carbon filters: formats vary enormously from one brand to another, even within a single range.

Cleaning the indoor heat exchanger

Beyond the filters, the finned coil of the indoor unit builds up a biofilm over the summer. A self-evaporating foam air conditioner cleaning spray makes it possible to treat the coil without dismantling anything: the foam settles between the fins, dissolves deposits, then drains away naturally through the condensate tray when the unit restarts.

One caveat, though: this type of product is no substitute for professional cleaning of the fan barrel. If you notice black spots on the fan roller or droplets being sprayed out at start-up, a full strip-down by a technician is needed, with a cleaning bag and a low-pressure washer.

Step 2: condensate drainage and the tray

Three months of condensate production leave their mark. The drain pipe, often made of flexible PVC, can become blocked by algae deposits or simply by a reverse gradient created when a support settles.

The symptoms of a partially blocked drain are easy to recognise:

• droplets beading at the bottom of the indoor unit;
• a "gurgling" noise during operation;
• automatic shutdown with an error code linked to the safety float (on ducted systems and cassettes).

In heating mode, condensate production shifts location: it is the outdoor unit that produces it, particularly during defrost cycles. So check that the lower drain of the outdoor unit is clear and that water will not build up under the chassis, where it will freeze on the first sub-zero nights. On ground-mounted installations, a drip tray with drainage directed towards a soakaway or free-draining area prevents a sheet of ice forming under the unit.