Air conditioning in a holiday home: the winterising protocol that stops you finding a breakdown come spring
L'équipe Proclimo
8 Oct 2026 - 08 min read
The last weekend of October looks the same everywhere: you empty the fridge, drain the water system, close the shutters, turn off the gas and the water. Then your eye lands on the reversible air conditioner remote sitting on the sideboard, and the question comes: do we switch it off, or leave it on?
The hesitant answer — "we're cutting everything at the panel anyway" — is exactly what fills HVAC technicians' diaries between March and May. An air-to-air heat pump left for six months with no power, no ventilation and no monitoring in an unheated house is not an appliance "at rest": it is an appliance exposed to frost, stagnant damp, rodents and power surges, with nobody around to notice.
What really happens inside a closed-up house
A holiday home left empty from November to April combines three conditions your equipment hates.
Relative humidity climbs. With no occupants, no heating and often reduced natural ventilation (shutters closed, air inlets blocked), the indoor air becomes saturated. In a seaside or mountain house, readings of 75 to 85% relative humidity in February are common. Yet the indoor unit of a split system contains a heat exchanger, a tray and a fan that still hold condensate residue from the summer. That is the perfect breeding ground for mould and bacteria. France's ANSES regularly points out in its work on indoor air quality that poorly maintained air treatment systems are a source of exposure to microbiological contaminants — and a machine shut up for six months with a damp exchanger is exactly that scenario.
Frost attacks the water points. People always think of the plumbing, never of the condensate drain. Yet in heating mode it is the outdoor unit that produces water — several litres on a cold, damp day during defrost cycles. If the machine was switched off in September, a residue of water can remain in the defrost tray or in a loop of the drain hose. Frost at -6 °C is enough to crack a plastic tray or burst a badly sloped hose.
Electrical networks are unstable. Winter storms, grid switching operations and restarts after an outage all cause surges. An outdoor unit's control board left energised without protection can be destroyed. Conversely, a machine that is completely de-energised loses its crankcase heater — more on that shortly; it is the most misunderstood point of the whole subject.
Cut the power or leave it on: the real trade-off
There is no universal answer. It depends on three things: the climate zone, the length of the absence and the type of appliance.
The crankcase heater, the manufacturers' argument
Most outdoor units on modern air-to-air heat pumps include a crankcase heater that keeps the compressor oil above ambient temperature. The aim: to stop the refrigerant dissolving into the oil while at rest in cold weather. On restart, diluted oil loses its lubricating power and the compressor can suffer liquid slugging.
That is why manuals from Daikin, Mitsubishi Electric or Toshiba overwhelmingly recommend re-energising the appliance 6 to 12 hours before starting it after a prolonged shutdown, and for several models not cutting the power supply during cold weather. This instruction appears in the installation manual, rarely in the consumer user guide — which is why almost nobody knows about it.
In practice: if you switch off the dedicated breaker for your split system over winter, write on the panel door "re-energise 12 h before use". It is the cheapest precaution in the whole protocol.
The "everything off" scenario
It makes sense in one specific case: a property in an area with no risk of sustained frost, a total absence of more than four months, and no frost-protection stakes (no pressurised water circuit, no wine cellar, no sensitive equipment).
Advantages: zero standby consumption, zero risk of a surge hitting the board, zero risk of a stray start-up if a shutter falls on the curious neighbour's remote. Drawbacks: loss of the crankcase heater, loss of all remote monitoring, and a cold restart in spring.
The "managed frost protection" scenario
This is now the majority choice among owners of mountain houses or homes in regions with harsh winters. It means leaving the reversible air conditioner energised, in heating mode, with a low setpoint of 8 to 12 °C. Most machines have a dedicated function (often called "8 °C heating", "frost protection" or "minimum maintenance").
What it delivers:
- the building fabric stays above the critical dew point and humidity stabilises;
- water pipes that cannot be drained are protected;
- the compressor runs regularly, which prevents refrigerant migration and keeps the seals lubricated;
- the outdoor unit continues its defrost cycles and does not accumulate standing water.
What it costs: in a moderately insulated 90 m² house in climate zone H2, holding 10 °C from November to March with an air-to-air heat pump rated SCOP 4 typically means 400 to 900 kWh over the season, or €80 to €180 at 2026 regulated tariffs. Compare that with an HVAC call-out and a compressor replacement, which starts at €1,200.
| Scenario | Winter cost | Frost risk | Humidity risk | Compressor risk |
|---|---|---|---|---|
| Everything off | €0 | High | High | Moderate to high |
| Standby with no setpoint | €5 to €15 | High | High | Low |
| Frost protection 8-12 °C | €80 to €180 | Low | Low | Very low |
The closing-up protocol, in order
1. A drying cycle before you close up
Never switch off an air conditioner right after using it in cooling mode. The indoor unit's exchanger is soaking wet. Run the appliance 2 to 3 hours in fan-only mode, or use the dry/self-clean function if it has one. The goal is to dry out the coil and the tray before putting the machine to rest.
2. Clean the filters and the tray
The filters come out, rinse under lukewarm water, and must be completely dry before refitting. A filter put back damp in a house closed up for six months comes out black with mould. For the exchanger fins, an air conditioner cleaning aerosol of the active-foam type, applied and then left to drain away through the condensate hose, handles most of the job. A soft brush and a vacuum with a narrow nozzle finish the work on the grille and the accessible fan.
Anything beyond that — removing the blower wheel, deep disinfection of the exchanger, checking the refrigerant charge — is a job for an HVAC technician and a maintenance contract.
3. Secure the condensate drainage
This is the point that causes the most damage in spring. Check:
- that the indoor unit's drain hose is clear, with no low point full of water;
- that there is no improvised trap in the outdoor section that could freeze;
- that the end of the outdoor hose is not blocked by leaves or an insect nest;
- that the outdoor unit's tray, if accessible, is empty and its outlet clear.
If you have a condensate pump, it must be drained. A self-regulating heating tape fitted to an exposed condensate drain is a cheap, effective solution in severe frost zones — provided you leave the power supply on.
4. Protect the outdoor unit without suffocating it
The classic mistake: wrapping the unit in an airtight plastic tarpaulin. You trap moisture, encourage corrosion, and if the machine restarts in frost-protection mode you block the airflow.
What works: a breathable protective cover for an outdoor unit, fitted over the top only in "roof" style if the machine stays in service, or fully if it is completely switched off. In the mountains, a rigid snow canopy makes more sense than a cover. Also remember to clear dead vegetation from around the unit: that is what attracts the rodents that come and chew through the low-voltage cables.
5. Protect the electronics
A modular surge protector for the electrical panel on the heat pump circuit is mandatory in certain French départements with a high lightning strike density (NF C 15-100) and remains an excellent idea everywhere else for a house left on its own. If you keep the machine energised, it is the logical complement to frost protection.
6. Set up remote monitoring
This is the genuine advance of the last five years. A WiFi-connected thermometer and hygrometer placed in the main room sends you a notification if the temperature drops below 5 °C or the humidity goes above 70%. Some models support a remote probe you can run into the plant room. Paired with a WiFi control module compatible with your air conditioner brand, it lets you restart the heating remotely ahead of a forecast cold snap, or pre-heat the house the day before you arrive.
For older installations with no connectivity, a smart plug with energy metering on an emergency backup heater is a useful safety net: if the heat pump faults, you still have backup frost protection.
The insurance angle: read your policy before you leave
This is where many owners get a nasty surprise. Most home insurance policies contain an unoccupancy clause: beyond a certain number of consecutive days of absence (often 30, 60 or 90 depending on the insurer), water damage and theft cover can be reduced, or even excluded, if certain measures have not been taken.
The conditions most often required for a holiday home during the frost season:
- draining or purging the water installations, or maintaining heating at a minimum temperature;
- closing the main stopcock;
- declaring the "holiday home" use and the length of unoccupancy.
Water damage caused by a condensate tray cracked by frost in a house left unheated and undrained is the textbook case where an insurer will dispute the claim. And as we have seen in our articles on insurance and air conditioning, the question of liability gets even more complicated if the water runs down into a neighbour's flat in a multi-unit building.
Practical tip: keep the invoice for the last service on your air-to-air heat pump and, if you opt for frost protection, a screenshot of the consumption history or the readings from your connected probe. These are very concrete pieces of evidence in the event of a dispute.
Reopening: what to check before pressing "on"
Spring is the second critical moment. Abruptly restarting a machine that has sat idle for six months causes more breakdowns than winter itself.
D-day minus 1. Switch the dedicated breaker back on if you had cut it, and leave the appliance energised for at least 12 hours without starting it. The crankcase heater does its job.
Visual inspection. Open the front of the indoor unit: black marks on the blower wheel, a musty smell, warped filters. Outside, look for rodent marks on the conduits, flattened fins, condensation at the base of the unit, and check that all four anti-vibration pads are still in place and not crushed. A new set of anti-vibration pads for an outdoor unit costs a few tens of euros and heads off a good number of budding complaints from the neighbours.
First start-up. Begin with fan-only for 15 minutes. Then cooling mode at a mid-range setpoint, and watch for:
- a difference of 8 to 12 °C between return air and supply air after 10 minutes;
- no metallic noise or rattling;
- water flowing from the end of the condensate hose after 20 to 30 minutes;
- no fault code on the display.
If the temperature difference is small or the outdoor unit ices up in cooling mode, suspect an insufficient refrigerant charge. Any work on the refrigerant circuit (R-32, R-290) is reserved for professionals holding a certificate of competence under the F-Gas regulations: check this before letting anyone connect a pressure gauge to your installation.
Three special cases
Mountain house above 1,000 m. Frost protection via an air-to-air heat pump reaches its limits: performance collapses below -10 °C and some machines switch over to a backup electric element, with a bill that rockets. Conventional electric frost protection, controlled remotely, is often more appropriate, with the heat pump staying energised but switched off.
Seaside apartment. No frost problem, but major salt corrosion on the outdoor unit. Rinsing the fins with clean water before closing up, plus an anti-corrosion treatment on the metal parts, significantly extends the unit's service life.
Property let out seasonally. Be careful: a machine left in the hands of successive tenants with no filter cleaning between stays deteriorates very quickly. A spare set of filters and written instructions posted in the property solve a good part of the problem — and protect your indoor air quality.
Key takeaways
Closing up a house for winter with a reversible air conditioner installed means weighing a few tens of euros of electricity against several hundred euros of potential repairs. In most cases, in a frost-exposed area, managed frost protection at 8-12 °C with connected monitoring remains the best cost/risk ratio. Everywhere, the non-negotiable minimum comes down to four actions: dry the exchanger before closing up, clean and dry the filters, clear the condensate drain, and write a note on the electrical panel reminding you to re-energise the day before restarting.
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