The general rule is simple: once a year. What is not simple is knowing when that rule does not fit, because the answer depends less on the calendar and more on how many hours the unit runs and the air it breathes.
The table, by type of use
| Situation | Deep clean | Why |
|---|---|---|
| Permanent residence, summer use | Once a year | Use concentrated in three or four months |
| Home with heating in winter | Once a year, before summer | Long use but a moderate dust load |
| Holiday home closed for several months | Once a year, before reopening | The humidity built up while the house is closed is the worst case |
| Holiday rental | 1 to 2 times a year | Intensive use and guest turnover |
| Hotel | 1 to 2 times a year, in low season | Continuous operation for months |
| Office | Once a year | Regular occupancy, moderate dust |
| Restaurant or kitchen | Twice a year | Grease in the air clogs the heat exchanger far faster |
| Home with pets | Twice a year | Hair and dander clog filters and fins |
| Recent building work in the property | As soon as the work is finished | Construction dust is the most aggressive of all |
Between deep cleans, the filters should be washed every two months during the season of use. That is five minutes' work and needs no technician.
The signs that count for more than the calendar
There are four indicators that are worth more than any table. If any appears, do not wait for the year to pass.
A smell when switching on. The clearest of all. It means there is already biofilm on the heat exchanger or standing water in the tray.
Less air coming out. Put your hand in front of the grille. If the airflow is weaker than you remember, the fins are blocked.
Having to lower the thermostat. If it used to cool well at 23° and now needs 19° for the same effect, the unit has lost capacity. It is not the heat outside.
Water dripping from the unit. Almost always it is the drainage clogged by dirt built up in the tray. It is solved in the cleaning.
What postponing costs
Here the numbers are clear and worth looking at.
An experimental study on split units measured performance with the heat exchanger progressively blocked. With around 30% of the surface covered, cooling capacity fell 19.1%, energy efficiency fell 13.5% and consumption rose 6.4%.
Research on dirt build-up in residential heat exchangers shows that a 5 to 10% reduction in airflow is enough for capacity and efficiency to fall by 2 to 4%. And that a typical heat exchanger accumulates enough dirt to double the pressure drop in about seven and a half years — against a service life of 15 to 30 years. In other words: the unit spends most of its life working below what it was sized to deliver.
Translated into the unit you have at home: a 12,000 BTU unit that loses 19% of capacity behaves like one of 9,700 BTU. It still cools, but it takes longer, runs more hours and consumes more to give the same result.
And there is the reverse effect, which pays off: every extra degree you can put on the thermostat represents about 279 kWh saved per year, according to the same research. Recovering capacity is recovering degrees.
A house closed for several months: the most critical case
It is the most common situation in the Algarve and the least guarded against.
A house closed from October to May has high relative humidity, still air and zero ventilation. The inside of the unit stays damp for months, at ambient temperature, with no air current. These are the ideal conditions for fungal growth — better, even, than during use.
Whoever reopens the house in May and switches on the air conditioning gets back everything that grew inside over the winter, concentrated in the first few minutes.
If you have a holiday home or manage other people's properties, the right time to clean is not August. It is April or May, before first use.
Why low season is better if you have many units
For hotels, holiday rentals and condominiums, the timing of the work matters as much as the frequency.
In high season there is no window: rooms are occupied, the units cannot stop and any work is done in a rush between departures and arrivals. In low season you plan by wing or by floor, with time to do the work properly.
From October to March is when we do it best, and it is also when we have availability to do it.
In short
Once a year for most homes. Twice for anyone who uses the unit many hours a day or has grease, pets or building work in the mix. And always before reopening a house that has been closed.
If yours is already showing any of the four signs above, it is not worth waiting for next summer.
