The hygrometer shows 65% with the unit on – should it not drop?
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Popești-Leordeni, new block, 12,000 BTU Daikin, 3 years old, bedroom. I bought a hygrometer for the child. With the unit on Cool at 24 °C, humidity sits at 62–65% and does not go below 60 even after 3 hours. The unit cools. Is that normal? I thought air conditioning dries the air.
What is really happening
Dehumidification happens on the evaporator: air passes over cold fins, water condenses and runs into the pan. When the fins are coated in dust and biofilm, heat transfer drops, the air rushes past the surface without reaching dew point and leaves cooled but not dried – and the water that does condense stays trapped in the felt and re-evaporates into the jet at the next compressor pause. Result: temperature drops, humidity does not. In a child's bedroom, 65% is above the 50% threshold at which dust mites multiply and above the 60% maximum EPA recommends. At 3 years in a bedroom, the evaporator is dirty enough for this effect. If it still does not drop after sanitising, we talk about sizing and sources (laundry, bathroom), but in practice cleaning most often fixes it.
- A hygrometer tracks room humidity.
- Drying laundry, cooking and other activities add moisture.
- Leaks and damp surfaces require separate inspection.
- Relative humidity above about 50% increases house dust mite levels; a clean air conditioner helps keep humidity below this threshold. [WHO / NCBI Bookshelf, 2009]
- EPA recommends keeping indoor humidity below 60%, ideally between 30 and 50%, to prevent mould. [US EPA, 2024]
- A typical evaporator fouls enough to double its pressure drop in about 7.5 years, reducing airflow by 5–7% and raising fan power by 1–10%. [Lawrence Berkeley National Laboratory / , 2002]
- The relative humidity range with the fewest adverse health effects is roughly 40–60%. [Environmental Health Perspectives / PubM, 1986]
What we do on site
- We measure humidity on arrival with our hygrometer and yours, as a baseline.
- Collection cover; cleaning solution on evaporator, roller, pan; pressure wash in place.
- Sanitising solution; drying on 'Fan'.
- Outdoor unit, washed in place.
- Test: 45 minutes on Cool 25 °C – we read the humidity together. See current rates on the Prices page.
What you can do yourself
- Evenings: Dry 30–60 minutes, then Cool 25–26 °C – Dry keeps the compressor longer in water-removal mode.
- Bathroom door closed, fan on after showers; no laundry in the bedroom.
- Filters every two weeks, 'Fan' 20 minutes before shut-off.
What NOT to do
- Do not drop to 21 °C 'to dry more' – it cools even faster, cycles off more often, humidity stays.
- Do not put a dehumidifier next to the unit as a permanent fix before sanitising.
- Do not keep the window open with the unit running.
Prevention
The hygrometer is the best dirty-unit indicator: when humidity no longer drops below 55% with the unit on, it is time for sanitising. Annually, as a rule.
When to call us
0764 012 943 / WhatsApp, Monday–Saturday 08:00–20:00. Popești-Leordeni.
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The information on this page is educational and based on cited sources; it does not replace medical advice. We do not do repairs, installation or refrigerant circuit work.